C# and Asp.Net Questions 面试题

C# and Asp.Net Questions

Introduction

My this article provides a collection of numerous .Net, C#, ADO.NET, Web Services, .Net Framework questions and answers for which a reader has to look around for entire internet on different community web sites. Most of the questions and answers you must have already read. The purpose of this article is to consolidate at the most study material related to .Net at one single place.

ASP.NET

What is view state and use of it? The current property settings of an ASP.NET page and those of any ASP.NET server controls contained within the page. ASP.NET can detect when a form is requested for the first time versus when the form is posted (sent to the server), which  allows you to program accordingly.

What are user controls and custom controls?Custom controls:
 A control authored by a user or a third-party software vendor that does not belong to   the .NET Framework class library. This is a generic term that includes user controls. A  custom server control is used in Web Forms (ASP.NET pages). A custom client control is used  in Windows Forms applications.

User Controls:
In ASP.NET: A user-authored server control that enables an ASP.NET page to be re-used   as a server control. An ASP.NET user control is authored declaratively  and persisted as a  text file with an .ascx extension. The ASP.NET page framework compiles a user control on  the fly to a class that derives from the        System.Web.UI.UserControl class.

What are the validation controls? A set of server controls included with ASP.NET that test user input in HTML and Web server  controls for programmer-defined requirements. Validation controls perform input checking in server code. If the user is working with a browser that supports DHTML, the validation  controls can also perform validation using client script.

What's the difference between Response.Write() andResponse.Output.Write()? The latter one allows you to write formattedoutput.

What methods are fired during the page load? Init ()  When the page is instantiated, Load() - when the page is loaded into server  memory,PreRender () - the brief moment before the page is displayed to the user  as HTML, Unload() - when page finishes loading.

Where does the Web page belong in the .NET Framework class hierarchy? System.Web.UI.Page

Where do you store the information about the user's locale? System.Web.UI.Page.Culture

What's the difference between Codebehind="MyCode.aspx.cs" and Src="MyCode.aspx.cs"? CodeBehind is relevant to Visual Studio.NET only.

What's a bubbled event?
When you have a complex control, like DataGrid, writing an event processing routine for each object (cell, button,row, etc.) is quite tedious. The controls can bubble up their eventhandlers, allowing the main DataGrid event handler to take care of its constituents.
Suppose you want a certain ASP.NET function executed on MouseOver over a certain button.

Where do you add an event handler? It's the Attributesproperty, the Add function inside that property.            
e.g. btnSubmit.Attributes.Add("onMouseOver","someClientCode();")

What data type does the RangeValidator control support? Integer,String and Date.

What are the different types of caching?
Caching is a technique widely used in computing to increase performance by keeping frequently accessed or expensive data in memory. In context of web application, caching is used to retain the pages or data across HTTP requests and reuse them without the expense of recreating them.ASP.NET has 3 kinds of caching strategiesOutput CachingFragment CachingData    

CachingOutput Caching: Caches the dynamic output generated by a request. Some times it is useful to cache  the output of a website even for a minute, which will result in a better  performance. For caching the whole page the page should have OutputCache directive.<%@ OutputCache Duration="60" VaryByParam="state" %>

Fragment Caching: Caches the portion of the  page generated by the request. Some times it is not practical to cache the entire page, in such cases we can cache a portion of page<%@ OutputCache Duration="120" VaryByParam="CategoryID;SelectedID"%>

Data Caching:  Caches the objects programmatically. For     data caching asp.net provides a cache object for eg: cache["States"] = dsStates;

What do you mean by authentication and authorization? Authentication is the process of validating a user on the credentials (username and     password) and authorization performs after authentication. After Authentication a user will     be verified for performing the various tasks, It access is limited it is known as       authorization.

What are different types of directives in .NET?
@Page : Defines page-specific attributes used by the ASP.NET page parser and compiler. Can       be included only in .aspx files <%@ Page AspCompat="TRUE" language="C#" %>  
@Control :Defines control-specific attributes used by the ASP.NET page parser and        compiler. Can be included only in .ascx files. <%@ Control Language="VB" EnableViewState="false" %>
@Import : Explicitly imports a namespace into a page or user control. The Import         directive cannot have more than one namespace attribute. To import multiple     namespaces,     use multiple @Import directives. <% @ Import Namespace="System.web" %>
@Implements : Indicates that the current page or user control implements the specified .NET      framework interface.<%@ Implements Interface="System.Web.UI.IPostBackEventHandler" %>
@Register : Associates aliases with namespaces and class names for concise notation in   custom server control syntax.<%@ Register Tagprefix="Acme" Tagname="AdRotator" Src="AdRotator.ascx" %>
@Assembly : Links an assembly to the current page during compilation, making all         the     assembly's classes and interfaces available for use on the      page. <%@ Assembly Name="MyAssembly" %><%@ Assembly Src="MySource.vb" %>
@OutputCache : Declaratively controls the output caching policies of an ASP.NET page or a        user control contained in a page<%@ OutputCache Duration="#ofseconds" Location="Any | Client | Downstream | Server | None" Shared="True | False" VaryByControl="controlname" VaryByCustom="browser | customstring" VaryByHeader="headers" VaryByParam="parametername" %>
@Reference : Declaratively indicates that another user control or page source file               should be dynamically compiled and linked against the page in which this directive is   declared.

Note: Few of the references are taken from other sites/sources

How do I debug an ASP.NET application that wasn't written with Visual Studio.NET and that doesn't use code-behind? Start the DbgClr debugger that comes with the .NET Framework SDK, open the file containing      the code you want to debug, and set your breakpoints. Start the ASP.NET application. Go back to DbgClr, choose Debug Processes from the Tools menu, and select  aspnet_wp.exe from  the list of processes. (If aspnet_wp.exe doesn't appear in the list,check the "Show system      processes" box.) Click the Attach button to attach to aspnet_wp.exe and begin debugging.
Be sure to enable debugging in the ASPX file before debugging it with DbgClr. You can   enable tell ASP.NET to build debug executables by placing a
<%@ Page Debug="true" %>   statement at the top of an ASPX file or a   statement in a Web.config file.

Can a user browsing my Web site read my Web.config or Global.asax files?
No. The section of Machine.config, which holds the master configuration  settings for ASP.NET, contains entries that map ASAX files, CONFIG files, and selected  other file types to an HTTP handler named HttpForbiddenHandler, which fails attempts to retrieve the associated file. You can modify it by editing  Machine.config or including an section in a local Web.config file.

What's the difference between Page.RegisterClientScriptBlock and Page.RegisterStartupScript? RegisterClientScriptBlock is for returning blocks of client-side script containing functions. RegisterStartupScript is for returning blocks of client-script not   packaged in functions-in other words, code that's to execute when the page is loaded. The latter positions script blocks near the end of the document so elements on the page that the script interacts are loaded before the script runs.<%@ Reference Control="MyControl.ascx" %>

Is it necessary to lock application state before accessing it? Only if you're performing a multistep update and want the update to be treated as an atomic     operation. Here's an example:

Application.Lock ();
                Application["ItemsSold"] = (int) Application["ItemsSold"] + 1;
                Application["ItemsLeft"] = (int) Application["ItemsLeft"] - 1;
                Application.UnLock ();

By locking application state before updating it and unlocking it afterwards, you ensure  that another request being processed on another thread doesn't read application state   at exactly the wrong time and see an inconsistent view of it. If I update session state, should I lock it, too? Are concurrent accesses by multiple requests executing on multiple threads a concern with session state?
Concurrent accesses aren't an issue with session state, for two reasons. One, it's unlikely that two requests from the same user will overlap. Two, if they do overlap, ASP.NET locks down session state during request processing so that two threads can't touch it at once. Session state is locked down when the HttpApplication instance that's processing the request fires an AcquireRequestState event and unlocked when it fires a ReleaseRequestState event.

Do ASP.NET forms authentication cookies provide any protection against replay attacks? Do they, for example, include the client's IP  address or anything else that would distinguish the real client from an attacker?
No. If an authentication cookie is stolen, it can be used by an attacker. It's up to you to     prevent this from happening by using an encrypted communications channel (HTTPS). Authentication cookies issued as session cookies, do, however,include a time-out valid that     limits their lifetime. So a stolen session cookie can only be used in replay attacks as long as the ticket inside the cookie is valid. The default time-out interval is 30 minutes.You can change that by modifying the timeout attribute accompanying the element in Machine.config or a local Web.config file. Persistent authentication cookies do not  time-out and therefore are a more serious security threat if stolen.

How do I send e-mail from an ASP.NET application?

MailMessage message = new MailMessage ();
        message.From = ;
        message.To = ;
        message.Subject = "Scheduled Power Outage";
        message.Body = "Our servers will be down tonight.";
        SmtpMail.SmtpServer = "localhost";
        SmtpMail.Send (message);

 MailMessage and SmtpMail are classes defined in the .NET Framework Class Library's  System.Web.Mail namespace. Due to a security change made to ASP.NET just before it shipped,  you need to set SmtpMail's SmtpServer property to "localhost" even though "localhost" is  the default. In addition, you must use the IIS configuration applet to enable localhost  (127.0.0.1) to relay messages through the local SMTP service.

What are VSDISCO files?

VSDISCO files are DISCO files that support dynamic discovery of Web services. If you place the following VSDISCO file in a directory on your Web server, for example, it returns   references to all ASMX and DISCO files in the host directory and any subdirectories not         noted in elements:

xml version="1.0" ?>
                <dynamicDiscovery
                  xmlns="urn:schemas-dynamicdiscovery:disco.2000-03-17">
                  <exclude path="_vti_cnf" />
                  <exclude path="_vti_pvt" />
                  <exclude path="_vti_log" />
                  <exclude path="_vti_script" />
                  <exclude path="_vti_txt" />
                </dynamicDiscovery>

How does dynamic discovery work?
ASP.NET maps the file name extension VSDISCO to an HTTP handler that scans the host  directory and subdirectories for ASMX and DISCO files and returns a dynamically generated DISCO document. A client who requests a VSDISCO file gets back what appears to be a static DISCO document.
Note that VSDISCO files are disabled in the release version of ASP.NET. You can reenable them by uncommenting the line   in the section of Machine.config that maps *.vsdisco to System.Web.Services.Discovery.DiscoveryRequestHandler and granting the ASPNET  user account permission to read the IIS metabase. However, Microsoft is actively discouraging the use of VSDISCO files because they could represent a threat to Web server security.

Is it possible to prevent a browser from caching an ASPX page?
Just call SetNoStore on the HttpCachePolicy object exposed through the Response object's Cache property, as demonstrated here:

<%@ Page Language="C#" %>
        <html>
          <body>
            <%
              Response.Cache.SetNoStore ();
              Response.Write (DateTime.Now.ToLongTimeString ());
            %>
          </body>
        </html>

SetNoStore works by returning a Cache-Control: private, no-store header in the HTTP response. In this example, it prevents caching of a Web page that shows the current time.

What does AspCompat="true" mean and when should I use it?
AspCompat is an aid in migrating ASP pages to ASPX pages. It defaults to false but should be set to true in any ASPX file that creates apartment-threaded COM objects--that is, COM objects registered ThreadingModel=Apartment. That includes all COM objects written with  Visual Basic 6.0. AspCompat should also be set to true (regardless of threading  model)  if the page creates COM objects that access intrinsic ASP objects such as Request and Response. The following directive sets AspCompat to true:

<%@ Page AspCompat="true" %> 

Setting AspCompat to true does two things. First, it makes intrinsic ASP objects available      to the COM components by placing unmanaged wrappers around the equivalent ASP.NET objects. Second, it improves the performance of calls that the page places to apartment- threaded COM objects by ensuring that the page (actually, the thread that processes the  request for the page) and the COM objects it creates share an apartment. AspCompat="true" forces ASP.NET request threads into single-threaded apartments (STAs). If those threads create COM objects marked ThreadingModel=Apartment, then the objects are created in the same STAs as the threads that created them. Without AspCompat="true," request threads run in a multithreaded apartment (MTA) and each call to an STA-based COM object incurs a performance hit when it's  marshaled across apartment boundaries.

Do not set AspCompat to true if your page uses no COM objects or if it uses COM objects that don't access ASP intrinsic objects and that are registered ThreadingModel=Free or  ThreadingModel=Both.

Explain the differences between Server-side and Client-side code?  Server side scripting means that all the script will be executed by the server and  interpreted as needed. ASP doesn't have some of the functionality like sockets, uploading,  etc. For these you have to make a custom components usually in VB or VC++. Client side  scripting means that the script will be executed immediately in the browser such as form field validation, clock, email validation, etc. Client side scripting is usually done in  VBScript or JavaScript. Download time, browser compatibility, and visible code - since  JavaScript and VBScript code is included in the HTML page, then anyone can see the code by viewing the page source. Also a possible security hazards for the client computer.

What type of code (server or client) is found in a Code-Behind class?
C#

Should validation (did the user enter a real date) occur server-side or client-side? Why? Client-side validation because there is no need to request a server side date when you  could obtain a date from the client machine.

What are ASP.NET Web Forms? How is this technology different than what is available though ASP?
Web Forms are the heart and soul of ASP.NET. Web Forms are the User Interface (UI) elements that give your Web applications their look and feel. Web Forms are similar to Windows Forms in that they provide properties, methods, and events for the controls that are placed onto  them. However, these UI elements render themselves in the appropriate markup language   required by the request, e.g. HTML. If you use Microsoft Visual Studio .NET, you will also get the familiar drag-and-drop interface used to create your UI for your Web application.

What is the difference between Server.Transfer and Response.Redirect? Why would I choose one over the other?
In earlier versions of IIS, if we wanted to send a user to a new Web page, the only option we had was Response.Redirect. While this method does accomplish our goal, it has several important drawbacks. The biggest problem is that this method causes each page to be treated as a separate transaction. Besides making it difficult to maintain your transactional   integrity, Response.Redirect introduces some additional headaches. First, it prevents good encapsulation of code. Second, you lose access to all of the properties in the Request  object. Sure, there are workarounds, but they're difficult. Finally, Response.Redirect  necessitates a round trip to the client, which, on high-volume sites, causes scalability problems.
As you might suspect, Server.Transfer fixes all of these problems. It does this by performing the transfer on the server without requiring a roundtrip to the client.

How can you provide an alternating color scheme in a Repeater control?
AlternatingItemTemplate Like the ItemTemplate element, but rendered for every other row (alternating items) in the Repeater control. You can specify a different appearance for the AlternatingItemTemplate element by setting its style properties.

Which template must you provide, in order to display data in a Repeater control?
ItemTemplate

What event handlers can I include in Global.asax?

Application_Start,Application_End, Application_AcquireRequestState, Application_AuthenticateRequest, Application_AuthorizeRequest, Application_BeginRequest, Application_Disposed,  Application_EndRequest, Application_Error, Application_PostRequestHandlerExecute, Application_PreRequestHandlerExecute,
Application_PreSendRequestContent, Application_PreSendRequestHeaders, Application_ReleaseRequestState, Application_ResolveRequestCache, Application_UpdateRequestCache, Session_Start,Session_End
You can optionally include "On" in any of method names. For example, you can name a BeginRequest event handler.Application_BeginRequest or Application_OnBeginRequest.You can also include event handlers in Global.asax for events fired by custom HTTP modules.Note that not all of the event handlers make sense for Web Services (they're designed for ASP.NET applications in general, whereas .NET XML Web Services are specialized instances of an ASP.NET app). For example, the Application_AuthenticateRequest and Application_AuthorizeRequest events are designed to be used with ASP.NET Forms authentication. 

What is different b/w  webconfig.xml & Machineconfig.xml

Web.config & machine.config both are configuration files.Web.config contains settings specific to an application where as machine.config contains settings to a computer. The Configuration system first searches settings in machine.config file & then looks in application configuration  files.Web.config, can appear in multiple directories on an ASP.NET Web application server. Each Web.config file applies configuration settings to its own directory and all child directories below it. There is only Machine.config file on a web server.

If I'm developing an application that must accomodate multiple security levels though secure login and my ASP.NET web appplication is spanned across three web-servers (using round-robbin load balancing) what would be the best approach to maintain login-in state for the users?
Use the state server or store the state in the database. This can be easily done through simple setting change in the web.config. 

<SESSIONSTATE
StateConnectionString="tcpip=127.0.0.1:42424"
sqlConnectionString="data source=127.0.0.1; user id=sa; password="
cookieless="false"
timeout="30"
/>

You can specify mode as "stateserver" or "sqlserver".

Where would you use an iHTTPModule, and what are the limitations of any approach you might take in implementing one
"One of ASP.NET's most useful features is the extensibility of the HTTP pipeline, the path that data takes between client and server. You can use them to extend your ASP.NET applications by adding pre- and post-processing to each HTTP request coming into your application. For example, if you wanted custom authentication facilities for your application, the best technique would be to intercept the request when it comes in and process the request in a custom HTTP module.

How do you turn off cookies for one page in your site?
Since no Page Level directive is present, I am afraid that cant be done.

How do you create a permanent cookie?  Permanent cookies are available until a specified expiration date, and are stored on the hard disk.So Set the 'Expires' property any value greater than DataTime.MinValue with respect to the current datetime. If u want the cookie which never expires set its Expires property equal to DateTime.maxValue.
Which method do you use to redirect the user to another page without performing a round trip to the client?
Server.Transfer and Server.Execute

What property do you have to set to tell the grid which page to go to when using the Pager object?
CurrentPageIndex

Should validation (did the user enter a real date) occur server-side or client-side? Why?  It should occur both at client-side and Server side.By using expression validator control with the specified expression ie.. the regular expression provides the facility of only validatating the date specified is in the correct format or not. But for checking the date where it is the real data or not should be done at the server side, by getting the system date ranges and checking the date whether it is in between that range or not.

What does the "EnableViewState" property do? Why would I want it on or off? 
Enable ViewState turns on the automatic state management feature that enables server controls to re-populate their values on a round trip without requiring you to write any code. This feature is not free however, since the state of a control is passed to and from the server in a hidden form field. You should be aware of when ViewState is helping you and when it is not. For example, if you are binding a control to data on every round trip, then you do not need the control to maintain it's view state, since you will wipe out any re-populated data in any case. ViewState is enabled for all server controls by default. To disable it, set the EnableViewState property of the control to false.

What is the difference between Server.Transfer and Response.Redirect? Why would I choose one over the other? 
Server.Transfer() : client is shown as it is on the requesting page only, but the all the content is of the requested page. Data can be persist accros the pages using Context.Item collection, which is one of the best way to transfer data from one page to another keeping the page state alive.

Response.Redirect() :client know the physical location (page name and query string as well). Context.Items loses the persisitance when nevigate to destination page. In earlier versions of IIS, if we wanted to send a user to a new Web page, the only option we had was Response.Redirect. While this method does accomplish our goal, it has several important drawbacks. The biggest problem is that this method causes each page to be treated as a separate transaction. Besides making it difficult to maintain your transactional integrity, Response.Redirect introduces some additional headaches. First, it prevents good encapsulation of code. Second, you lose access to all of the properties in the Request object. Sure, there are workarounds, but they're difficult. Finally, Response.Redirect necessitates a round trip to the client, which, on high-volume sites, causes scalability problems. As you might suspect, Server.Transfer fixes all of these problems. It does this by performing the transfer on the server without requiring a roundtrip to the client.

Can you give an example of when it would be appropriate to use a web service as opposed to a non-serviced .NET component? 

• Communicating through a Firewall When building a distributed application with 100s/1000s of users spread over multiple locations, there is always the problem of communicating between client and server because of firewalls and proxy servers. Exposing your middle tier components as Web Services and invoking the directly from a Windows UI is a very valid option. 
• Application Integration When integrating applications written in various languages and running on disparate systems. Or even applications running on the same platform that have been written by separate vendors. 
• Business-to-Business Integration This is an enabler for B2B intergtation which allows one to expose vital business processes to authorized supplier and customers. An example would be exposing electronic ordering and invoicing, allowing customers to send you purchase orders and suppliers to send you invoices electronically. 
• Software Reuse This takes place at multiple levels. Code Reuse at the Source code level or binary componet-based resuse. The limiting factor here is that you can reuse the code but not the data behind it. Webservice overcome this limitation. A scenario could be when you are building an app that aggregates the functionality of serveral other Applicatons. Each of these functions could be performed by individual apps, but there is value in perhaps combining the the multiple apps to present a unifiend view in a Portal or Intranet. 
• When not to use Web Services: Single machine Applicatons When the apps are running on the same machine and need to communicate with each other use a native API. You also have the options of using component technologies such as COM or .NET Componets as there is very little overhead. 
• Homogeneous Applications on a LAN If you have Win32 or Winforms apps that want to communicate to their server counterpart. It is much more efficient to use DCOM in the case of Win32 apps and .NET Remoting in the case of .NET Apps

Can you give an example of what might be best suited to place in the Application_Start and Session_Start subroutines? The Application_Start event is guaranteed to occur only once throughout the lifetime of the application. It's a good place to initialize global variables. For example, you might want to retrieve a list of products from a database table and place the list in application state or the Cache object. SessionStateModule exposes both Session_Start and Session_End events.

What are the advantages and  disadvantages of viewstate? The primary advantages of the ViewState feature in ASP.NET are:

1. Simplicity. There is no need to write possibly complex code to store form data between page submissions.
2. Flexibility. It is possible to enable, configure, and disable ViewState on a control-by-control basis, choosing to persist the values of some fields but not others.

There are, however a few disadvantages that are worth pointing out:

1. Does not track across pages. ViewState information does not automatically transfer from page to page. With the session approach, values can be stored in the session and accessed from other pages. This is not possible with ViewState, so storing data into the session must be done explicitly.
2. ViewState is not suitable for transferring data for back-end systems. That is, data still has to be transferred to the back end using some form of data object.

Describe session handling in a webfarm, how does it work and what are the limits?  ASP.NET Session supports storing of session data in 3 ways, i] in In-Process ( in the same memory that ASP.NET uses) , ii] out-of-process using Windows NT Service )in separate memory from ASP.NET ) or iii] in SQL Server (persistent storage). Both the Windows Service and SQL Server solution support a webfarm scenario where all the web-servers can be configured to share common session state store.

1. Windows Service :
We can start this service by Start | Control Panel | Administrative Tools | Services | . In that we service names  ASP.NET State Service. We can start or stop service by manually or configure to start automatically. Then we have to configure our web.config file

<CONFIGURATION><configuration>
 <system.web>
  <SessionState
   mode = “StateServer”
   stateConnectionString = “tcpip=127.0.0.1:42424”
   stateNetworkTimeout = “10”
   sqlConnectionString=”data source = 127.0.0.1; uid=sa;pwd=”
   cookieless =”Flase”
   timeout= “20” />
 </system.web>
</configuration> </SYSTEM.WEB>
</CONFIGURATION>

Here ASP.Net Session is directed to use Windows Service for state management on local server (address : 127.0.0.1 is TCP/IP loop-back address). The default port is 42424. we can configure to any port but for that we have to manually edit the registry.
 Follow these simple steps

• In a webfarm make sure you have the same config file in all your web servers.
• Also make sure your objects are serializable.
• For session state to be maintained across different web servers in the webfarm, the application path of the web-site in the IIS Metabase should be identical in all the web-servers in the webfarm.

Which template must you provide, in order to display data in a Repeater control?  You have to use the ItemTemplate to Display data. Syntax is as follows,

< ItemTemplate >
   < div class =”rItem” >
        < img src=”images/<%# Container.DataItem(“ImageURL”)%>”  hspace=”10” />
    < b > <% # Container.DataItem(“Title”)%>
   < /div >
  < ItemTemplate >

How can you provide an alternating color scheme in a Repeater control? 
Using the AlternatintItemTemplate

What property must you set, and what method must you call in your code, in order to bind the data from some data source to the Repeater control?
  Set the DataMember property to the name of the table to bind to. (If this property is not set, by default the first table in the dataset is used.)
DataBind method, use this method to bind data from a source to a server control. This method is commonly used after retrieving a data set through a database query.

What method do you use to explicitly kill a user s session? You can dump (Kill) the session yourself by calling the method Session.Abandon.

ASP.NET automatically deletes a user's Session object, dumping its contents, after it has been idle for a configurable timeout interval. This interval, in minutes, is set in the section of the web.config file. The default is 20 minutes.

How do you turn off cookies for one page in your site? 
Use Cookie.Discard property, Gets or sets the discard flag set by the server. When true, this property instructs the client application not to save the Cookie on the user's hard disk when a session ends.

Which two properties are on every validation control?  We have two common properties for every validation controls

1. Control to Validate,
2. Error Message.

What tags do you need to add within the asp:datagrid tags to bind columns manually?

< asp:DataGrid id="dgCart" AutoGenerateColumns="False" CellPadding="4" Width="448px" runat="server"  >
 < Columns >
< asp:ButtonColumn HeaderText="SELECT" Text="SELECT" CommandName="select" >< /asp:ButtonColumn >
< asp:BoundColumn DataField="ProductId" HeaderText="Product ID" >< /asp:BoundColumn >
< asp:BoundColumn DataField="ProductName" HeaderText="Product Name" >< /asp:BoundColumn >
< asp:BoundColumn DataField="UnitPrice" HeaderText="UnitPrice" >< /asp:BoundColumn >
< /Columns >
< /asp:DataGrid >

How do you create a permanent cookie? 
Permanent cookies are the ones that are most useful. Permanent cookies are available until a specified expiration date, and are stored on the hard disk. The location of cookies differs with each browser, but this doesn’t matter, as this is all handled by your browser and the server. If you want to create a permanent cookie called Name with a value of Nigel, which expires in one month, you’d use the following code

Response.Cookies ("Name") = "Nigel"
Response.Cookies ("Name"). Expires = DateAdd ("m", 1, Now ())

What tag do you use to add a hyperlink column to the DataGrid?

< asp:HyperLinkColumn > </ asp:HyperLinkColumn>

Which method do you use to redirect the user to another page without performing a round trip to the client?

Server.transfer

What is the transport protocol you use to call a Web service SOAP ?
HTTP Protocol

Explain role based security ?
Role Based Security lets you identify groups of users to allow or deny based on their role in the organization.In Windows NT and Windows XP, roles map to names used to identify user groups. Windows defines several built-in groups, including Administrators, Users, and Guests.To allow or deny access to certain groups of users, add the element to the authorization list in your Web application's Web.config file.e.g.

<AUTHORIZATION>< authorization >
   < allow roles="Domain Name\Administrators" / >   < !-- Allow Administrators in domain. -- >
   < deny users="*"  / >                            < !-- Deny anyone else. -- >
< /authorization >

How do you register JavaScript for webcontrols ?

You can register javascript for controls using Attribtues.Add(scriptname,scripttext) method.

When do you set "" ?
Identity is a webconfig declaration under System.web, which helps to control the application Identity of the web applicaton. Which can be at any level(Machine,Site,application,subdirectory,or page), attribute impersonate with "true" as value specifies that client impersonation is used.

What are different templates available in Repeater,DataList and Datagrid ?

Templates enable one to apply complicated formatting to each of the items displayed by a control.Repeater control supports five types of templates.HeaderTemplate controls  how the header of the repeater control is formatted.ItemTemplate controls the formatting of each  item displayed.AlternatingItemTemplate controls how alternate items are formatted and the  SeparatorTemplate displays a separator between each item displyed.FooterTemplate is used for  controlling how the footer of the repeater control is formatted.The DataList and Datagrid supports two templates in addition to the above five.SelectedItem Template controls how a selected item is  formatted and EditItemTemplate controls how an item selected for editing is formatted.

What is ViewState ? and how it is managed ?

ASP.NET ViewState is a new kind of state service that developers can use to track  UI state on a per-user basis. Internally it uses an an old Web programming trick-roundtripping  state in a hidden form field and bakes it right into the page-processing framework.It needs less code to write and maintain state in your Web-based forms.

What is web.config file ?

Web.config file is the configuration file for the Asp.net web application. There is one web.config  file for one asp.net application which configures
the particular application. Web.config file is  written in XML with specific tags having specific meanings.It includes databa which includes
connections,Session States,Error Handling,Security etc.
For example :

< configuration >

< configuration >
 < appSettings >
    < add key="ConnectionString"
         value="server=localhost;uid=sa;pwd=;database=MyDB" / >
 < /appSettings >
< /configuration >

What is advantage of viewstate and what are benefits?

When a form is submitted in classic ASP, all form values are cleared. Suppose you have submitted a  form with a lot of information and the server comes back with an error. You will have to go back to  the form and correct the information. You click the back button, and what happens.......ALL form values are CLEARED, and you will have to start all over again! The site did not maintain your  ViewState.With ASP .NET, the form reappears in the browser window together with all form  values.This is because ASP .NET maintains your ViewState. The ViewState indicates the status of the  page when submitted to the server.

What tags do you need to add within the asp:datagrid tags to bind columns manually?

Set AutoGenerateColumns Property to false on the datagrid tag and then use Column tag and an ASP:databound tag 

< asp:DataGrid runat="server" id="ManualColumnBinding" AutoGenerateColumns="False" >
  < Columns >
      < asp:BoundColumn HeaderText="Column1" DataField="Column1"/ >
        < asp:BoundColumn HeaderText="Column2" DataField="Column2"/ >
  < /Columns >
< /asp:DataGrid >
<asp:DataGrid id=ManualColumnBinding runat="server" AutoGenerateColumns="False">
<COLUMNS>   <asp:BoundColumn HeaderText="Column2" DataField="Column2"></asp:BoundColumn>
</asp:DataGrid>

Which property on a Combo Box do you set with a column name, prior to setting the DataSource, to display data in the combo box? 
DataTextField and  DataValueField

Which control would you use if you needed to make sure the values in two different controls matched?

CompareValidator is used to ensure that two fields are identical.

What is validationsummary server control?where it is used?.
The ValidationSummary control allows you to summarize the error messages from all validation controls on a Web page in a single location. The summary can be displayed as a list, a bulleted list, or a single paragraph, based on the value of the DisplayMode property. The error message displayed in the ValidationSummary control for each validation control on the page is specified by the ErrorMessage property of each validation control. If the ErrorMessage property of the validation control is not set, no error message is displayed in the ValidationSummary control for that validation control. You can also specify a custom title in the heading section of the ValidationSummary control by setting the HeaderText property.
You can control whether the ValidationSummary control is displayed or hidden by setting the ShowSummary property. The summary can also be displayed in a message box by setting the ShowMessageBox property to true.

What is the sequence of operation takes place when a page is loaded? BeginTranaction  - only if the request is transacted
Init    - every time a page is processed
LoadViewState  - Only on postback
ProcessPostData1  - Only on postback
Load    - every time
ProcessData2   - Only on Postback
RaiseChangedEvent  - Only on Postback
RaisePostBackEvent  - Only on Postback
PreRender   - everytime
BuildTraceTree  - only if tracing is enabled
SaveViewState  - every time
Render   - Everytime
End Transaction  - only if the request is transacted
Trace.EndRequest  - only when tracing is enabled
UnloadRecursive  - Every request

Difference between asp and asp.net?.
"ASP (Active Server Pages) and ASP.NET are both server side technologies for building web sites and web applications, ASP.NET is Managed compiled code - asp is interpreted. and ASP.net is fully Object oriented. ASP.NET has been entirely re-architected to provide a highly productive programming experience based on the .NET Framework, and a robust infrastructure for building reliable and scalable web
applications."

Name the validation control available in asp.net?. RequiredField, RangeValidator,RegularExpression,Custom validator,compare Validator

What are the various ways of securing a web site that could prevent from hacking etc .. ?

1. Authentication/Authorization
2. Encryption/Decryption
3. Maintaining web servers outside the corporate firewall. etc.,

What is the difference between in-proc and out-of-proc? 
An inproc is one which runs in the same process area as that of the client giving tha advantage of speed but the disadvantage of stability becoz if it crashes it takes the client application also with it.Outproc is one which works outside the clients memory thus giving stability to the client, but we have to compromise a bit on speed.

When you’re running a component within ASP.NET, what process is it running within on Windows XP? Windows 2000? Windows 2003?

On Windows 2003 (IIS 6.0) running in native mode, the component is running within the w3wp.exe process associated with the application pool which has been configured for the web application containing the component.

On Windows 2003 in IIS 5.0 emulation mode, 2000, or XP, it's running within the IIS helper process whose name I do not remember, it being quite a while since I last used IIS 5.0.

What does aspnet_regiis -i do ?

Aspnet_regiis.exe is The ASP.NET IIS Registration tool allows an administrator or installation program to easily update the script maps for an ASP.NET application to point to the ASP.NET ISAPI version associated with the tool. The tool can also be used to display the status of all installed versions of ASP. NET, register the ASP.NET version coupled with the tool, create client-script directories, and perform other configuration operations.

When multiple versions of the .NET Framework are executing side-by-side on a single computer, the ASP.NET ISAPI version mapped to an ASP.NET application determines which version of the common language runtime is used for the application.

The tool can be launched with a set of optional parameters. Option "i" Installs the version of ASP.NET associated with Aspnet_regiis.exe and updates the script maps at the IIS metabase root and below. Note that only applications that are currently mapped to an earlier version of ASP.NET are affected

What is a PostBack? 
The process in which a Web page sends data back to the same page on the server.

What is ViewState? How is it encoded? Is it encrypted? Who uses ViewState?  ViewState is the mechanism ASP.NET uses to keep track of server control state values that don't otherwise post back as part of the HTTP form. ViewState Maintains the UI State of a Page
  ViewState is base64-encoded. 
 It is not encrypted but it can be encrypted by setting EnableViewStatMAC="true" & setting the machineKey validation type to 3DES.  If you want to NOT maintain the ViewState, include the directive < %@ Page EnableViewState="false" % > at the top of an .aspx page or add the attribute EnableViewState="false" to any control.\

What is the < machinekey > element and what two ASP.NET technologies is it used for?

Configures keys to use for encryption and decryption of forms authentication cookie data and view state data, and for verification of out-of-process session state identification.There fore 2 ASP.NET technique in which it is used are Encryption/Decryption & Verification

What three Session State providers are available in ASP.NET 1.1? What are the pros and cons of each?

ASP.NET provides three distinct ways to store session data for your application: in-process session state, out-of-process session state as a Windows service, and out-of-process session state in a SQL Server database. Each has it advantages. 

1. In-process session-state mode
   Limitations:
    * When using the in-process session-state mode, session-state data is lost if aspnet_wp.exe or the application domain restarts.
    * If you enable Web garden mode in the < processModel > element of the application's Web.config file, do not use in-process session-state mode. Otherwise, random data loss can occur.
   Advantage:
    * in-process session state is by far the fastest solution. If you are storing only small amounts of volatile data in session state, it is recommended that you use the in-process provider.
2. The State Server simply stores session state in memory when in out-of-proc mode. In this mode the worker process talks directly to the State Server
3. SQL mode, session states are stored in a SQL Server database and the worker process talks directly to SQL. The ASP.NET worker processes are then able to take advantage of this simple storage service by serializing and saving (using .NET serialization services) all objects within a client's Session collection at the end of each Web request
    Both these out-of-process solutions are useful primarily if you scale your application across multiple processors or multiple computers, or where data cannot be lost if a server or process is restarted.

What is the difference between HTTP-Post and HTTP-Get?
As their names imply, both HTTP GET and HTTP POST use HTTP as their underlying protocol. Both of these methods encode request parameters as name/value pairs in the HTTP request.
The GET method creates a query string and appends it to the script's URL on the server that handles the request.
The POST method creates a name/value pairs that are passed in the body of the HTTP request message.

Name and describe some HTTP Status Codes and what they express to the requesting client.

When users try to access content on a server that is running Internet Information Services (IIS) through HTTP or File Transfer Protocol (FTP), IIS returns a numeric code that indicates the status of the request. This status code is recorded in the IIS log, and it may also be displayed in the Web browser or FTP client. The status code can indicate whether a particular request is successful or unsuccessful and can also reveal the exact reason why a request is unsuccessful. There are 5 groups ranging from 1xx - 5xx of http status codes exists.
101 - Switching protocols.
200 - OK. The client request has succeeded
302 - Object moved.
400 - Bad request.
500.13 - Web server is too busy.

Explain < @OutputCache% > and the usage of VaryByParam, VaryByHeader.

OutputCache is used to control the caching policies of an ASP.NET page or user control. To cache a page @OutputCache directive should be defined as follows < %@ OutputCache Duration="100" VaryByParam="none" % >

VaryByParam: A semicolon-separated list of strings used to vary the output cache. By default, these strings correspond to a query string value sent with GET method attributes, or a parameter sent using the POST method. When this attribute is set to multiple parameters, the output cache contains a different version of the requested document for each specified parameter. Possible values include none, *, and any valid query string or POST parameter name.

VaryByHeader: A semicolon-separated list of HTTP headers used to vary the output cache. When this attribute is set to multiple headers, the output cache contains a different version of the requested document for each specified header.

What is the difference between repeater over datalist and datagrid?

The Repeater class is not derived from the WebControl class, like the DataGrid and DataList. Therefore, the Repeater lacks the stylistic properties common to both the DataGrid and DataList. What this boils down to is that if you want to format the data displayed in the Repeater, you must do so in the HTML markup.
The Repeater control provides the maximum amount of flexibility over the HTML produced. Whereas the DataGrid wraps the DataSource contents in an HTML < table >, and the DataList wraps the contents in either an HTML < table > or < span > tags (depending on the DataList's RepeatLayout property), the Repeater adds absolutely no HTML content other than what you explicitly specify in the templates.
While using Repeater control, If we wanted to display the employee names in a bold font we'd have to alter the "ItemTemplate" to include an HTML bold tag, Whereas with the DataGrid or DataList, we could have made the text appear in a bold font by setting the control's ItemStyle-Font-Bold property to True.
The Repeater's lack of stylistic properties can drastically add to the development time metric. For example, imagine that you decide to use the Repeater to display data that needs to be bold, centered, and displayed in a particular font-face with a particular background color. While all this can be specified using a few HTML tags, these tags will quickly clutter the Repeater's templates. Such clutter makes it much harder to change the look at a later date. Along with its increased development time, the Repeater also lacks any built-in functionality to assist in supporting paging, editing, or editing of data. Due to this lack of feature-support, the Repeater scores poorly on the usability scale.

However, The Repeater's performance is slightly better than that of the DataList's, and is more noticeably better than that of the DataGrid's. Following figure shows the number of requests per second the Repeater could handle versus the DataGrid and DataList

Can we handle the error and redirect to some pages using web.config?

Yes, we can do this, but to handle errors, we must know the error codes; only then we can take the user to a proper error message page, else it may confuse the user.
CustomErrors Configuration section in web.config file:
The default configuration is:

< customErrors mode="RemoteOnly"
defaultRedirect="Customerror.aspx" >
  < error statusCode="404" redirect="Notfound.aspx" / >
< /customErrors >

If mode is set to Off, custom error messages will be disabled. Users will receive detailed exception error messages.
If mode is set to On, custom error messages will be enabled.
If mode is set to RemoteOnly, then users will receive custom errors, but users accessing the site locally will receive detailed error messages.
Add an < error > tag for each error you want to handle. The error tag will redirect the user to the Notfound.aspx page when the site returns the 404 (Page not found) error.

[Example]

There is a page MainForm.aspx

Private
Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles MyBase.Load
     'Put user code to initialize the
page here
Dim str As System.Text.StringBuilder
str.Append("hi") ' Error Line as str is not instantiated
Response.Write(str.ToString)
End Sub

[Web.Config]

< customErrors mode="On" defaultRedirect="Error.aspx"/ >
' a simple redirect will take the user to Error.aspx [user defined] error file.

< customErrors mode="RemoteOnly" defaultRedirect="Customerror.aspx" >
  < error statusCode="404" redirect="Notfound.aspx" / >
< /customErrors >
'This will take the user to NotFound.aspx defined in IIS.

How do you implement Paging in .Net?

The DataGrid provides the means to display a group of records from the data source (for example, the first 10), and then navigate to the "page" containing the next 10 records, and so on through the data.

Using Ado.Net we can explicit control over the number of records returned from the data source, as well as how much data is to be cached locally in the DataSet.

1. Using DataAdapter.fill method give the value of 'Maxrecords' parameter
   (Note: - Don't use it because query will return all records but fill the dataset based on value of 'maxrecords' parameter).
2. For SQL server database, combines a WHERE clause and a ORDER BY clause with TOP predicate.
3. If Data does not change often just cache records locally in DataSet and just take some records from the DataSet to display.

What is the difference between Server.Transfer and Response.Redirect?

Server.Transfer() : client is shown as it is on the requesting page only, but the all the content is of the requested page. Data can be persist across the pages using Context.Item collection, which is one of the best way to transfer data from one page to another keeping the page state alive.

Response.Dedirect() :client knows the physical location (page name and query string as well). Context.Items loses the persistence when navigate to destination page. In earlier versions of IIS, if we wanted to send a user to a new Web page, the only option we had was Response.Redirect. While this method does accomplish our goal, it has several important drawbacks. The biggest problem is that this method causes each page to be treated as a separate transaction. Besides making it difficult to maintain your transactional integrity, Response.Redirect introduces some additional headaches. First, it prevents good encapsulation of code. Second, you lose access to all of the properties in the Request object. Sure, there are workarounds, but they're difficult. Finally, Response.Redirect necessitates a round trip to the client, which, on high-volume sites, causes scalability problems. As you might suspect, Server.Transfer fixes all of these problems. It does this by performing the transfer on the server without requiring a roundtrip to the client.

Response.Redirect sends a response to the client browser instructing it to request the second page. This requires a round-trip to the client, and the client initiates the Request for the second page. Server.Transfer transfers the process to the second page without making a round-trip to the client. It also transfers the HttpContext to the second page, enabling the second page access to all the values in the HttpContext of the first page.

Can you create an app domain?

Yes, We can create user app domain by calling on of the following overload static methods of the System.AppDomain class

1. Public static AppDomain CreateDomain(String friendlyName)
2. Public static AppDomain CreateDomain(String friendlyName, Evidence securityInfo)
3. Public static AppDomain CreateDomain(String friendlyName, Evidence securityInfo, AppDomainSetup info)
4. Public static AppDomain CreateDomain(String friendlyName, Evidence securityInfo, String appBasePath, String appRelativeSearchPath, bool shadowCopyFiles)

What are the various security methods which IIS Provides apart from .NET ?

The various security methods which IIS provides are

1. Authentication Modes
2. IP Address and Domain Name Restriction
3. DNS Lookups DNS Lookups
4. The Network ID and Subnet Mask
5. SSL

What is Web Gardening? How would using it affect a design?  

The Web Garden Model
The Web garden model is configurable through the section of the machine.config file. Notice that the section is the only configuration section that cannot be placed in an application-specific web.config file. This means that the Web garden mode applies to all applications running on the machine. However, by using the node in the machine.config source, you can adapt machine-wide settings on a per-application basis.

Two attributes in the section affect the Web garden model. They are webGarden and cpuMask. The webGarden attribute takes a Boolean value that indicates whether or not multiple worker processes (one per each affinitized CPU) have to be used. The attribute is set to false by default. The cpuMask attribute stores a DWORD value whose binary representation provides a bit mask for the CPUs that are eligible to run the ASP.NET worker process. The default value is -1 (0xFFFFFF), which means that all available CPUs can be used. The contents of the cpuMask attribute is ignored when the webGarden attribute is false. The cpuMask attribute also sets an upper bound to the number of copies of aspnet_wp.exe that are running.

Web gardening enables multiple worker processes to run at the same time. However, you should note that all processes will have their own copy of application state, in-process session state, ASP.NET cache, static data, and all that is needed to run applications. When the Web garden mode is enabled, the ASP.NET ISAPI launches as many worker processes as there are CPUs, each a full clone of the next (and each affinitized with the corresponding CPU). To balance the workload, incoming requests are partitioned among running processes in a round-robin manner. Worker processes get recycled as in the single processor case. Note that ASP.NET inherits any CPU usage restriction from the operating system and doesn't include any custom semantics for doing this.

All in all, the Web garden model is not necessarily a big win for all applications. The more stateful applications are, the more they risk to pay in terms of real performance. Working data is stored in blocks of shared memory so that any changes entered by a process are immediately visible to others. However, for the time it takes to service a request, working data is copied in the context of the process. Each worker process, therefore, will handle its own copy of working data, and the more stateful the application, the higher the cost in performance. In this context, careful and savvy application benchmarking is an absolute must.

Changes made to the section of the configuration file are effective only after IIS is restarted. In IIS 6, Web gardening parameters are stored in the IIS metabase; the webGarden and cpuMask attributes are ignored.

What is view state?.where it stored?.can we disable it?

The web is state-less protocol, so the page gets instantiated, executed, rendered and then disposed on every round trip to the server. The developers code to add "statefulness" to the page by using Server-side storage for the state or posting the page to itself. When require to persist and read the data in control on webform, developer had to read the values and store them in hidden variable (in the form), which were then used to restore the values. With advent of .NET framework, ASP.NET came up with ViewState mechanism, which tracks the data values of server controls on ASP.NET webform. In effect,ViewState can be viewed as "hidden variable managed by ASP.NET framework!". When ASP.NET page is executed, data values from all server controls on page are collected and encoded as single string, which then assigned to page's hidden atrribute  "< input type=hidden >", that is part of page sent to the client.

ViewState value is temporarily saved in the client's browser.ViewState can be disabled for a single control, for an entire page orfor an entire web application. The syntax is:

Disable ViewState for control (Datagrid in this example)

< asp:datagrid EnableViewState="false" ... / >

Disable ViewState for a page, using Page directive

< %@ Page EnableViewState="False" ... % >

Disable ViewState for application through entry in web.config

< Pages EnableViewState="false" ... / >

.NET FrameWork FAQ's

When was .NET announced?

Bill Gates delivered a keynote at Forum 2000, held June 22, 2000, outlining the .NET 'vision'. The July 2000 PDC had a number of sessions on .NET technology, and delegates were given CDs containing a pre-release version of the .NET framework/SDK and Visual Studio.NET.

When was the first version of .NET released?

The final version of the 1.0 SDK and runtime was made publicly available around 6pm PST on 15-Jan-2002. At the same time, the final version of Visual Studio.NET was made available to MSDN subscribers.

What platforms does the .NET Framework run on?

The runtime supports Windows XP, Windows 2000, NT4 SP6a and Windows ME/98. Windows 95 is not supported. Some parts of the framework do not work on all platforms - for example, ASP.NET is only supported on Windows XP and Windows 2000. Windows 98/ME cannot be used for development.

IIS is not supported on Windows XP Home Edition, and so cannot be used to host ASP.NET. However, the ASP.NET Web Matrix

web server does run on XP Home.
The Mono project is attempting to implement the .NET framework on Linux.

What is the CLR?

CLR = Common Language Runtime. The CLR is a set of standard resources that (in theory) any .NET program can take advantage of, regardless of programming language. Robert Schmidt (Microsoft) lists the following CLR resources in his MSDN PDC# article:

Object-oriented programming model (inheritance, polymorphism, exception handling, garbage collection)
 Security model 
 Type system 
 All .NET base classes 
 Many .NET framework classes 
 Development, debugging, and profiling tools 
 Execution and code management 
 IL-to-native translators and optimizers 
What this means is that in the .NET world, different programming languages will be more equal in capability than they have ever been before, although clearly not all languages will support all CLR services.

What is the CTS?

CTS = Common Type System. This is the range of types that the .NET runtime understands, and therefore that .NET applications can use. However note that not all .NET languages will support all the types in the CTS. The CTS is a superset of the CLS.

What is the CLS?

CLS = Common Language Specification. This is a subset of the CTS which all .NET languages are expected to support. The idea is that any program which uses CLS-compliant types can interoperate with any .NET program written in any language.
In theory this allows very tight interop between different .NET languages - for example allowing a C# class to inherit from a VB class.

What is IL?

IL = Intermediate Language. Also known as MSIL (Microsoft Intermediate Language) or CIL (Common Intermediate Language). All .NET source code (of any language) is compiled to IL. The IL is then converted to machine code at the point where the software is installed, or at run-time by a Just-In-Time (JIT) compiler.

What does 'managed' mean in the .NET context?

The term 'managed' is the cause of much confusion. It is used in various places within .NET, meaning slightly different things.Managed code: The .NET framework provides several core run-time services to the programs that run within it - for example exception handling and security. For these services to work, the code must provide a minimum level of information to the runtime.

Such code is called managed code. All C# and Visual Basic.NET code is managed by default. VS7 C++ code is not managed by default, but the compiler can produce managed code by specifying a command-line switch (/com+).
Managed data: This is data that is allocated and de-allocated by the .NET runtime's garbage collector. C# and VB.NET data is always managed. VS7 C++ data is unmanaged by default, even when using the /com+ switch, but it can be marked as managed using the __gc keyword.Managed classes: This is usually referred to in the context of Managed Extensions (ME) for C++. When using ME C++, a class can be marked with the __gc keyword. As the name suggests, this means that the memory for instances of the class is managed by the garbage collector, but it also means more than that. The class becomes a fully paid-up member of the .NET community with the benefits and restrictions that brings. An example of a benefit is proper interop with classes written in other languages - for example, a managed C++ class can inherit from a VB class. An example of a restriction is that a managed class can only inherit from one base class.

What is reflection?

All .NET compilers produce metadata about the types defined in the modules they produce. This metadata is packaged along with the module (modules in turn are packaged together in assemblies), and can be accessed by a mechanism called reflection. The System.Reflection namespace contains classes that can be used to interrogate the types for a module/assembly.
Using reflection to access .NET metadata is very similar to using ITypeLib/ITypeInfo to access type library data in COM, and it is used for similar purposes - e.g. determining data type sizes for marshaling data across context/process/machine boundaries.
Reflection can also be used to dynamically invoke methods (see System.Type.InvokeMember ) ,  or even create types dynamically at run-time (see System.Reflection.Emit.TypeBuilder).

What is the difference between Finalize and Dispose (Garbage collection) ?

Class instances often encapsulate control over resources that are not managed by the runtime, such as window handles (HWND), database connections, and so on. Therefore, you should provide both an explicit and an implicit way to free those resources. Provide implicit control by implementing the protected Finalize Method on an object (destructor syntax in C# and the Managed Extensions for C++). The garbage collector calls this method at some point after there are no longer any valid references to the object. In some cases, you might want to provide programmers using an object with the ability to explicitly release these external resources before the garbage collector frees the object. If an external resource is scarce or expensive, better performance can be achieved if the programmer explicitly releases resources when they are no longer being used. To provide explicit control, implement the Dispose method provided by the IDisposable Interface. The consumer of the object should call this method when it is done using the object.

Dispose can be called even if other references to the object are alive. Note that even when you provide explicit control by way of Dispose, you should provide implicit cleanup using the Finalize method. Finalize provides a backup to prevent resources from permanently leaking if the programmer fails to call Dispose.

What is Partial Assembly References?

Full Assembly reference: A full assembly reference includes the assembly's text name, version, culture, and public key token (if the assembly has a strong name). A full assembly reference is required if you reference any assembly that is part of the common

language runtime or any assembly located in the global assembly cache.

Partial Assembly reference: We can dynamically reference an assembly by providing only partial information, such as specifying only the assembly name. When you specify a partial assembly reference, the runtime looks for the assembly only in the application

directory.
We can make partial references to an assembly in your code one of the following ways:
-> Use a method such as System.Reflection.Assembly.Load and specify only a partial reference. The runtime checks for the assembly in the application directory.
-> Use the System.Reflection.Assembly.LoadWithPartialName method and specify only a partial reference. The runtime checks for the assembly in the application directory and in the global assembly cache

Changes to which portion of version number indicates an incompatible change? Major or minor. Changes to the major or minor portion of the version number indicate an incompatible change. Under this convention then, version 2.0.0.0 would be considered incompatible with version 1.0.0.0. Examples of an incompatible change would be a change to the types of some method parameters or the removal of a type or method altogether. Build. The Build number is typically used to distinguish between daily builds or smaller compatible releases. Revision. Changes to the revision number are typically reserved for an incremental build needed to fix a particular bug. You'll sometimes hear this referred to as the "emergency bug fix" number in that the revision is what is often changed when a fix to a specific bug is shipped to a customer.

What is side-by-side execution? Can two application one using private assembly and other using Shared assembly be stated as a side-by-side executables?

Side-by-side execution is the ability to run multiple versions of an application or component on the same computer. You can have multiple versions of the common language runtime, and multiple versions of applications and components that use a version of the runtime, on the same computer at the same time. Since versioning is only applied to shared assemblies, and not to private assemblies, two application one using private assembly and one using shared assembly cannot be stated as side-by-side executables.

Why string are called Immutable data Type ?

The memory representation of string is an Array of Characters, So on re-assigning the new array of Char is formed & the start address is changed . Thus keeping the Old string in Memory for Garbage Collector to be disposed.

What does assert() method do?

In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false. The program proceeds without any interruption if the condition is true.

What's the difference between the Debug class and Trace class?

Documentation looks the same.  Use Debug class for debug builds, use Trace class for both debug and release builds.

Why are there five tracing levels in System.Diagnostics.TraceSwitcher?

The tracing dumps can be quite verbose.  For applications that are constantly running you run the risk of overloading the machine and the hard drive.  Five levels range from None to Verbose, allowing you to fine-tune the tracing activities.

Where is the output of TextWriterTraceListener redirected?

To the Console or a text file depending on the parameter passed to the constructor.

How do assemblies find each other?

By searching directory paths. There are several factors which can affect the path (such as the AppDomain host, and application configuration files), but for private assemblies the search path is normally the application's directory and its sub-directories. For shared assemblies, the search path is normally same as the private assembly path plus the shared assembly cache.

How does assembly versioning work?

Each assembly has a version number called the compatibility version. Also each reference to an assembly (from another assembly) includes both the name and version of the referenced assembly.The version number has four numeric parts (e.g. 5.5.2.33). Assemblies with either of the first two parts different are normally viewed as incompatible. If the first two parts are the same, but the third is different, the assemblies are deemed as 'maybe compatible'. If only the fourth part is different, the assemblies are deemed compatible. However, this is just the default guideline - it is the version policy that decides to what extent these rules are enforced. The version policy can be specified via the application configuration file.

What is garbage collection?

Garbage collection is a system whereby a run-time component takes responsibility for managing the lifetime of objects and the heap memory that they occupy. This concept is not new to .NET - Java and many other languages/runtimes have used garbage collection for some time.

Why doesn't the .NET runtime offer deterministic destruction?

Because of the garbage collection algorithm. The .NET garbage collector works by periodically running through a list of all the objects that are currently being referenced by an application. All the objects that it doesn't find during this search are ready to be destroyed and the memory reclaimed. The implication of this algorithm is that the runtime doesn't get notified immediately when the final reference on an object goes away - it only finds out during the next sweep of the heap.

Futhermore, this type of algorithm works best by performing the garbage collection sweep as rarely as possible. Normally heap exhaustion is the trigger for a collection sweep.

Is the lack of deterministic destruction in .NET a problem?

It's certainly an issue that affects component design. If you have objects that maintain expensive or scarce resources (e.g. database locks), you need to provide some way for the client to tell the object to release the resource when it is done. Microsoft recommend that you provide a method called Dispose() for this purpose. However, this causes problems for distributed objects - in a distributed system who calls the Dispose() method? Some form of reference-counting or ownership-management mechanism is needed to handle distributed objects - unfortunately the runtime offers no help with this.

What is serialization?

Serialization is the process of converting an object into a stream of bytes. Deserialization is the opposite process of creating an object from a stream of bytes. Serialization / Deserialization is mostly used to transport objects (e.g. during remoting), or to persist

objects (e.g. to a file or database).

Does the .NET Framework have in-built support for serialization?

There are two separate mechanisms provided by the .NET class library - XmlSerializer and SoapFormatter/BinaryFormatter. Microsoft uses XmlSerializer for Web Services, and uses SoapFormatter/BinaryFormatter for remoting. Both are available for use in your own code.

Can I customise the serialization process?

Yes. XmlSerializer supports a range of attributes that can be used to configure serialization for a particular class. For example, a field or property can be marked with the [XmlIgnore] attribute to exclude it from serialization. Another example is the [XmlElement] attribute, which can be used to specify the XML element name to be used for a particular property or field.
Serialization via SoapFormatter/BinaryFormatter can also be controlled to some extent by attributes. For example, the [NonSerialized] attribute is the equivalent of XmlSerializer's [XmlIgnore] attribute. Ultimate control of the serialization process can be acheived by implementing the the ISerializable interface on the class whose instances are to be serialized.

Why is XmlSerializer so slow?

There is a once-per-process-per-type overhead with XmlSerializer. So the first time you serialize or deserialize an object of a given type in an application, there is a significant delay. This normally doesn't matter, but it may mean, for example, that XmlSerializer is a poor choice for loading configuration settings during startup of a GUI application.

Why do I get errors when I try to serialize a Hashtable?

XmlSerializer will refuse to serialize instances of any class that implements IDictionary, e.g. Hashtable. SoapFormatter and BinaryFormatter do not have this restriction.

What are attributes?

There are at least two types of .NET attribute. The first type I will refer to as a metadata attribute - it allows some data to be attached to a class or method. This data becomes part of the metadata for the class, and (like other class metadata) can be accessed via reflection.
The other type of attribute is a context attribute. Context attributes use a similar syntax to metadata attributes but they are fundamentally different. Context attributes provide an interception mechanism whereby instance activation and method calls can be pre- and/or post-processed.

How does CAS work? The CAS security policy revolves around two key concepts - code groups and permissions. Each .NET assembly is a member of a particular code group, and each code group is granted the permissions specified in a named permission set.
For example, using the default security policy, a control downloaded from a web site belongs to the 'Zone - Internet' code group, which adheres to the permissions defined by the 'Internet' named permission set. (Naturally the 'Internet' named permission set represents a very restrictive range of permissions.)

Who defines the CAS code groups? Microsoft defines some default ones, but you can modify these and even create your own. To see the code groups defined on your system, run 'caspol -lg' from the command-line. On my system it looks like this:
Level = Machine

Code Groups:

1.  All code: Nothing
   1.1.  Zone - MyComputer: FullTrust
      1.1.1.  Honor SkipVerification requests: SkipVerification
   1.2.  Zone - Intranet: LocalIntranet
   1.3.  Zone - Internet: Internet
   1.4.  Zone - Untrusted: Nothing
   1.5.  Zone - Trusted: Internet
   1.6.  StrongName - 0024000004800000940000000602000000240000525341310004000003
000000CFCB3291AA715FE99D40D49040336F9056D7886FED46775BC7BB5430BA4444FEF8348EBD06
F962F39776AE4DC3B7B04A7FE6F49F25F740423EBF2C0B89698D8D08AC48D69CED0FC8F83B465E08
07AC11EC1DCC7D054E807A43336DDE408A5393A48556123272CEEEE72F1660B71927D38561AABF5C
AC1DF1734633C602F8F2D5: 

Note the hierarchy of code groups - the top of the hierarchy is the most general ('All code'), which is then sub-divided into several

groups, each of which in turn can be sub-divided. Also note that (somewhat counter-intuitively) a sub-group can be associated with a more permissive permission set than its parent.

How do I define my own code group?

Use caspol. For example, suppose you trust code from  www.mydomain.com and you want it have full access to your system, but you want to keep the default restrictions for all other internet sites. To achieve this, you would add a new code group as a sub-group of the 

'Zone - Internet' group, like this:
caspol -ag 1.3 -site  www.mydomain.com FullTrust
Now if you run caspol -lg you will see that the new group has been added as group 1.3.1:

   1.3.  Zone - Internet: Internet
      1.3.1.  Site -  www.mydomain.com: FullTrust

Note that the numeric label (1.3.1) is just a caspol invention to make the code groups easy to manipulate from the command-line. The underlying runtime never sees it.

How do I change the permission set for a code group?

Use caspol. If you are the machine administrator, you can operate at the 'machine' level - which means not only that the changes you make become the default for the machine, but also that users cannot change the permissions to be more permissive. If you are a normal (non-admin) user you can still modify the permissions, but only to make them more restrictive. For example, to allow intranet code to do what it likes you might do this:
caspol -cg 1.2 FullTrust
Note that because this is more permissive than the default policy (on a standard system), you should only do this at the machine level - doing it at the user level will have no effect.

I can't be bothered with all this CAS stuff. Can I turn it off?

Yes, as long as you are an administrator. Just run: caspol -s off

Can I look at the IL for an assembly?

Yes. MS supply a tool called Ildasm which can be used to view the metadata and IL for an assembly.

Can source code be reverse-engineered from IL?

Yes, it is often relatively straightforward to regenerate high-level source (e.g. C#) from IL.

How can I stop my code being reverse-engineered from IL?

There is currently no simple way to stop code being reverse-engineered from IL. In future it is likely that IL obfuscation tools will become available, either from MS or from third parties. These tools work by 'optimising' the IL in such a way that reverse-engineering becomes much more difficult.
Of course if you are writing web services then reverse-engineering is not a problem as clients do not have access to your IL.

Is there built-in support for tracing/logging?

Yes, in the System.Diagnostics namespace. There are two main classes that deal with tracing - Debug and Trace. They both work in a similar way - the difference is that tracing from the Debug class only works in builds that have the DEBUG symbol defined, whereas tracing from the Trace class only works in builds that have the TRACE symbol defined. Typically this means that you should use System.Diagnostics.Trace.WriteLine for tracing that you want to work in debug and release builds, and System.Diagnostics.Debug.WriteLine for tracing that you want to work only in debug builds.

Can I redirect tracing to a file?

Yes. The Debug and Trace classes both have a Listeners property, which is a collection of sinks that receive the tracing that you send via Debug.WriteLine and Trace.WriteLine respectively. By default the Listeners collection contains a single sink, which is an instance of the DefaultTraceListener class. This sends output to the Win32 OutputDebugString() function and also the System.Diagnostics.Debugger.Log() method. This is useful when debugging, but if you're trying to trace a problem at a customer site, redirecting the output to a file is more appropriate. Fortunately, the TextWriterTraceListener class is provided for this purpose.

What are the contents of assembly?

In general, a static assembly can consist of four elements:
  The assembly manifest, which contains assembly metadata. 
 Type metadata. 
 Microsoft intermediate language (MSIL) code that implements the types. 
 A set of resources.

What is GC (Garbage Collection) and how it works

One of the good features of the CLR is Garbage Collection, which runs in the background collecting unused object references, freeing us from having to ensure we always destroy them. In reality the time difference between you releasing the object instance and it being garbage collected is likely to be very small, since the GC is always running.

[The process of transitively tracing through all pointers to actively used objects in order to locate all objects that can be referenced, and then arranging to reuse any heap memory that was not found during this trace. The common language runtime garbage collector also compacts the memory that is in use to reduce the working space needed for the heap.]

Heap:

A portion of memory reserved for a program to use for the temporary storage of data structures whose existence or size cannot be determined until the program is running.

Differnce between Managed code and unmanaged code ?

Managed Code:

Code that runs under a "contract of cooperation" with the common language runtime. Managed code must supply the metadata necessary for the runtime to provide services such as memory management, cross-language integration, code access security, and automatic lifetime control of objects. All code based on Microsoft intermediate language (MSIL) executes as managed code.

Un-Managed Code:

Code that is created without regard for the conventions and requirements of the common language runtime. Unmanaged code executes in the common language runtime environment with minimal services (for example, no garbage collection, limited debugging, and so on).

What is MSIL, IL, CTS and, CLR ?

MSIL: (Microsoft intermediate language)
When compiling to managed code, the compiler translates your source code into Microsoft intermediate language (MSIL), which is a CPU-independent set of instructions that can be efficiently converted to native code. MSIL includes instructions for loading, storing, initializing, and calling methods on objects, as well as instructions for arithmetic and logical operations, control flow, direct memory access, exception handling, and other operations. Before code can be executed, MSIL must be converted to CPU-specific code, usually by a just-in-time (JIT) compiler. Because the common language runtime supplies one or more JIT compilers for each computer architecture it supports, the same set of MSIL can be JIT-compiled and executed on any supported architecture.
When a compiler produces MSIL, it also produces metadata. Metadata describes the types in your code, including the definition of each type, the signatures of each type's members, the members that your code references, and other data that the runtime uses at execution time. The MSIL and metadata are contained in a portable executable (PE) file that is based on and extends the published Microsoft PE and Common Object File Format (COFF) used historically for executable content. This file format, which accommodates

MSIL or native code as well as metadata, enables the operating system to recognize common language runtime images. The presence of metadata in the file along with the MSIL enables your code to describe itself, which means that there is no need for type libraries or Interface Definition Language (IDL). The runtime locates and extracts the metadata from the file as needed during execution.

IL: (Intermediate Language)
A language used as the output of a number of compilers and as the input to a just-in-time (JIT) compiler. The common language

runtime includes a JIT compiler for converting MSIL to native code.

CTS: (Common Type System)
The specification that determines how the common language runtime defines, uses, and manages types

CLR: (Common Language Runtime)
The engine at the core of managed code execution. The runtime supplies managed code with services such as cross-language integration, code access security, object lifetime management, and debugging and profiling support.

What is Reference type and value type ?

Reference Type:
Reference types are allocated on the managed CLR heap, just like object types.
A data type that is stored as a reference to the value's location. The value of a reference type is the location of the sequence of bits that represent the type's data. Reference types can be self-describing types, pointer types, or interface types

Value Type:
Value types are allocated on the stack just like primitive types in VBScript, VB6 and C/C++. Value types are not instantiated using new go out of scope when the function they are defined within returns.
Value types in the CLR are defined as types that derive from system.valueType.

A data type that fully describes a value by specifying the sequence of bits that constitutes the value's representation. Type information for a value type instance is not stored with the instance at run time, but it is available in metadata. Value type instances can be treated as objects using boxing.

What is Boxing and unboxing ?
Boxing:

The conversion of a value type instance to an object, which implies that the instance will carry full type information at run time and will be allocated in the heap. The Microsoft intermediate language (MSIL) instruction set's box instruction converts a value type to an object by making a copy of the value type and embedding it in a newly allocated object.

Un-Boxing:
The conversion of an object instance to a value type.

What is JIT and how is works ?

An acronym for "just-in-time," a phrase that describes an action that is taken only when it becomes necessary, such as just-in-time compilation or just-in-time object activation

What is portable executable (PE) ?

The file format used for executable programs and for files to be linked together to form executable programs

What is strong name?

A name that consists of an assembly's identity—its simple text name, version number, and culture information (if provided)—strengthened by a public key and a digital signature generated over the assembly. Because the assembly manifest contains file hashes for all the files that constitute the assembly implementation, it is sufficient to generate the digital signature over just the one file in the assembly that contains the assembly manifest. Assemblies with the same strong name are expected to be identical

What is global assembly cache?

A machine-wide code cache that stores assemblies specifically installed to be shared by many applications on the computer. Applications deployed in the global assembly cache must have a strong name.

What is difference between constants, readonly and, static ? Constants: The value can’t be changed

Read-only: The value will be initialized only once from the constructor of the class.
Static: Value can be initialized once.

What is difference between shared and public?

An assembly that can be referenced by more than one application. An assembly must be explicitly built to be shared by giving it a cryptographically strong name.

What is namespace used for loading assemblies at run time and name the methods? System.Reflection

What are the types of authentication in .net?
We have three types of  authentication:

1. Form authentication
2. Windows authentication
3. Passport

This has to be declared in web.config file.

What is the difference between a Struct and a Class ?

The struct type is suitable for representing lightweight objects such as Point, Rectangle, and Color. Although it is possible to represent a point as a class, a struct is more efficient in some scenarios. For example, if you declare an array of 1000 Point objects, you will allocate additional memory for referencing each object. In this case, the struct is less expensive. When you create a struct object using the new operator, it gets created and the appropriate constructor is called. Unlike classes, structs can be instantiated without using the new operator. If you do not use new, the fields will remain unassigned and the object cannot be used until all of the fields are initialized. It is an error to declare a default (parameterless) constructor for a struct. A default constructor is always provided to initialize the struct members to their default values.

It is an error to initialize an instance field in a struct. There is no inheritance for structs as there is for classes. A struct cannot inherit from another struct or class, and it cannot be the base of a class. Structs, however, inherit from the base class Object. A struct can implement interfaces, and it does that exactly as classes do. A struct is a value type, while a class is a reference type.

How big is the datatype int in .NET?

32 bits.

How big is the char?

16 bits (Unicode).

How do you initiate a string without escaping each backslash?

Put an @ sign in front of the double-quoted string.

What's the access level of the visibility type internal?

Current application.

Explain encapsulation ?

The implementation is hidden, the interface is exposed.

What data type should you use if you want an 8-bit value that's signed?

sbyte.

Speaking of Boolean data types, what's different between C# and C/C++?

There's no conversion between 0 and false, as well as any other number and true, like in C/C++.

Where are the value-type variables allocated in the computer RAM?

Stack.

Where do the reference-type variables go in the RAM?

The references go on the stack, while the objects themselves go on the heap.

What is the difference between the value-type variables and reference-type variables in terms of garbage collection?

The value-type variables are not garbage-collected, they just fall off the stack when they fall out of scope, the reference-type objects are picked up by GC when their references go null.

How do you convert a string into an integer in .NET?

Int32.Parse(string)

How do you box a primitive data type variable?

Assign it to the object, pass an object.

Why do you need to box a primitive variable?

To pass it by reference.

What's the difference between Java and .NET garbage collectors?

Sun left the implementation of a specific garbage collector up to the JRE developer, so their performance varies widely, depending on whose JRE you're using. Microsoft standardized on their garbage collection.

How do you enforce garbage collection in .NET?

System.GC.Collect();

What's different about namespace declaration when comparing that to package declaration in Java?  No semicolon.

What's the difference between const and readonly?

You can initialize readonly variables to some runtime values. Let's say your program uses current date and time as one of the values that won't change. This way you declare public readonly string DateT = new DateTime().ToString().

What happens when you encounter a continue statement inside the for loop?

The code for the rest of the loop is ignored, the control is transferred back to the beginning of the loop.

What's the advantage of using System.Text.StringBuilder over System.String?

StringBuilder is more efficient in the cases, where a lot of manipulation is done to the text. Strings are immutable, so each time it's being operated on, a new instance is created.

Can you store multiple data types in System.Array?

No.

What's the difference between the System.Array.CopyTo() and System.Array.Clone()?

The first one performs a deep copy of the array, the second one is shallow.

How can you sort the elements of the array in descending order?

By calling Sort() and then Reverse() methods.

What's the .NET datatype that allows the retrieval of data by a unique key?

HashTable.

What's class SortedList underneath?

A sorted HashTable.

Will finally block get executed if the exception had not occurred?

Yes.

Can multiple catch blocks be executed?

No, once the proper catch code fires off, the control is transferred to the finally block (if there are any), and then whatever follows the finally block.

Why is it a bad idea to throw your own exceptions?

Well, if at that point you know that an error has occurred, then why not write the proper code to handle that error instead of passing a new Exception object to the catch block? Throwing your own exceptions signifies some design flaws in the project.

What's a delegate?

A delegate object encapsulates a reference to a method. In C++ they were referred to as function pointers.

What's a multicast delegate?

It's a delegate that points to and eventually fires off several methods.

How's the DLL Hell problem solved in .NET?

Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32),  but also the version of the assembly.

What are the ways to deploy an assembly?

An MSI installer, a CAB archive, and XCOPY command.

What's a satellite assembly?

When you write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.

What namespaces are necessary to create a localized application?

System.Globalization, System.Resources.

What does assert() do?

In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is  false. The program proceeds without any interruption if the condition is true.

What's the difference between the Debug class and Trace class?

Documentation looks the same. Use Debug class for debug builds, use Trace class for both debug and release builds.

Why are there five tracing levels in System.Diagnostics.TraceSwitcher?

The tracing dumps can be quite verbose and for some applications that are constantly running you run the risk of overloading the machine and the hard drive there. Five levels range from None to Verbose, allowing to fine-tune the tracing activities.

Where is the output of TextWriterTraceListener redirected?

To the Console or a text file depending on the parameter passed to the constructor.

What namespaces are necessary to create a localized application?

System.Globalization, System.Resources.

What are three test cases you should go through in unit testing? 
Positive test cases (correct data, correct output), negative test cases (broken or missing data, proper handling), exception  test cases (exceptions are thrown and caught properly).

Can you change the value of a variable while debugging a C# application?

Yes, if you are debugging via Visual Studio.NET, just go to Immediate window.

What's the implicit name of the parameter that gets passed into the class' set method?

Value, and it's datatype depends on whatever variable we're changing.

How do you inherit from a class in C#?

Place a colon and then the name of the base class. Notice that it's double colon in C++.

Does C# support multiple inheritance?

No, use interfaces instead.

When you inherit a protected class-level variable, who is it available to?

Derived Classes.

What's the top .NET class that everything is derived from?

System.Object.

How's method overriding different from overloading?

When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the  same name within the class.

What does the keyword virtual mean in the method definition?

The method can be over-ridden.

Can you declare the override method static while the original method is non-static?

No, you can't, the signature of the virtual method must remain the same, only the keyword virtual is changed to keyword  override.

Can you override private virtual methods?

No, moreover, you cannot access private methods in inherited classes, have to be protected in the base class to allow any sort of access.

Can you prevent your class from being inherited and becoming a base class for some other classes?

Yes, that's what keyword sealed in the class definition is for. The developer trying to derive from your class will get a  message: cannot inherit from Sealed class WhateverBaseClassName. It's the same concept as final class in Java.

Can you allow class to be inherited, but prevent the method from being over-ridden?

Yes, just leave the class public and make the method sealed.

Why can't you specify the accessibility modifier for methods inside the interface?

They all must be public. Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, it's public by default.

Can you inherit multiple interfaces?

Yes, why not.

And if they have conflicting method names?

It's up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you're okay.

What's the difference between an interface and abstract class?

In the interface all methods must be abstract, in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes.

How can you overload a method?

Different parameter data types, different number of parameters, different order of parameters.

If a base class has a bunch of overloaded constructors, and an inherited class has another bunch of overloaded constructors, can you enforce a call from an inherited constructor to an arbitrary base constructor?

Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

What's the difference between System.String and System.StringBuilder classes?

System.String is immutable, System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

Does C# support multiple-inheritance?

No, use interfaces instead.

When you inherit a protected class-level variable, who is it available to?

The derived class.

Are private class-level variables inherited?

Yes, but they are not accessible.  Although they are not visible or accessible via the class interface, they are inherited.

Describe the accessibility modifier "protected internal".

It is available to derived classes and classes within the same Assembly (and naturally from the base class it's declared in).

What's the top .NET class that everything is derived from?

System.Object.

What's the advantage of using System.Text.StringBuilder over System.String?

StringBuilder is more efficient in cases where there is a large amount of string manipulation.  Strings are immutable, so each time it's being operated on, a new instance is created.

Can you store multiple data types in System.Array?

No.

What's the .NET class that allows the retrieval of a data element using a unique key?

HashTable.

Will the finally block get executed if an exception has not occurred?

Yes.

What's an abstract class? A class that cannot be instantiated.  An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.

When do you absolutely have to declare a class as abstract?

1. When at least one of the methods in the class is abstract.
2. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.

What's an interface?

It's an abstract class with public abstract methods all of which must be implemented in the inherited classes.

Why can't you specify the accessibility modifier for methods inside the interface?

They all must be public.  Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, it's public by default.

What's the difference between an interface and abstract class?

In an interface class, all methods must be abstract.  In an abstract class some methods can be concrete.  In an interface class, no accessibility modifiers are allowed, which is ok in an abstract class.

How is method overriding different from method overloading?

When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.

Can you declare an override method to be static if the original method is non-static?

No. The signature of the virtual method must remain the same, only the keyword virtual is changed to keyword override.

Can you override private virtual methods?

No.  Private methods are not accessible outside the class.

Can you write a class without specifying namespace? Which namespace does it belong to by default? 

Yes, you can, then the class belongs to global namespace which has no name. For commercial products, naturally, you  wouldn't want global namespace.

What is a formatter?  

A formatter is an object that is responsible for encoding and serializing data into messages on one end, and deserializing  and decoding messages into data on the other end.

Different b/w .NET & J2EE ?
Differences between J2EE and the .NET Platform

Vendor Neutrality

The .NET platform is not vendor neutral, it is tied to the Microsoft operating systems. But neither are any of the J2EE implementations Many companies buy into J2EE believing that it will give them vendor neutrality. And, in fact, this is a stated goal of Sun's vision:
A wide variety of J2EE product configurations and implementations, all of which meet the requirements of this specification, are possible. A portable J2EE application will function correctly when successfully deployed in any of these products. (ref : Java 2 Platform Enterprise Edition Specification, v1.3, page 2-7 available at http://java.sun.com/j2ee/)

Overall Maturity

Given that the .NET platform has a three year lead over J2EE, it should be no surprise to learn that the .NET platform is far more mature than the J2EE platform. Whereas we have high volume highly reliable web sites using .NET technologies (NASDAQ and Dell being among many examples)

Interoperability and Web Services

The .NET platform eCollaboration model is, as I have discussed at length, based on the UDDI and SOAP standards. These standards are widely supported by more than 100 companies. Microsoft, along with IBM and Ariba, are the leaders in this area. Sun is a member of the UDDI consortium and recognizes the importance of the UDDI standards. In a recent press release, Sun's George Paolini, Vice President for the Java Community Development,  says:

"Sun has always worked to help establish and support open, standards-based technologies that facilitate the growth of network-based applications, and we see UDDI as an important project to establish a registry framework for business-to-business e-commerce

But while Sun publicly says it believes in the UDDI standards, in reality, Sun has done nothing whatsoever to incorporate any of the UDDI standards into J2EE.

Scalability

Typical Comparision w.r.t Systems and their costs

J2EE

Company System Total Sys.	Cost
Bull	Escala T610 c/s	16,785 $1,980,179
IBM	RS/6000 Enterprise Server F80	16,785 $2,026,681
Bull	Escala EPC810 c/s	33,375 $3,037,499
IBM	RS/6000 Enterprise Server M80	33,375 $3,097,055
Bull	Escala EPC2450	110,403 $9,563,263
IBM	IBM eServer pSeries 680 Model 7017-S85	110,403 $9,560,594

.NET platform systems

Company System	Total Sys.	Cost
Dell	PowerEdge 4400	16,263 $273,487
Compaq	ProLiant ML-570-6/700-3P	20,207 $201,717
Dell	PowerEdge 6400	30,231 $334,626
IBM	Netfinity 7600 c/s	32,377 $443,463
Compaq	ProLiant 8500-X550-64P	161,720 $3,534,272
Compaq	ProLiant 8500-X700-64P	179,658 $3,546,582
Compaq	ProLiant 8500-X550-96P	229,914 $5,305,571
Compaq	ProLiant 8500-X700-96P	262,244 $5,305,571
Compaq	ProLiant 8500-700-192P	505,303 $10,003,826

Framework Support

The .NET platform includes such an eCommerce framework called Commerce Server. At this point, there is no equivalent vendor-neutral framework in the J2EE space. With J2EE, you should assume that you will be building your new eCommerce solution from scratch

Moreover, no matter what [J2EE] vendor you choose, if you expect a component framework that will allow you to quickly field complete e-business applications, you are in for a frustrating experience

Language

In the language arena, the choice is about as simple as it gets. J2EE supports Java, and only Java. It will not support any other language in the foreseeable future. The .NET platform supports every language except Java (although it does support a language that is syntactically and functionally equivalent to Java, C#). In fact, given the importance of the .NET platform as a language independent vehicle, it is likely that any language that comes out in the near future will include support for the .NET platform.

Some companies are under the impression that J2EE supports other languages. Although both IBM's WebSphere and BEA's WebLogic support other languages, neither does it through their J2EE technology. There are only two official ways in the J2EE platform to access other languages, one through the Java Native Interface and the other through CORBA interoperability. Sun recommends the later approach. As Sun's Distinguished Scientist and Java Architect Rick Cattell said in a recent interview.

Portability

The reason that operating system portability is a possibility with J2EE is not so much because of any inherent portability of J2EE, as it is that most of the J2EE vendors support multiple operating systems. Therefore as long as one sticks with a given J2EE vendor and a given database vendor, moving from one operating system to another should be possible. This is probably the single most important benefit in favor of J2EE over the .NET platform, which is limited to the Windows operating system. It is worth noting, however, that Microsoft has submitted the specifications for C# and a subset of the .NET Framework (called the common language infrastructure) to ECMA, the group that standardizes JavaScript.

J2EE offers an acceptable solution to ISVs when the product must be marketed to non-Windows customers, particularly when the J2EE platform itself can be bundled with the ISV's product as an integrated offering.

If the primary customer base for the ISV is Windows customers, then  the .NET platform should be chosen.  It will provide much better performance at a much lower cost.

Client device independence

The major difference being that with Java, it is the presentation tier programmer that determines the ultimate HTML that will be delivered to the client, and with .NET, it is a Visual Studio.NET control.

This Java approach has three problems. First, it requires a lot of code on the presentation tier, since every possible thin client system requires a different code path. Second, it is very difficult to test the code with every possible thin client system. Third, it is very difficult to add new thin clients to an existing application, since to do so involves searching through, and modifying a tremendous amount of presentation tier logic.

The .NET Framework approach is to write device independent code that interacts with visual controls. It is the control, not the programmer, that is responsible for determining what HTML to deliver, based on the capabilities of the client device.. In the .NET Framework model, one can forget that such a thing as HTML even exists!

Conclusion

Sun's J2EE vision is based on a family of specifications that can be implemented by many vendors. It is open in the sense that any company can license and implement the technology, but closed in the sense that it is controlled by a single vendor, and a self contained architectural island with very limited ability to interact outside of itself. One of J2EE's major disadvantages is that the choice of the platform dictates the use of a single programming language, and a programming language that is not well suited for most businesses. One of J2EE's major advantages is that most of the J2EE vendors do offer operating system portability.

Microsoft's .NET platform vision is a family of products rather than specifications, with specifications used primarily to define points of interoperability. The major disadvantage of this approach is that if is limited to the Windows platform, so applications written for the .NET platform can only be run on .NET platforms. Their are several important advantages to the .NET platform:

* The cost of developing applications is much lower, since standard business languages can be used and device independent presentation tier logic can be written.

* The cost of running applications is much lower, since commodity hardware platforms (at 1/5 the cost of their Unix counterparts) can be used.

* The ability to scale up is much greater, with the proved ability to support at least ten times the number of clients any J2EE platform has shown itself able to support.

* Interoperability is much stronger, with industry standard eCollaboration built into the platform.

What are the Main Features of .NET platform?

Features of .NET Platform are :-

Common Language Runtime
Explains the features and benefits of the common language runtime, a run-time environment that manages the execution of code and provides services that simplify the development process.

Assemblies
Defines the concept of assemblies, which are collections of types and resources that form logical units of functionality. Assemblies are the fundamental units of deployment, version control, reuse, activation scoping, and security permissions.

Application Domains
Explains how to use application domains to provide isolation between applications.

Runtime Hosts
Describes the runtime hosts supported by the .NET Framework, including ASP.NET, Internet Explorer, and shell executables.

Common Type System
Identifies the types supported by the common language runtime.

Metadata and Self-Describing Components
Explains how the .NET Framework simplifies component interoperation by allowing compilers to emit additional declarative information, or metadata, into all modules and assemblies.

Cross-Language Interoperability
Explains how managed objects created in different programming languages can interact with one another.

.NET Framework Security
Describes mechanisms for protecting resources and code from unauthorized code and unauthorized users.

.NET Framework Class Library
Introduces the library of types provided by the .NET Framework, which expedites and optimizes the development process and gives you access to system functionality.

What is the use of JIT ?
JIT (Just - In - Time) is a compiler which converts MSIL code to Native Code (ie.. CPU-specific code that runs on the same computer architecture).

Because the common language runtime supplies a JIT compiler for each supported CPU architecture, developers can write a set of MSIL that can be JIT-compiled and run on computers with different architectures. However, your managed code will run only on a specific operating system if it calls platform-specific native APIs, or a platform-specific class library.

JIT compilation takes into account the fact that some code might never get called during execution. Rather than using time and memory to convert all the MSIL in a portable executable (PE) file to native code, it converts the MSIL as needed during execution and stores the resulting native code so that it is accessible for subsequent calls. The loader creates and attaches a stub to each of a type's methods when the type is loaded. On the initial call to the method, the stub passes control to the JIT compiler, which converts the MSIL for that method into native code and modifies the stub to direct execution to the location of the native code. Subsequent calls of the JIT-compiled method proceed directly to the native code that was previously generated, reducing the time it takes to JIT-compile and run the code.

What meant of assembly & global assembly cache (gac) & Meta data.
Assembly :-- An assembly is the primary building block of a .NET based application. It is a collection of functionality that is built, versioned, and deployed as a single implementation unit (as one or more files). All managed types and resources are marked either as accessible only within their implementation unit, or as accessible by code outside that unit. It overcomes the problem of 'dll Hell'.The .NET Framework uses assemblies as the fundamental unit for several purposes: 

•  Security 
•  Type Identity 
•  Reference Scope 
• Versioning 
•  Deployment

Global Assembly Cache :-- Assemblies can be shared among multiple applications on the machine by registering them in global Assembly cache(GAC). GAC is a machine wide a local cache of assemblies maintained by the .NET Framework. We can register the assembly to global assembly cache by using gacutil command.
We can Navigate to the GAC directory, C:\winnt\Assembly in explore. In the tools menu select the cache properties; in the windows displayed you can set the memory limit in MB used by the GAC
MetaData :--Assemblies have Manifests. This Manifest contains Metadata information of the Module/Assembly as well as it contains detailed Metadata of other assemblies/modules references (exported). It's the Assembly Manifest which differentiates between an Assembly and a Module.

What are the mobile devices supported by .net platform

The Microsoft .NET Compact Framework is designed to run on mobile devices such as mobile phones, Personal Digital Assistants (PDAs), and embedded devices. The easiest way to develop and test a Smart Device Application is to use an emulator.

These devices are divided into two main divisions:

1. Those that are directly supported by .NET (Pocket PCs, i-Mode phones, and WAP devices)
2. Those that are not (Palm OS and J2ME-powered devices).

What is GUID , why we use it and where?

GUID :-- GUID is Short form of Globally Unique Identifier, a unique 128-bit number that is produced by the Windows OS or by some Windows applications to identify a particular component, application, file, database entry, and/or user. For instance, a Web site may generate a GUID and assign it to a user's browser to record and track the session. A GUID is also used in a Windows registry to identify COM DLLs. Knowing where to look in the registry and having the correct GUID yields a lot information about a COM object (i.e., information in the type library, its physical location, etc.). Windows also identifies user accounts by a username (computer/domain and username) and assigns it a GUID. Some database administrators even will use GUIDs as primary key values in databases.

GUIDs can be created in a number of ways, but usually they are a combination of a few unique settings based on specific point in time (e.g., an IP address, network MAC address, clock date/time, etc.).

Describe the difference between inline and code behind - which is best in a loosely coupled solution

ASP.NET supports two modes of page development: Page logic code that is written inside runat="server"> blocks within an .aspx file and dynamically compiled the first time the page is requested on the server. Page logic code that is written within an external class that is compiled prior to deployment on a server and linked ""behind"" the .aspx file at run time.

Whats MSIL, and why should my developers need an appreciation of it if at all?

When compiling the source code to managed code, the compiler translates the source into Microsoft intermediate language (MSIL). This is a CPU-independent set of instructions that can efficiently be converted to native code. Microsoft intermediate language (MSIL) is a translation used as the output of a number of compilers. It is the input to a just-in-time (JIT) compiler. The Common Language Runtime includes a JIT compiler for the conversion of MSIL to native code.

Before Microsoft Intermediate Language (MSIL) can be executed it, must be converted by the .NET Framework just-in-time (JIT) compiler to native code. This is CPU-specific code that runs on the same computer architecture as the JIT compiler. Rather than using time and memory to convert all of the MSIL in a portable executable (PE) file to native code. It converts the MSIL as needed whilst executing, then caches the resulting native code so its accessible for any subsequent calls.

How many .NET languages can a single .NET DLL contain?
One

What type of code (server or client) is found in a Code-Behind class?

Server

Whats an assembly?

Assemblies are the building blocks of .NET Framework applications; they form the fundamental unit of deployment, version control, reuse, activation scoping, and security permissions. An assembly is a collection of types and resources that are built to work together and form a logical unit of functionality. An assembly provides the common language runtime with the information it needs to be aware of type implementations. To the runtime, a type does not exist outside the context of an assembly.

How many classes can a single .NET DLL contain?

Unlimited.

What is the difference between string and String ?

No difference

What is manifest?

It is the metadata that describes the assemblies.

What is metadata?

Metadata is machine-readable information about a resource, or ""data about data."" Such information might include details on content, format, size, or other characteristics of a data source. In .NET, metadata includes type definitions, version information, external assembly references, and other standardized information.

What are the types of assemblies?

There are four types of assemblies in .NET:

Static assemblies These are the .NET PE files that you create at compile time.

Dynamic assemblies
These are PE-formatted, in-memory assemblies that you dynamically create at runtime using the classes in the System.Reflection.Emit namespace.

Private assemblies
These are static assemblies used by a specific application.

Public or shared assemblies
These are static assemblies that must have a unique shared name and can be used by any application.

An application uses a private assembly by referring to the assembly using a static path or through an XML-based application configuration file. While the CLR doesn't enforce versioning policies-checking whether the correct version is used-for private assemblies, it ensures that an
application uses the correct shared assemblies with which the application was built. Thus, an application uses a specific shared assembly by referring to the specific shared assembly, and the CLR ensures that the correct version is loaded at runtime.

In .NET, an assembly is the smallest unit to which you can associate a version number;

What are delegates?where are they used ?

A delegate defines a reference type that can be used to encapsulate a method with a specific signature. A delegate instance encapsulates a static or an instance method. Delegates are roughly similar to function pointers in C++; however, delegates are type-safe and secure.

When do you use virutal keyword?.

When we need to override a method of the base class in the sub class, then we give the virtual keyword in the base class method. This makes the method in the base class to be overridable. Methods, properties, and indexers can be virtual, which means that their implementation can be overridden in derived classes.

What are class access modifiers ?
Access modifiers are keywords used to specify the declared accessibility of a member or a type. This section introduces the four access modifiers:

• Public  - Access is not restricted.
• Protected  - Access is limited to the containing class or types derived from the containing class.
• Internal - Access is limited to the current assembly.
• Protected inertnal - Access is limited to the current assembly or types derived · from the containing class.
• Private - Access is limited to the containing type.

What Is Boxing And Unboxing? Boxing :- Boxing is an implicit conversion of a value type to the type object type

Eg:-
Consider the following declaration of a value-type variable:
int i = 123;
object o = (object) i;
Boxing Conversion

UnBoxing :- Unboxing is an explicit conversion from the type object to a value type
Eg:

int i = 123;
object o = (object) i;
Boxing Conversion

What is Value type and refernce type in .Net?.

Value Type : A variable of a value type always contains a value of that type. The assignment to a variable of a value type creates a copy of the assigned value, while the assignment to a variable of a reference type creates a copy of the reference but not of the referenced object.

The value types consist of two main categories:

• Stuct Type
• Enumeration Type

Reference Type :Variables of reference types, referred to as objects, store references to the actual data. This section introduces the following keywords used to declare reference types: 

• Class
• Interface
• Delegate

This section also introduces the following built-in reference types:

• object
• string

Note: Few of the references are taken from other sites/sources

What is the difference between structures and enumeration?.

Unlike classes, structs are value types and do not require heap allocation. A variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains a reference to the data. They are derived from System.ValueType class.

Enum->An enum type is a distinct type that declares a set of named constants.They  are strongly typed constants. They are unique types that allow to declare symbolic names to integral values. Enums are value types, which means they contain their own value, can't inherit or be inherited from and assignment copies the value of one enum to another.

public enum Grade
{
   A,
   B,
   C
}

What is namespaces?.

Namespace is a logical naming scheme for group related types.Some class types that logically belong together they can be put into a common namespace. They prevent namespace collisions and they provide scoping. They are imported as "using" in C# or "Imports" in Visual Basic. It seems as if these directives specify a particular assembly, but they don't. A namespace can span multiple assemblies, and an assembly can define multiple namespaces. When the compiler needs the definition for a class type, it tracks  through each of the different imported namespaces to the type name and searches each referenced assembly until it is found.
Namespaces can be nested. This is very similar to packages in Java as far as scoping is concerned.

How do you create shared assemblies?.

Just look through the definition of Assemblies..

• An Assembly is a  logical unit of code
• Assembly physically exist as DLLs or EXEs
• One assembly can contain one or more files
• The constituent files can include any file types like image files, text files etc. along with DLLs or EXEs
• When you compile your source code by default the exe/dll generated is actually an assembly
• Unless your code is bundled as assembly it can not be used in any other application
• When you talk about version of a component you are actually talking about version of the assembly to which the component belongs.
• Every assembly file contains information about itself. This information is called as Assembly Manifest.

Following steps are involved in creating shared assemblies :

• Create your DLL/EXE source code
• Generate unique assembly name using SN utility
• Sign your DLL/EXE with the private key by modifying AssemblyInfo file
• Compile your DLL/EXE
• Place the resultant DLL/EXE in global assembly cache using AL utility

What is global assembly cache?

Each computer where the common language runtime is installed has a machine-wide code cache called the global assembly cache. The global assembly cache stores assemblies specifically designated to be shared by several applications on the computer.
 There are several ways to deploy an assembly into the global assembly cache:

• Use an installer designed to work with the global assembly cache. This is the preferred option for installing assemblies into the global assembly cache.
• Use a developer tool called the Global Assembly Cache tool (Gacutil.exe), provided by the .NET Framework SDK.
• Use Windows Explorer to drag assemblies into the cache.  

What is MSIL?.

When compiling to managed code, the compiler translates your source code into Microsoft intermediate language (MSIL), which is a CPU-independent set of instructions that can be efficiently converted to native code. MSIL includes instructions for loading, storing, initializing, and calling methods on objects, as well as instructions for arithmetic and logical operations, control flow, direct memory access, exception handling, and other operations. Before code can be run, MSIL must be converted to CPU-specific code, usually by a just-in-time (JIT) compiler. Because the common language runtime supplies one or more JIT compilers for each computer architecture it supports, the same set of MSIL can be JIT-compiled and run on any supported architecture.
When a compiler produces MSIL, it also produces metadata. Metadata describes the types in your code, including the definition of each type, the signatures of each type's members, the members that your code references, and other data that the runtime uses at execution time. The MSIL and metadata are contained in a portable executable (PE) file that is based on and extends the published Microsoft PE and common object file format (COFF) used historically for executable content. This file format, which accommodates MSIL or native code as well as metadata, enables the operating system to recognize common language runtime images. The presence of metadata in the file along with the MSIL enables your code to describe itself, which means that there is no need for type libraries or Interface Definition Language (IDL). The runtime locates and extracts the metadata from the file as needed during execution.

What is Jit compilers?.how many are available in clr?

Just-In-Time compiler- it converts the language that you write in .Net into machine language that a computer can understand. there are tqo types of JITs one  is memory optimized & other  is performace optimized.

What is tracing?Where it used.Explain few methods available

Tracing refers to collecting information about the application while it is running. You use tracing  information to troubleshoot an application.
Tracing allows us to observe and correct programming errors. Tracing enables you to record information in various log files about the errors that might occur at run time. You can analyze these log files to find the cause of the errors.

In .NET we have objects called Trace Listeners. A listener is an object that receives the trace output and outputs it somewhere; that somewhere could be a window in your development environment, a file on your hard drive, a Windows Event log, a SQL Server or Oracle database, or any other customized data store.

The System.Diagnostics namespace provides the interfaces, classes, enumerations and structures that are used for tracing The System.Diagnostics namespace provides two classes named Trace and Debug that are used for writing errors and application execution information in logs.

All Trace Listeners have the following functions. Functionality of these functions is same except that the target media for the tracing output is determined by the Trace Listener.

Method Name

Result Fail   Outputs the specified text with the Call Stack.
Write   Outputs the specified text.
WriteLine    Outputs the specified text and a carriage return.
Flush   Flushes the output buffer to the target media.
Close   Closes the output stream in order to not receive the tracing/debugging output.

How to set the debug mode? Debug Mode for ASP.NET applications - To set ASP.NET appplication in debugging mode, edit the application's web.config and assign the "debug" attribute in < compilation > section to "true" as show below:

< configuration >
  < system.web >
    < compilation defaultLanguage="vb" debug="true" / >
....
...
..
< / configuration >

This case-sensitive attribute 'debug tells ASP.NET to generate symbols for dynamically generated files and enables the debugger to attach to the ASP.NET application. ASP.NET will detect this change automatically, without the need to restart the server. Debug Mode for ASP.NET Webservices - Debugging an XML Web service created with ASP.NET is similar to the debugging an ASP.NET Web application.

What is the property available to check if the page posted or not?

The Page_Load event handler in the page checks for IsPostBack property value, to ascertain whether the page is posted. The Page.IsPostBack gets a value indicating whether the page is being loaded in response to the client postback, or it is for the first time. The value of Page.IsPostBack is True, if the page is being loaded in response to the client postback; while its value is False, when the page is loaded for the first time. The Page.IsPostBack property facilitates execution of certain routine in Page_Load, only once (for e.g. in Page load, we need to set default value in controls, when page is loaded for the first time. On post back, we check for true value for IsPostback value and then invoke server-side code to
update data).

Which are the abstract classes available under system.xml namespace?

The System.XML namespace provides XML related processing ability in .NET framework. XmlReader and XMLWriter are the two abstract classes at the core of .NET Framework XML classes:

1. XmlReader provides a fast, forward-only, read-only cursor for processing an XML document stream.
2. XmlWriter provides an interface for producing XML document streams that conform to the W3C's XML standards.

Both XmlReader and XmlWriter are abstract base classes, which define the functionality that all derived classes must support.

Is it possible to use multipe inheritance in .net?

Multiple Inheritance is an ability to inherit from more than one base class i.e.  ability of a class to have more than one superclass, by inheriting from different sources and thus combine separately-defined behaviors in a single class. There are two types of multiple inheritance: multiple type/interface inheritance and multiple implementation inheritance. C# & VB.NET supports only multiple type/interface inheritance, i.e. you can derive an class/interface from multiple interfaces. There is no support for multiple implementation inheritance in .NET. That means a class can only derived from one class.

What are the derived classes from xmlReader and xmlWriter?

Both XmlReader and XmlWriter are abstract base classes, which define the functionality that all derived classes must support.
There are three concrete implementations of XmlReader:
       1.XmlTextReader
       2.XmlNodeReader
       3.XmlValidatingReader
There are two concrete implementations of XmlWriter:
       1.XmlTextWriter
       2.XmlNodeWriter
XmlTextReader and XmlTextWriter support reading data to/from text-based stream, while XmlNodeReader and XmlNodeWriter are designed for working with in-memory DOM tree structure. The custom readers and writers can also be developed to extend the built-in functionality of XmlReader and XmlWriter.

What is managed and unmanaged code?

The .NET framework provides several core run-time services to the programs that run within it - for example exception handling and security. For these services to work, the code must provide a minimum level of information to the runtime. i.e., code executing under the control of the CLR is called managed code. For example, any code written in C# or Visual Basic .NET is managed code.

Code that runs outside the CLR is referred to as "unmanaged code." COM components, ActiveX components, and Win32 API functions are examples of unmanaged code.

How you deploy .NET assemblies?

One way is simply use xcopy. others are use and the setup projects in .net. and one more way is use of nontuch deployment.

What is Globalizationa and Localization ?

Globalization is the process of creating an application that meets the needs of users from multiple cultures. It includes using the correct
currency, date and time format, calendar, writing direction, sorting rules, and other issues. Accommodating these cultural differences in an application is called localization.Using classes of System.Globalization namespace, you can set application's current culture.

This can be achieved by using any of the following 3 approaches.

1. Detect and redirect
2. Run-time adjustment
3. Using Satellite assemblies.

Whate are Resource Files ? How are they used in .NET?

Resource files are the files containing data that is logically deployed with an application.These files can contain data in a number of formats including strings, images and persisted objects. It has the main advantage of If we store data in these files then we don't need to compile these if the data get changed. In .NET we basically require them storing culture specific informations by localizing application's resources. You can deploy your resources using satellite assemblies.

Difference between Dispose and Finallize method?

Finalize method is used to free the memory used by some unmanaged resources like window handles (HWND). It's similar to the destructor syntax in C#. The GC calls this method when it founds no more references to the object. But, In some cases we may need release the memory used by the resources explicitely.To release the memory explicitly we need to implement the Dispose method of IDisposable interface.

What is encapsulation ?

Encapsulation is the ability to hide the internal workings of an object's behavior and its data. For instance, let's say you have a object named Bike and this object has a method named start(). When you create an instance of a Bike object and call its start() method you are not worried about what happens to accomplish this, you just want to make sure the state of the bike is changed to 'running' afterwards. This kind of behavior hiding is encapsulation and it makes programming much easier.

How can you prevent your class to be inherated further?

By setting Sealed - Key word

public sealed class Planet
 {
             //code goes here
 }

class Moon:Planet
 {
     //Not allowed as base class is sealed
 }

What is GUID and why we need to use it and in what condition? How this is created.

A GUID is a 128-bit integer (16 bytes) that can be used across all computers and networks wherever  a unique identifier is required. Such an identifier has a very low probability of being duplicated. Visual Studio .NET IDE has a utility under the tools menu to generate GUIDs.

Why do you need to serialize.?

We need to serialize the object,if you want to pass object from one  computer/application domain to another.Process of converting complex objects into stream of bytes that can be persisted or  transported.Namespace for serialization is System.Runtime.Serialization.The  ISerializable interface allows you to make any class Serializable..NET framework features 2 serializing method.
1.Binary Serialization 2.XML Serialization

What is inline schema, how does it works?

Schemas can be included inside of XML file is called Inline Schemas.This is useful  when it is inconvenient to physically seprate the schema and the XML document.A  schema is an XML document that defines the structure, constraints, data types, and  relationships of the elements that constitute the data contained inside the XML  document or in another XML document.Schema can be an external file which uses the  XSD or XDR extension called external schema. Inline schema can take place even when validation is turned off.

Describe the advantages of writing a managed code application instead of unmanaged one. What's involved in certain piece of code being managed?"Advantage includes automatic garbage collection,memory management,security,type checking,versioning

Managed code is compiled for the .NET run-time environment. It runs in the Common Language Runtime (CLR), which is the heart of the .NET Framework. The CLR provides services such as security,
memory management, and cross-language integration. Managed applications written to take advantage of the features of the CLR perform more efficiently and safely, and take better advantage of developers existing expertise in languages that support the .NET Framework.

Unmanaged code includes all code written before the .NET Framework was introduced—this includes code written to use COM, native Win32, and Visual Basic 6. Because it does not run inside the .NET environment, unmanaged code cannot make use of any .NET managed facilities."

What are multicast delegates ? give me an example ?

Delegate that can have more than one element in its invocation List.

namespace SampleMultiCastDelegate
{
  class MultiCast
  {
    public delegate string strMultiCast(string s);
  }
}

MainClass defines the static methods having same signature as delegate.

using System;

namespace SampleMultiCastDelegate
{
 
  public class MainClass
  {
    public MainClass()
    { 
    }

    public static string Jump(string s)
    {
      Console.WriteLine("Jump");
      return String.Empty;
    }

    public static string Run(string s)
    {
      Console.WriteLine("Run");
      return String.Empty;
    }

    public static string Walk(string s)
    {
      Console.WriteLine("Walk");
      return String.Empty;      
    }
  }
}

The Main class:

using System;
using System.Threading;
namespace SampleMultiCastDelegate
{
 
  public class MainMultiCastDelegate
  {
    public static void Main()
    {
      MultiCast.strMultiCast  Run,Walk,Jump;

      MultiCast.strMultiCast    myDelegate;

       ///here mydelegate used the Combine method of System.MulticastDelegate
      ///and the delegates combine 
      myDelegate=(MultiCast.strMultiCast)System.Delegate.Combine(Run,Walk); 
               
    }
  }
}

Can a nested object be used in Serialization ?

Yes. If a class that is to be serialized contains references to objects of other classes, and if those classes have been marked as serializable, then their objects are serialized too.

Difference between int and int32 ?

Both are same. System.Int32 is a .NET class. Int is an alias name for System.Int32.

Describe the difference between a Thread and a Process?

A Process is an instance of an running application. And a thread is the Execution stream of the Process. A process can have multiple Thread.

When a process starts a specific memory area is allocated to it. When there is multiple thread in a process, each thread gets a memory for storing the variables in it and plus they can access to the global variables which is common for all the thread. Eg.A Microsoft Word is a Application. When you open a word file,an instance of the Word starts and a process is allocated to this instance which has one thread.

What is the difference between an EXE and a DLL?

You can create an objects of Dll but not of the EXE.
Dll is an In-Process Component whereas EXE is an OUt-Process Component.
Exe is for single use whereas you can use Dll for multiple use.
Exe can be started as standalone where dll cannot be.

What is strong-typing versus weak-typing? Which is preferred? Why?

Strong typing implies that the types of variables involved in operations are associated to the variable, checked at compile-time, and require explicit conversion; weak typing implies that they are associated to the value, checked at run-time, and are implicitly converted as required. (Which is preferred is a disputable point, but I personally prefer strong typing because I like my errors to be found as soon as possible.)

What is a PID? How is it useful when troubleshooting a system?

PID is the process Id of the application in Windows. Whenever a process starts running in the Windows environment, it is associated with an individual process Id or PID.

The PID (Process ID) a unique number for each item on the Process Tab, Image Name list. How do you get the PID to appear? In Task Manger, select the View menu, then select columns and check PID (Process Identifier).

In Linux, PID is used to debug a process explicitly. However we cannot do this in a windows environment.

Microsoft has launched a SDK called as Microsoft Operations Management (MOM). This uses the PID to find out which dll’s have been loaded by a process in the memory. This is essentially helpful in situations where the Process which has a memory leak is to be traced to a erring dll. Personally I have never used a PID, our Windows debugger does the things required to find out.

What is the GAC? What problem does it solve?

Each computer where the common language runtime is installed has a machine-wide code cache called the global assembly cache. The global assembly cache stores assemblies that are to be shared by several applications on the computer. This area is typically the folder under windows or winnt in the machine.

All the assemblies that need to be shared across applications need to be done through the Global assembly Cache only. However it is not necessary to install assemblies into the global assembly cache to make them accessible to COM interop or unmanaged code.

There are several ways to deploy an assembly into the global assembly cache:

• Use an installer designed to work with the global assembly cache. This is the preferred option for installing assemblies into the global assembly cache.
• Use a developer tool called the Global Assembly Cache tool (Gacutil.exe), provided by the .NET Framework SDK.
• Use Windows Explorer to drag assemblies into the cache.

GAC solves the problem of DLL Hell and DLL versioning. Unlike earlier situations, GAC can hold two assemblies of the same name but different version. This ensures that the applications which access a particular assembly continue to access the same assembly even if another version of that assembly is installed on that machine.

Describe what an Interface is and how it’s different from a Class.

An interface is a structure of code which is similar to a class. An interface is a prototype for a class and is useful from a logical design perspective. Interfaces provide a means to define the protocols for a class without worrying about the implementation details. The syntax for creating interfaces follows:

interface Identifier {
  InterfaceBody
}

Identifier is the name of the interface and InterfaceBody refers to the abstract methods and static final variables that make up the interface. Because it is assumed that all the methods in an interface are abstract, it isn't necessary to use the abstract keyword

An interface is a description of some of the members available from a class. In practice, the syntax typically looks similar to a class definition, except that there's no code defined for the methods — just their name, the arguments passed and the type of the value returned.
So what good is it? None by itself. But you create an interface so that classes will implement it.

But what does it mean to implement an interface. The interface acts as a contract or promise. If a class implements an interface, then it must have the properties and methods of the interface defined in the class. This is enforced by the compiler.

Broadly the differentiators between classes and interfaces is as follows

• Interface should not have any implementation.
• Interface can not create any instance.
• Interface should provide high level abstraction from the implementation.
• Interface can have multiple inheritances.
• Default access level of the interface is public.

What is the difference between XML Web Services using ASMX and .NET Remoting using SOAP?

ASP.NET Web services and .NET Remoting provide a full suite of design options for cross-process and cross-plaform communication in distributed applications. In general, ASP.NET Web services provide the highest levels of interoperability with full support for WSDL and SOAP over HTTP, while .NET Remoting is designed for common language runtime type-system fidelity and supports additional data format and communication channels. Hence if we looking cross-platform communication than web services is the choice coz for .NET remoting .Net framework is requried which may or may not present for the other platform.

Serialization and Metadata

ASP.NET Web services rely on the System.Xml.Serialization.XmlSerializer class to marshal data to and from SOAP messages at runtime. For metadata, they generate WSDL and XSD definitions that describe what their messages contain. The reliance on pure WSDL and XSD makes ASP.NET Web services metadata portable; it expresses data structures in a way that other Web service toolkits on different platforms and with different programming models can understand. In some cases, this imposes constraints on the types you can expose from a Web service—XmlSerializer will only marshal things that can be expressed in XSD. Specifically, XmlSerializer will not marshal object graphs and it has limited support for container types.

.NET Remoting relies on the pluggable implementations of the IFormatter interface used by the System.Runtime.Serialization engine to marshal data to and from messages. There are two standard formatters, System.Runtime.Serialization.Formatters.Binary.BinaryFormatter and System.Runtime.Serialization.Formatters.Soap.SoapFormatter. The BinaryFormatter and SoapFormatter, as the names suggest, marshal types in binary and SOAP format respectively. For metadata, .NET Remoting relies on the common language runtime assemblies, which contain all the relevant information about the data types they implement, and expose it via reflection. The reliance on the assemblies for metadata makes it easy to preserve the full runtime type-system fidelity. As a result, when the .NET Remoting plumbing marshals data, it includes all of a class's public and private members; handles object graphs correctly; and supports all container types (e.g., System.Collections.Hashtable). However, the reliance on runtime metadata also limits the reach of a .NET Remoting system—a client has to understand .NET constructs in order to communicate with a .NET Remoting endpoint. In addition to pluggable formatters, the .NET Remoting layer supports pluggable channels, which abstract away the details of how messages are sent. There are two standard channels, one for raw TCP and one for HTTP. Messages can be sent over either channel independent of format.

Distributed Application Design: ASP.NET Web Services vs. .NET Remoting
ASP.NET Web services favor the XML Schema type system, and provide a simple programming model with broad cross-platform reach. .NET Remoting favors the runtime type system, and provides a more complex programming model with much more limited reach. This essential difference is the primary factor in determining which technology to use. However, there are a wide range of other design factors, including transport protocols, host processes, security, performance, state management, and support for transactions to consider as well.

Security
Since ASP.NET Web services rely on HTTP, they integrate with the standard Internet security infrastructure. ASP.NET leverages the security features available with IIS to provide strong support for standard HTTP authentication schemes including Basic, Digest, digital certificates, and even Microsoft® .NET Passport. (You can also use Windows Integrated authentication, but only for clients in a trusted domain.) One advantage of using the available HTTP authentication schemes is that no code change is required in a Web service; IIS performs authentication before the ASP.NET Web services are called. ASP.NET also provides support for .NET Passport-based authentication and other custom authentication schemes. ASP.NET supports access control based on target URLs, and by integrating with the .NET code access security (CAS) infrastructure. SSL can be used to ensure private communication over the wire.

Although these standard transport-level techniques to secure Web services are quite effective, they only go so far. In complex scenarios involving multiple Web services in different trust domains, you have to build custom ad hoc solutions. Microsoft and others are working on a set of security specifications that build on the extensibility of SOAP messages to offer message-level security capabilities. One of these is the XML Web Services Security Language (WS-Security), which defines a framework for message-level credential transfer, message integrity, and message confidentiality.

As noted in the previous section, the .NET Remoting plumbing does not secure cross-process invocations in the general case. A .NET Remoting endpoint hosted in IIS with ASP.NET can leverage all the same security features available to ASP.NET Web services, including support for secure communication over the wire using SSL. If you are using the TCP channel or the HTTP channel hosted in processes other than aspnet_wp.exe, you have to implement authentication, authorization and privacy mechanisms yourself.

One additional security concern is the ability to execute code from a semi-trusted environment without having to change the default security policy. ASP.NET Web Services client proxies work in these environments, but .NET Remoting proxies do not. In order to use a .NET Remoting proxy from a semi-trusted environment, you need a special serialization permission that is not given to code loaded from your intranet or the Internet by default. If you want to use a .NET Remoting client from within a semi-trusted environment, you have to alter the default security policy for code loaded from those zones. In situations where you are connecting to systems from clients running in a sandbox—like a downloaded Windows Forms application, for instance—ASP.NET Web Services are a simpler choice because security policy changes are not required.

Conceptually, what is the difference between early-binding and late-binding?
Early binding – Binding at Compile Time
Late Binding – Binding at Run Time

Early binding implies that the class of the called object is known at compile-time; late-binding implies that the class is not known until run-time, such as a call through an interface or via Reflection.

Early binding is the preferred method. It is the best performer because your application binds directly to the address of the function being called and there is no extra overhead in doing a run-time lookup. In terms of overall execution speed, it is at least twice as fast as late binding.

Early binding also provides type safety. When you have a reference set to the component's type library, Visual Basic provides IntelliSense support to help you code each function correctly. Visual Basic also warns you if the data type of a parameter or return value is incorrect, saving a lot of time when writing and debugging code.

Late binding is still useful in situations where the exact interface of an object is not known at design-time. If your application seeks to talk with multiple unknown servers or needs to invoke functions by name (using the Visual Basic 6.0 CallByName function for example) then you need to use late binding. Late binding is also useful to work around compatibility problems between multiple versions of a component that has improperly modified or adapted its interface between versions.

What is an Asssembly Qualified Name? Is it a filename? How is it different?

An assembly qualified name isn't the filename of the assembly; it's the internal name of the assembly combined with the assembly version, culture, and public key, thus making it unique.

e.g. (""System.Xml.XmlDocument, System.Xml, Version=1.0.3300.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"")

How is a strongly-named assembly different from one that isn’t strongly-named?  Strong names are used to enable the stricter naming requirements associated with shared assemblies. These strong names are created by a .NET utility – sn.exe

Strong names have three goals:

• Name uniqueness. Shared assemblies must have names that are globally unique. 
• Prevent name spoofing. Developers don't want someone else releasing a subsequent version of one of your assemblies and falsely claim it came from you, either by accident or intentionally. 
• Provide identity on reference. When resolving a reference to an assembly, strong names are used to guarantee the assembly that is loaded came from the expected publisher.

Strong names are implemented using standard public key cryptography. In general, the process works as follows: The author of an assembly generates a key pair (or uses an existing one), signs the file containing the manifest with the private key, and makes the public key available to callers. When references are made to the assembly, the caller records the public key corresponding to the private key used to generate the strong name.

Weak named assemblies are not suitable to be added in GAC and shared. It is essential for an assembly to be strong named.

Strong naming prevents tampering and enables assemblies to be placed in the GAC alongside other assemblies of the same name.

How does the generational garbage collector in the .NET CLR manage object lifetime? What is non-deterministic finalization?

The hugely simplistic version is that every time it garbage-collects, it starts by assuming everything to be garbage, then goes through and builds a list of everything reachable. Those become not-garbage, everything else doesn't, and gets thrown away. What makes it generational is that every time an object goes through this process and survives, it is noted as being a member of an older generation (up to 2, right now). When the garbage-collector is trying to free memory, it starts with the lowest generation (0) and only works up to higher ones if it can't free up enough space, on the grounds that shorter-lived objects are more likely to have been freed than longer-lived ones.

Non-deterministic finalization implies that the destructor (if any) of an object will not necessarily be run (nor its memory cleaned up, but that's a relatively minor issue) immediately upon its going out of scope. Instead, it will wait until first the garbage collector gets around to finding it, and then the finalisation queue empties down to it; and if the process ends before this happens, it may not be finalised at all. (Although the operating system will usually clean up any process-external resources left open - note the usually there, especially as the exceptions tend to hurt a lot.)

What is the difference between Finalize() and Dispose()?

Dispose() is called by the user of an object to indicate that he is finished with it, enabling that object to release any unmanaged resources it holds. Finalize() is called by the run-time to allow an object which has not had Dispose() called on it to do the same. However, Dispose() operates determinalistically, whereas there is no guarantee that Finalize() will be called immediately when an object goes out of scope - or indeed at all, if the program ends before that object is GCed - and as such Dispose() is generally preferred.

How is the using() pattern useful? What is IDisposable? How does it support deterministic finalization?

The using() pattern is useful because it ensures that Dispose() will always be called when a disposable object (defined as one that implements IDisposable, and thus the Dispose() method) goes out of scope, even if it does so by an exception being thrown, and thus that resources are always released.

What does this useful command line do? tasklist /m "mscor*" 
Lists all the applications and associated tasks/process currently  running on the system with a module whose name begins "mscor" loaded into them; which in nearly all cases means "all the .NET processes".

What’s wrong with a line like this? DateTime.Parse(myString);

Therez nothing wrong with this declaration.Converts the specified string representation of a date and time to its DateTime equivalent.But If the string is not a valid DateTime,It throws an exception.

What are PDBs? Where must they be located for debugging to work?

A program database (PDB) files holds debugging and project state information that allows incremental linking of debug configuration of your program.There are several different types of symbolic debugging information. The default type for Microsoft compiler is the so-called PDB file. The compiler setting for creating this file is /Zi, or /ZI for C/C++(which creates a PDB file with additional information that enables a feature called ""Edit and Continue"") or a Visual Basic/C#/JScript .NET program with /debug.

A PDB file is a separate file, placed by default in the Debug project subdirectory, that has the same name as the executable file with the extension .pdb. Note that the Visual C++ compiler by default creates an additional PDB file called VC60.pdb for VisulaC++6.0 and VC70.PDB file for VisulaC++7.0. The compiler creates this file during compilation of the source code, when the compiler isn't aware of the final name of the executable. The linker can merge this temporary PDB file into the main one if you tell it to, but it won't do it by default. The PDB file can be useful to display the detailed stack trace with source files and line numbers.

What is FullTrust? Do GAC’ed assemblies have FullTrust?

Before the .NET Framework existed, Windows had two levels of trust for downloaded code. This old model was a binary trust model. You only had two choices: Full Trust, and No Trust. The code could either do anything you could do, or it wouldn't run at all.

The permission sets in .NET include FullTrust, SkipVerification, Execution, Nothing, LocalIntranet, Internet and Everything. Full Trust Grants unrestricted permissions to system resources. Fully trusted code run by a normal, nonprivileged user cannot do administrative tasks, but can access any resources the user can access, and do anything the user can do. From a security standpoint, you can think of fully trusted code as being similar to native, unmanaged code, like a traditional ActiveX control.
GAC assemblies are granted FullTrust. In v1.0 and 1.1, the fact that assemblies in the GAC seem to always get a FullTrust grant is actually a side effect of the fact that the GAC lives on the local machine.  If anyone were to lock down the security policy by changing the grant set of the local machine to something less than FullTrust, and if your assembly did not get extra permission from some other code group, it would no longer have FullTrust even though it lives in the GAC.

What does this do? gacutil /l | find /i "Corillian"

The Global Assembly Cache tool allows you to view and manipulate the contents of the global assembly cache and download cache. The tool comes with various optional params to do that.

""/l"" option Lists the contents of the global assembly cache. If you specify the assemblyName parameter(/l [assemblyName]), the tool lists only the assemblies matching that name.

What does this do .. sn -t foo.dll ?

Sn -t option displays the token for the public key stored in infile. The contents of infile must be previously generated using -p.
Sn.exe computes the token using a hash function from the public key. To save space, the common language runtime stores public key tokens in the manifest as part of a reference to another assembly when it records a dependency to an assembly that has a strong name. The -tp option displays the public key in addition to the token.

How do you generate a strong name?

.NET provides an utility called strong name tool. You can run this toolfrom the VS.NET command prompt to generate a strong name with an option "-k" and providing the strong key file name. i.e. sn- -k < file-name >

What is the difference between a Debug and Release build? Is there a significant speed difference? Why or why not?

The Debug build is the program compiled with full symbolic debug information and no optimization. The Release build is the program compiled employing  optimization and contains no symbolic debug information. These settings can be changed as per need from Project Configuration properties. The release runs faster since it does not have any debug symbols and is optimized.

Explain the use of virtual, sealed, override, and abstract.

Abstract: The keyword can be applied for a class or method.
1. Class: If we use abstract keyword for a class it makes the
class an abstract class, which means it cant be instantiated. Though
it is not nessacary to make all the method within the  abstract class to be virtual. ie, Abstract class can have concrete methods
2. Method: If we make a method as abstract, we dont need to provide implementation
of the method in the class but the derived class need to implement/override this method.

Sealed: It can be applied on a class and methods. It stops the type from further derivation i.e no one can derive class
from a sealed class,ie A sealed class cannot be inherited.A sealed class cannot be a abstract class.A compile time error is thrown if you try to specify sealed class as a  base class.
When an instance method declaration includes a sealed modifier, that method is said to be a sealed method. If an instance method declaration includes the sealed modifier, it must also include the override modifier. Use of the sealed modifier prevents a derived class from further overriding the method  For Egs: sealed override public void Sample() { Console.WriteLine("Sealed Method"); }Virtual & Override: Virtual & Override keywords provides runtime polymorphism. A base class can make some of its methods
as virtual which allows the derived class a chance to override the base class implementation by using override keyword.

For e.g. class Shape

{
  int a
  public virtual void Display()
  {
   Console.WriteLine("Shape");
  }
 }
 
 class Rectangle:Shape
 {
  public override void Display()
  {
   Console.WriteLine("Derived");
  }
 }

Explain the importance and use of each, Version, Culture and PublicKeyToken for an assembly.

This three alongwith name of the assembly provide a strong name or fully qualified name to the assembly. When a assebly is referenced with all three.

PublicKeyToken: Each assembly can have a public key embedded in its manifest that identifies the developer. This ensures that once the assembly ships, no one can modify the code or other resources contained in the assembly.

Culture: Specifies which culture the assembly supports

Version: The version number of the assembly.It is of the following form major.minor.build.revision.

Explain the differences between public, protected, private and internal. These all are access modifier and they governs the access level. They can be applied to class, methods, fields.

Public: Allows class, methods, fields to be accessible from anywhere i.e. within and outside an assembly.
Private: When applied to field and method allows to be accessible within a class.
Protected: Similar to private but can be accessed by members of derived class also.
Internal: They are public within the assembly i.e. they can be accessed by anyone within an assembly but outside assembly they are not visible.

What is the difference between typeof(foo) and myFoo.GetType()?

Typeof is operator which applied to a object returns System.Type object. Typeof cannot be overloaded white GetType has lot of overloads.GetType is a method which also returns System.Type of an object. GetType is used to get the runtime type of the object.

Example from MSDN showing Gettype used to retrive type at untime:-

public class MyBaseClass: Object {
}

public class MyDerivedClass: MyBaseClass {
}

public class Test {

   public static void Main() {
      MyBaseClass myBase = new MyBaseClass();
      MyDerivedClass myDerived = new MyDerivedClass();
      object o = myDerived;
      MyBaseClass b = myDerived;

      Console.WriteLine("mybase: Type is {0}", myBase.GetType());
      Console.WriteLine("myDerived: Type is {0}", myDerived.GetType());
      Console.WriteLine("object o = myDerived: Type is {0}", o.GetType());
      Console.WriteLine("MyBaseClass b = myDerived: Type is {0}", b.GetType());
   }
}


/*

This code produces the following output.

mybase: Type is MyBaseClass
myDerived: Type is MyDerivedClass
object o = myDerived: Type is MyDerivedClass
MyBaseClass b = myDerived: Type is MyDerivedClass

*/

Can "this" be used within a static method?

No 'This' cannot be used in a static method. As only static variables/methods can be used in a static method.

What is the purpose of XML Namespaces?

An XML Namespace is a collection of element types and attribute names. It consists of 2 parts
1) The first part is the URI used to identify the namespace
2) The second part is the element type or attribute name itself.
Together they form a unique name. The various purpose of XML Namespace are

1. Combine fragments from different documents without any naming conflicts. (See example below.)
2. Write reusable code modules that can be invoked for specific elements and attributes. Universally unique names guarantee that
such modules are invoked only for the correct elements and attributes.
3. Define elements and attributes that can be reused in other schemas or instance documents without fear of name collisions. For
example, you might use XHTML elements in a parts catalog to provide part descriptions. Or you might use the nil attribute
defined in XML Schemas to indicate a missing value.

< Department >
     < Name >DVS1< /Name >
     < addr:Address xmlns:addr="http://www.tu-darmstadt.de/ito/addresses" >
        < addr:Street >Wilhelminenstr. 7< /addr:Street >
        < addr:City >Darmstadt< /addr:City >
        < addr:State >Hessen< /addr:State >
        < addr:Country >Germany< /addr:Country >
        < addr:PostalCode >D-64285< /addr:PostalCode >
     < /addr:Address >
     < serv:Server xmlns:serv="http://www.tu-darmstadt.de/ito/servers" >
        < serv:Name >OurWebServer< /serv:Name >
        < serv:Address >123.45.67.8< /serv:Address >
     < /serv:Server >
  < /Department >

What is difference between MetaData and Manifest ?

Metadata and Manifest forms an integral part of an assembly( dll / exe ) in .net framework .
Out of which Metadata is a mandatory component , which as the name suggests gives the details about various components of IL code viz : Methods , properties , fields , class etc.

Essentially Metadata maintains details in form of tables like Methods Metadata tables , Properties Metadata tables , which maintains the list of given type and other details like access specifier , return type etc.

Now Manifest is a part of metadata only , fully called as "manifest metadata tables" , it contains the details of the references needed by the assembly of any other external assembly / type , it could be a custom assembly or standard System namespace .

Now for an assembly that can independently exists and used in the .Net world both the things ( Metadata with Manifest ) are mandatory , so that it can be fully described assembly and can be ported anywhere without any system dependency . Essentially .Net framework can read all assembly related information from assembly itself at runtime .

But for .Net modules , that can’t be used independently , until they are being packaged as a part of an assembly , they don’t contain Manifest but their complete structure is defined by their respective metadata .

Ultimately . .Net modules use Manifest Metadata tables of parent assembly which contain them .

What is the use of Internal keyword?

Internal keyword is one of the access specifier available in .Net framework , that makes a type visible in a  given assembly , for e.g : a single dll can contain multiple modules , essentially a multi file assembly , but it forms a single binary component , so any type with internal keyword will be visible throughout the assembly and can be used in any of the modules .

What actually happes when you add a something to arraylistcollection ?

Following things will happen :

Arraylist is a dynamic array class in c# in System.Collections namespace derived from interfaces – ICollection , IList , ICloneable , IConvertible  . It terms of in memory structure following is the implementation .

a. Check up the total space if there’s any free space on the declared list .
b. If yes add the new item and increase count by 1 .
c. If No Copy the whole thing to a temporary Array of Last Max. Size .
d. Create new Array with size ( Last Array Size + Increase Value )
e. Copy back values from temp and reference this new array as original array .
f. Must doing Method updates too , need to check it up .

What is Boxing and unboxing? Does it occure automaatically or u need to write code to box and unbox?

Boxing – Process of converting a System.ValueType to Reference Type , Mostly base class System.Object type and allocating it memory on Heap .Reverse is unboxing , but can only be done with prior boxed variables.

Boxing is always implicit but Unboxing needs to be explicitly done via casting , thus ensuring the value type contained inside .

How Boxing and unboxing occures in memory?

Boxing converts value type to reference type , thus allocating memory on Heap . Unboxing converts already boxed reference types to value types through explicit casting , thus  allocating memory on stack .

Why only boxed types can be unboxed?

Unboxing is the process of converting a Reference type variable to Value type and thus allocating memory on the stack . It happens only to those Reference type variables that have been earlier created by Boxing of a Value Type , therefore internally they contain a value type , which can be obtained through explicit casting . For any other Reference type , they don’t internally contain a Value type to Unboxed via explicit casting . This is why only boxed types can be unboxed .

Com +

What are different transaction options available for services components ?

There are 5 transactions types that can be used with COM+. Whenever an object is registered with COM+ it has to abide either to these 5 transaction types.

Disabled: - There is no transaction. COM+ does not provide transaction support for this component.

Not Supported: - Component does not support transactions. Hence even if the calling component in the hierarchy is transaction enabled this component will not participate in the transaction.

Supported: - Components with transaction type supported will be a part of the transaction if the calling component has an active transaction.
If the calling component is not transaction enabled this component will not start a new transaction.

Required: - Components with this attribute require a transaction i.e. either the calling should have a transaction in place else this component will start a new transaction.

Required New: - Components enabled with this transaction type always require a new transaction. Components with required new transaction type instantiate a new transaction for themselves every time.

Can we use com Components in .net?.How ?.can we use .net components in vb?.Explain how ?
COM components have different internal architecture from .NET components hence they are not innately compatible. However .NET  framework supports invocation of unmanaged code from managed code (and vice-versa) through COM/.NET interoperability. .NET application communicates with a COM component through a managed wrapper of the component called Runtime Callable Wrapper (RCW); it acts as managed proxy to the unmanaged COM component. When a method call is made to COM object, it goes onto RCW and not the object itself. RCW manages the lifetime management of the COM component. Implementation Steps -

Create Runtime Callable Wrapper out of COM component. Reference the metadata assembly Dll in the project and use its methods & properties RCW can be created using Type Library Importer utility or through VS.NET. Using VS.NET, add reference through COM tab to select the desired DLL. VS.NET automatically generates metadata assembly putting the classes provided by that component into a namespace with the same name as COM dll (XYZRCW.dll)

.NET components can be invoked by unmanaged code through COM Callable Wrapper (CCW) in COM/.NET interop. The unmanaged code will talk to this proxy, which translates call to managed environment. We can use COM components in .NET through COM/.NET interoperability. When managed code calls an unmanaged component, behind the scene, .NET creates proxy called COM Callable wrapper (CCW), which accepts commands from a COM client, and forwards it to .NET component. There are two prerequisites to creating .NET component, to be used in unmanaged code:
1. .NET class should be implement its functionality through interface. First define interface in code, then have the class to imlpement it. This way, it prevents breaking of COM client, if/when .NET component changes.

2.Secondly, .NET class, which is to be visible to COM clients must be declared public. The tools that create the CCW only define types based
on public classes. The same rule applies to methods, properties, and events that will be used by COM clients.

Implementation Steps -
1. Generate type library of .NET component, using TLBExporter utility. A type library is the COM equivalent of the metadata contained within
a .NET assembly. Type libraries are generally contained in files with the extension .tlb. A type library contains the necessary information to allow a COM client to determine which classes are located in a particular server, as well as the methods, properties, and events supported by those classes.
2. Secondly,  use Assembly Registration tool (regasm) to create the type library and register it.
3. Lastly install .NET assembly in GAC, so it is available as shared assembly.

What is Runtime Callable wrapper?.when it will created?.
The common language runtime exposes COM objects through a proxy called the runtime callable wrapper (RCW). Although the RCW appears to be an ordinary object to .NET clients, its primary function is to marshal calls between a .NET client and a COM object. This wrapper turns the COM interfaces exposed by the COM component into .NET-compatible interfaces. For oleautomation (attribute indicates that an interface is compatible with Automation) interfaces, the RCW can be generated automatically from a type library. For non-oleautomation interfaces, it may be necessary to develop a custom RCW which manually maps the types exposed by the COM interface to .NET-compatible types.

What is Com Callable wrapper?when it will created?

.NET components are accessed from COM via a COM Callable Wrapper (CCW). This is similar to a RCW, but works in the opposite direction. Again, if the wrapper cannot be automatically generated by the .NET development tools, or if the automatic behaviour is not desirable, a custom CCW can be developed. Also, for COM to 'see' the .NET component, the .NET component must be registered in the registry.CCWs also manage the object identity and object lifetime of the managed objects they wrap.

What is a primary interop ?
A primary interop assembly is a collection of types that are deployed, versioned, and configured as a single unit. However, unlike other managed assemblies, an interop assembly contains type definitions (not implementation) of types that have already been defined in COM. These type definitions allow managed applications to bind to the COM types at compile time and provide information to the common language runtime
about how the types should be marshaled at run time.

What are tlbimp and tlbexp tools used for ?

The Type Library Exporter generates a type library that describes the types defined in a common language runtime assembly.
The Type Library Importer converts the type definitions found within a COM type library into equivalent definitions in a common language runtime assembly. The output of Tlbimp.exe is a binary file (an assembly) that contains runtime metadata for the types defined within the original type library.

What benefit do you get from using a Primary Interop Assembly (PIA)? PIAs are important because they provide unique type identity. The PIA distinguishes the official type definitions from counterfeit definitions provided by other interop assemblies. Having a single type identity ensures type compatibility between applications that share the types defined in the PIA. Because the PIA is signed by its  publisher and labeled with the PrimaryInteropAssembly attribute, it can be differentiated from other interop assemblies that define the same types.

ADO.NET

Explain what a diffgram is and its usage ?
A DiffGram is an XML format that is used to identify current and original versions of data elements. The DataSet uses the DiffGram format to load and persist its contents, and to serialize its contents for transport across a network connection. When a DataSet is written as a DiffGram, it populates the DiffGram with all the necessary information to accurately recreate the contents, though not the schema, of the DataSet, including column values from both the Original and Current row versions, row error information, and row order.

When sending and retrieving a DataSet from an XML Web service, the DiffGram format is implicitly used. Additionally, when loading the contents of a DataSet from XML using the ReadXml method, or when writing the contents of a DataSet in XML using the WriteXml method, you can select that the contents be read or written as a DiffGram.

The DiffGram format is divided into three sections: the current data, the original (or "before") data, and an errors section, as shown in the following example.

xml version="1.0"?>
<diffgr:diffgram
         xmlns:msdata="urn:schemas-microsoft-com:xml-msdata"
         xmlns:diffgr="urn:schemas-microsoft-com:xml-diffgram-v1"
         xmlns:xsd="http://www.w3.org/2001/XMLSchema">

   <DataInstance>
   </DataInstance>

  <diffgr:before>
  </diffgr:before>

  <diffgr:errors>
  </diffgr:errors>
</diffgr:diffgram>

The DiffGram format consists of the following blocks of data:

The name of this element, DataInstance, is used for explanation purposes in this documentation. A DataInstance element represents a DataSet or a row of a DataTable. Instead of DataInstance, the element would contain the name of the DataSet or DataTable. This block of the DiffGram format contains the current data, whether it has been modified or not. An element, or row, that has been modified is identified with the diffgr:hasChanges annotation.

This block of the DiffGram format contains the original version of a row. Elements in this block are matched to elements in the DataInstance block using the diffgr:id annotation.

This block of the DiffGram format contains error information for a particular row in the DataInstance block. Elements in this block are matched to elements in the DataInstance block using the diffgr:id annotation.

Which method do you invoke on the DataAdapter control to load your generated dataset with data?
You have to use the Fill method of the DataAdapter control and pass the dataset object as an argument to load the generated data.

Can you edit data in the Repeater control?  NO.

Which are the different IsolationLevels ?
Following are the various IsolationLevels:

• Serialized   Data read by a current transaction cannot be changed by another transaction until the current transaction finishes. No new data can be inserted that would affect the current transaction. This is the safest isolation level and is the default. 
• Repeatable Read   Data read by a current transaction cannot be changed by another transaction until the current transaction finishes. Any type of new data can be inserted during a transaction. 
• Read Committed   A transaction cannot read data that is being modified by another transaction that has not committed. This is the default isolation level in Microsoft® SQL Server. 
• Read Uncommitted   A transaction can read any data, even if it is being modified by another transaction. This is the least safe isolation level but allows the highest concurrency. 
• Any   Any isolation level is supported. This setting is most commonly used by downstream components to avoid conflicts. This setting is useful because any downstream component must be configured with an isolation level that is equal to or less than the isolation level of its immediate upstream component. Therefore, a downstream component that has its isolation level configured as Any always uses the same isolation level that its immediate upstream component uses. If the root object in a transaction has its isolation level configured to Any, its isolation level becomes Serialized.

How xml files and be read and write using dataset?. DataSet exposes method like ReadXml and WriteXml to read and write xml

What are the different rowversions available?
There are four types of Rowversions.
Current:
The current values for the row. This row version does not exist for rows with a RowState of Deleted.

Default :
The row the default version for the current DataRowState. For a DataRowState value of Added, Modified or Current, the default version is Current. For a DataRowState of Deleted, the version is Original. For a DataRowState value of Detached, the version is Proposed.

Original:
The row contains its original values.

Proposed:
The proposed values for the row. This row version exists during an edit operation on a row, or for a row that is not part of a DataRowCollection

Explain acid properties?. The term ACID conveys the role transactions play in mission-critical applications. Coined by transaction processing pioneers, ACID stands for atomicity, consistency, isolation, and durability.

These properties ensure predictable behavior, reinforcing the role of transactions as all-or-none propositions designed to reduce the management load when there are many variables.

Atomicity
A transaction is a unit of work in which a series of operations occur between the BEGIN TRANSACTION and END TRANSACTION statements of an application. A transaction executes exactly once and is atomic — all the work is done or none of it is.

Operations associated with a transaction usually share a common intent and are interdependent. By performing only a subset of these operations, the system could compromise the overall intent of the transaction. Atomicity eliminates the chance of processing a subset of operations.

Consistency
A transaction is a unit of integrity because it preserves the consistency of data, transforming one consistent state of data into another consistent state of data.

Consistency requires that data bound by a transaction be semantically preserved. Some of the responsibility for maintaining consistency falls to the application developer who must make sure that all known integrity constraints are enforced by the application. For example, in developing an application that transfers money, you should avoid arbitrarily moving decimal points during the transfer.

Isolation
A transaction is a unit of isolation — allowing concurrent transactions to behave as though each were the only transaction running in the system.

Isolation requires that each transaction appear to be the only transaction manipulating the data store, even though other transactions may be running at the same time. A transaction should never see the intermediate stages of another transaction.

Transactions attain the highest level of isolation when they are serializable. At this level, the results obtained from a set of concurrent transactions are identical to the results obtained by running each transaction serially. Because a high degree of isolation can limit the number of concurrent transactions, some applications reduce the isolation level in exchange for better throughput.

Durability
A transaction is also a unit of recovery. If a transaction succeeds, the system guarantees that its updates will persist, even if the computer crashes immediately after the commit. Specialized logging allows the system's restart procedure to complete unfinished operations, making the transaction durable.

Whate are different types of Commands available with DataAdapter ? The SqlDataAdapter has SelectCommand, InsertCommand, DeleteCommand and UpdateCommand

What is a Dataset?
Datasets are the result of bringing together ADO and XML. A dataset contains one or more data of tabular XML, known as DataTables, these data can be treated separately, or can have relationships defined between them. Indeed these relationships give you ADO data SHAPING without needing to master the SHAPE language, which many people are not comfortable with.

The dataset is a disconnected in-memory cache database. The dataset object model looks like this:

Dataset
  DataTableCollection
  DataTable
   DataView
   DataRowCollection
    DataRow
   DataColumnCollection
    DataColumn
   ChildRelations
   ParentRelations
   Constraints
   PrimaryKey
DataRelationCollection

Let’s take a look at each of these:

DataTableCollection: As we say that a DataSet is an in-memory database. So it has this collection, which holds data from multiple tables in a single DataSet object.

DataTable: In the DataTableCollection, we have DataTable objects, which represents the individual tables of the dataset.

DataView: The way we have views in database, same way we can have DataViews. We can use these DataViews to do Sort, filter data.

DataRowCollection: Similar to DataTableCollection, to represent each row in each Table we have DataRowCollection.

DataRow:  To represent each and every row of the DataRowCollection, we have DataRows.

DataColumnCollection: Similar to DataTableCollection, to represent each column in each Table we have DataColumnCollection.

DataColumn: To represent each and every Column of the DataColumnCollection, we have DataColumn.

PrimaryKey: Dataset defines Primary key for the table and the primary key validation will take place without going to the database.

Constraints: We can define various constraints on the Tables, and can use Dataset.Tables(0).enforceConstraints. This will execute all the constraints, whenever we enter data in DataTable.

DataRelationCollection: as we know that we can have more than 1 table in the dataset, we can also define relationship between these tables using this collection and maintain a parent-child relationship.

Explain the ADO . Net Architecture ( .Net Data Provider)

ADO.Net is the data access model for .Net –based applications. It can be used to access relational database systems such as SQL SERVER 2000, Oracle, and many other data sources for which there is an OLD DB or ODBC provider. To a certain extent, ADO.NET represents the latest evolution of ADO technology. However, ADO.NET introduces some major changes and innovations that are aimed at the loosely coupled and inherently disconnected – nature of web applications.

A .Net Framework data provider is used to connecting to a database, executing commands, and retrieving results. Those results are either processed directly, or placed in an ADO.NET DataSet in order to be exposed to the user in an ad-hoc manner, combined with data from multiple sources, or remoted between tiers. The .NET Framework data provider is designed to be lightweight, creating a minimal layer between the data source and your code, increasing performance without sacrificing functionality.

Following are the 4 core objects of .Net Framework Data provider:

• Connection: Establishes a connection to a specific data source
• Command: Executes a command against a data source. Exposes Parameters and can execute within the scope of a Transaction from a Connection.
• DataReader: Reads a forward-only, read-only stream of data from a data source.
• DataAdapter: Populates a DataSet and resolves updates with the data source.

The .NET Framework includes the .NET Framework Data Provider for SQL Server (for Microsoft SQL Server version 7.0 or later), the .NET Framework Data Provider for OLE DB, and the .NET Framework Data Provider for ODBC.

The .NET Framework Data Provider for SQL Server:  The .NET Framework Data Provider for SQL Server uses its own protocol to communicate with SQL Server. It is lightweight and performs well because it is optimized to access a SQL Server directly without adding an OLE DB or Open Database Connectivity (ODBC) layer. The following illustration contrasts the .NET Framework Data Provider for SQL Server with the .NET Framework Data Provider for OLE DB. The .NET Framework Data Provider for OLE DB communicates to an OLE DB data source through both the OLE DB Service component, which provides connection pooling and transaction services, and the OLE DB Provider for the data source

The .NET Framework Data Provider for OLE DB: The .NET Framework Data Provider for OLE DB uses native OLE DB through COM interoperability to enable data access. The .NET Framework Data Provider for OLE DB supports both local and distributed transactions. For distributed transactions, the .NET Framework Data Provider for OLE DB, by default, automatically enlists in a transaction and obtains transaction details from Windows 2000 Component Services.

The .NET Framework Data Provider for ODBC: The .NET Framework Data Provider for ODBC uses native ODBC Driver Manager (DM) through COM interoperability to enable data access. The ODBC data provider supports both local and distributed transactions. For distributed transactions, the ODBC data provider, by default, automatically enlists in a transaction and obtains transaction details from Windows 2000 Component Services.

The .NET Framework Data Provider for Oracle: The .NET Framework Data Provider for Oracle enables data access to Oracle data sources through Oracle client connectivity software. The data provider supports Oracle client software version 8.1.7 and later. The data provider supports both local and distributed transactions (the data provider automatically enlists in existing distributed transactions, but does not currently support the EnlistDistributedTransaction method).

The .NET Framework Data Provider for Oracle requires that Oracle client software (version 8.1.7 or later) be installed on the system before you can use it to connect to an Oracle data source.
.NET Framework Data Provider for Oracle classes are located in the System.Data.OracleClient namespace and are contained in the System.Data.OracleClient.dll assembly. You will need to reference both the System.Data.dll and the System.Data.OracleClient.dll when compiling an application that uses the data provider.
Choosing a .NET Framework Data Provider

.NET Framework Data Provider for SQL Server: Recommended for middle-tier applications using Microsoft SQL Server 7.0 or later. Recommended for single-tier applications using Microsoft Data Engine (MSDE) or Microsoft SQL Server 7.0 or later.
Recommended over use of the OLE DB Provider for SQL Server (SQLOLEDB) with the .NET Framework Data Provider for OLE DB. For Microsoft SQL Server version 6.5 and earlier, you must use the OLE DB Provider for SQL Server with the .NET Framework Data Provider for OLE DB.

.NET Framework Data Provider for OLE DB: Recommended for middle-tier applications using Microsoft SQL Server 6.5 or earlier, or any OLE DB provider. For Microsoft SQL Server 7.0 or later, the .NET Framework Data Provider for SQL Server is recommended. Recommended for single-tier applications using Microsoft Access databases. Use of a Microsoft Access database for a middle-tier application is not recommended.

.NET Framework Data Provider for ODBC: Recommended for middle-tier applications using ODBC data sources. Recommended for single-tier applications using ODBC data sources.

.NET Framework Data Provider for Oracle: Recommended for middle-tier applications using Oracle data sources. Recommended for single-tier applications using Oracle data sources. Supports Oracle client software version 8.1.7 and later. The .NET Framework Data Provider for Oracle classes are located in the System.Data.OracleClient namespace and are contained in the System.Data.OracleClient.dll assembly. You need to reference both the System.Data.dll and the System.Data.OracleClient.dll when compiling an application that uses the data provider.

Can you explain the difference between an ADO.NET Dataset and an ADO Recordset?
Let’s take a look at the differences between ADO Recordset and ADO.Net DataSet:

1. Table Collection: ADO Recordset provides the ability to navigate through a single table of information. That table would have been formed with a join of multiple tables and returning columns from multiple tables. ADO.NET DataSet is capable of holding instances of multiple tables. It has got a Table Collection, which holds multiple tables in it. If the tables are having a relation, then it can be manipulated on a Parent-Child relationship. It has the ability to support multiple tables with keys, constraints and interconnected relationships. With this ability the DataSet can be considered as a small, in-memory relational database cache.

2. Navigation: Navigation in ADO Recordset is based on the cursor mode. Even though it is specified to be a client-side Recordset, still the navigation pointer will move from one location to another on cursor model only. ADO.NET DataSet is an entirely offline, in-memory, and cache of data. All of its data is available all the time. At any time, we can retrieve any row or column, constraints or relation simply by accessing it either ordinarily or by retrieving it from a name-based collection.

3. Connectivity Model: The ADO Recordset was originally designed without the ability to operate in a disconnected environment. ADO.NET DataSet is specifically designed to be a disconnected in-memory database. ADO.NET DataSet follows a pure disconnected connectivity model and this gives it much more scalability and versatility in the amount of things it can do and how easily it can do that.

4. Marshalling and Serialization: In COM, through Marshalling, we can pass data from 1 COM component to another component at any time. Marshalling involves copying and processing data so that a complex type can appear to the receiving component the same as it appeared to the sending component. Marshalling is an expensive operation. ADO.NET Dataset and DataTable components support Remoting in the form of XML serialization. Rather than doing expensive Marshalling, it uses XML and sent data across boundaries.

5. Firewalls and DCOM and Remoting: Those who have worked with DCOM know that how difficult it is to marshal a DCOM component across a router. People generally came up with workarounds to solve this issue. ADO.NET DataSet uses Remoting, through which a DataSet / DataTable component can be serialized into XML, sent across the wire to a new AppDomain, and then Desterilized back to a fully functional DataSet. As the DataSet is completely disconnected, and it has no dependency, we lose absolutely nothing by serializing and transferring it through Remoting.

How do you handle data concurrency in .NET ?

One of the key features of the ADO.NET DataSet is that it can be a self-contained and disconnected data store. It can contain the schema and data from several rowsets in DataTable objects as well as information about how to relate the DataTable objects-all in memory. The DataSet neither knows nor cares where the data came from, nor does it need a link to an underlying data source. Because it is data source agnostic you can pass the DataSet around networks or even serialize it to XML and pass it across the Internet without losing any of its features. However, in a disconnected model, concurrency obviously becomes a much bigger problem than it is in a connected model.

In this column, I'll explore how ADO.NET is equipped to detect and handle concurrency violations. I'll begin by discussing scenarios in which concurrency violations can occur using the ADO.NET disconnected model. Then I will walk through an ASP.NET application that handles concurrency violations by giving the user the choice to overwrite the changes or to refresh the out-of-sync data and begin editing again. Because part of managing an optimistic concurrency model can involve keeping a timestamp (rowversion) or another type of flag that indicates when a row was last updated, I will show how to implement this type of flag and how to maintain its value after each database update.

Is Your Glass Half Full?

There are three common techniques for managing what happens when users try to modify the same data at the same time: pessimistic, optimistic, and last-in wins. They each handle concurrency issues differently.

The pessimistic approach says: "Nobody can cause a concurrency violation with my data if I do not let them get at the data while I have it." This tactic prevents concurrency in the first place but it limits scalability because it prevents all concurrent access. Pessimistic concurrency generally locks a row from the time it is retrieved until the time updates are flushed to the database. Since this requires a connection to remain open during the entire process, pessimistic concurrency cannot successfully be implemented in a disconnected model like the ADO.NET DataSet, which opens a connection only long enough to populate the DataSet then releases and closes, so a database lock cannot be held.

Another technique for dealing with concurrency is the last-in wins approach. This model is pretty straightforward and easy to implement-whatever data modification was made last is what gets written to the database. To implement this technique you only need to put the primary key fields of the row in the UPDATE statement's WHERE clause. No matter what is changed, the UPDATE statement will overwrite the changes with its own changes since all it is looking for is the row that matches the primary key values. Unlike the pessimistic model, the last-in wins approach allows users to read the data while it is being edited on screen. However, problems can occur when users try to modify the same data at the same time because users can overwrite each other's changes without being notified of the collision. The last-in wins approach does not detect or notify the user of violations because it does not care. However the optimistic technique does detect violations.

In optimistic concurrency models, a row is only locked during the update to the database. Therefore the data can be retrieved and updated by other users at any time other than during the actual row update operation. Optimistic concurrency allows the data to be read simultaneously by multiple users and blocks other users less often than its pessimistic counterpart, making it a good choice for ADO.NET. In optimistic models, it is important to implement some type of concurrency violation detection that will catch any additional attempt to modify records that have already been modified but not committed. You can write your code to handle the violation by always rejecting and canceling the change request or by overwriting the request based on some business rules. Another way to handle the concurrency violation is to let the user decide what to do. The sample application that is shown in Figure 1 illustrates some of the options that can be presented to the user in the event of a concurrency violation.

Where Did My Changes Go?

When users are likely to overwrite each other's changes, control mechanisms should be put in place. Otherwise, changes could be lost. If the technique you're using is the last-in wins approach, then these types of overwrites are entirely possible.For example, imagine Julie wants to edit an employee's last name to correct the spelling. She navigates to a screen which loads the employee's information into a DataSet and has it presented to her in a Web page. Meanwhile, Scott is notified that the same employee's phone extension has changed. While Julie is correcting the employee's last name, Scott begins to correct his extension. Julie saves her changes first and then Scott saves his.Assuming that the application uses the last-in wins approach and updates the row using a SQL WHERE clause containing only the primary key's value, and assuming a change to one column requires the entire row to be updated, neither Julie nor Scott may immediatelyrealize the concurrency issue that just occurred. In this particular situation, Julie's changes were overwritten by Scott's changes because he saved last, and the last name reverted to the misspelled version.

So as you can see, even though the users changed different fields, their changes collided and caused Julie's changes to be lost. Without some sort of concurrency detection and handling, these types of overwrites can occur and even go unnoticed.When you run the sample application included in this column's download, you should open two separate instances of Microsoft® Internet Explorer. When I generated the conflict, I opened two instances to simulate two users with two separate sessions so that a concurrency violation would occur in the sample application. When you do this, be careful not to use Ctrl+N because if you open one instance and then use the Ctrl+N technique to open another instance, both windows will share the same session.

Detecting Violations

The concurrency violation reported to the user in Figure 1 demonstrates what can happen when multiple users edit the same data at the same time. In Figure 1, the user attempted to modify the first name to "Joe" but since someone else had already modified the last name to "Fuller III," a concurrency violation was detected and reported. ADO.NET detects a concurrency violation when a DataSet containing changed values is passed to a SqlDataAdapter's Update method and no rows are actually modified. Simply using the primary key (in this case the EmployeeID) in the UPDATE statement's WHERE clause will not cause a violation to be detected because it still updates the row (in fact, this technique has the same outcome as the last-in wins technique). Instead, more conditions must be specified in the WHERE clause in order for ADO.NET to detect the violation.

The key here is to make the WHERE clause explicit enough so that it not only checks the primary key but that it also checks for another appropriate condition. One way to accomplish this is to pass in all modifiable fields to the WHERE clause in addition to the primary key. For example, the application shown in Figure 1 could have its UPDATE statement look like the stored procedure that's shown in Figure 2.

Notice that in the code in Figure 2 nullable columns are also checked to see if the value passed in is NULL. This technique is not only messy but it can be difficult to maintain by hand and it requires you to test for a significant number of WHERE conditions just to update a row. This yields the desired result of only updating rows where none of the values have changed since the last time the user got the data, but there are other techniques that do not require such a huge WHERE clause.

Another way to make sure that the row is only updated if it has not been modified by another user since you got the data is to add a timestamp column to the table. The SQL Server(tm) TIMESTAMP datatype automatically updates itself with a new value every time a value in its row is modified. This makes it a very simple and convenient tool to help detect concurrency violations.

A third technique is to use a DATETIME column in which to track changes to its row. In my sample application I added a column called LastUpdateDateTime to the Employees table.

ALTER
TABLE Employees ADD LastUpdateDateTime DATETIME

There I update the value of the LastUpdateDateTime field automatically in the UPDATE stored procedure using the built-in SQL Server GETDATE function.

The binary TIMESTAMP column is simple to create and use since it automatically regenerates its value each time its row is modified, but since the DATETIME column technique is easier to display on screen and demonstrate when the change was made, I chose it for my sample application. Both of these are solid choices, but I prefer the TIMESTAMP technique since it does not involve any additional code to update its value.

Retrieving Row Flags

One of the keys to implementing concurrency controls is to update the timestamp or datetime field's value back into the DataSet. If the same user wants to make more modifications, this updated value is reflected in the DataSet so it can be used again. There are a few different ways to do this. The fastest is using output parameters within the stored procedure. (This should only return if @@ROWCOUNT equals 1.) The next fastest involves selecting the row again after the UPDATE within the stored procedure. The slowest involves selecting the row from another SQL statement or stored procedure from the SqlDataAdapter's RowUpdated event.

I prefer to use the output parameter technique since it is the fastest and incurs the least overhead. Using the RowUpdated event works well, but it requires me to make a second call from the application to the database. The following code snippet adds an output parameter to the SqlCommand object that is used to update the Employee information:

oUpdCmd.Parameters.Add(new SqlParameter("@NewLastUpdateDateTime",

  SqlDbType.DateTime, 8, ParameterDirection.Output,

  false, 0, 0, "LastUpdateDateTime", DataRowVersion.Current, null));

oUpdCmd.UpdatedRowSource = UpdateRowSource.OutputParameters;

The output parameter has its sourcecolumn and sourceversion arguments set to point the output parameter's return value back to the current value of the LastUpdateDateTime column of the DataSet. This way the updated DATETIME value is retrieved and can be returned to the user's .aspx page.

Saving Changes

Now that the Employees table has the tracking field (LastUpdateDateTime) and the stored procedure has been created to use both the primary key and the tracking field in the WHERE clause of the UPDATE statement, let's take a look at the role of ADO.NET. In order to trap the event when the user changes the values in the textboxes, I created an event handler for the TextChanged event for each TextBox control:

private void txtLastName_TextChanged(object sender, System.EventArgs e)

{

    // Get the employee DataRow (there is only 1 row, otherwise I could

    // do a Find)

    dsEmployee.EmployeeRow oEmpRow =

           (dsEmployee.EmployeeRow)oDsEmployee.Employee.Rows[0];

    oEmpRow.LastName = txtLastName.Text;

    // Save changes back to Session

    Session["oDsEmployee"] = oDsEmployee;

}

This event retrieves the row and sets the appropriate field's value from the TextBox. (Another way of getting the changed values is to grab them when the user clicks the Save button.) Each TextChanged event executes after the Page_Load event fires on a postback, so assuming the user changed the first and last names, when the user clicks the Save button, the events could fire in this order: Page_Load, txtFirstName_TextChanged, txtLastName_TextChanged, and btnSave_Click.

The Page_Load event grabs the row from the DataSet in the Session object; the TextChanged events update the DataRow with the new values; and the btnSave_Click event attempts to save the record to the database. The btnSave_Click event calls the SaveEmployee method (shown in Figure 3) and passes it a bLastInWins value of false since we want to attempt a standard save first. If the SaveEmployee method detects that changes were made to the row (using the HasChanges method on the DataSet, or alternatively using the RowState property on the row), it creates an instance of the Employee class and passes the DataSet to its SaveEmployee method. The Employee class could live in a logical or physical middle tier. (I wanted to make this a separate class so it would be easy to pull the code out and separate it from the presentation logic.)

Notice that I did not use the GetChanges method to pull out only the modified rows and pass them to the Employee object's Save method. I skipped this step here since there is only one row. However, if there were multiple rows in the DataSet's DataTable, it would be better to use the GetChanges method to create a DataSet that contains only the modified rows.

If the save succeeds, the Employee.SaveEmployee method returns a DataSet containing the modified row and its newly updated row version flag (in this case, the LastUpdateDateTime field's value). This DataSet is then merged into the original DataSet so that the LastUpdateDateTime field's value can be updated in the original DataSet. This must be done because if the user wants to make more changes she will need the current values from the database merged back into the local DataSet and shown on screen. This includes the LastUpdateDateTime value which is used in the WHERE clause. Without this field's current value, a false concurrency violation would occur.

Reporting Violations

If a concurrency violation occurs, it will bubble up and be caught by the exception handler shown in Figure 3 in the catch block for DBConcurrencyException. This block calls the FillConcurrencyValues method, which displays both the original values in the DataSet that were attempted to be saved to the database and the values currently in the database. This method is used merely to show the user why the violation occurred. Notice that the exDBC variable is passed to the FillConcurrencyValues method. This instance of the special database concurrency exception class (DBConcurrencyException) contains the row where the violation occurred. When a concurrency violation occurs, the screen is updated to look like Figure 1.

The DataSet not only stores the schema and the current data, it also tracks changes that have been made to its data. It knows which rows and columns have been modified and it keeps track of the before and after versions of these values. When accessing a column's value via the DataRow's indexer, in addition to the column index you can also specify a value using the DataRowVersion enumerator. For example, after a user changes the value of the last name of an employee, the following lines of C# code will retrieve the original and current values stored in the LastName column:

string sLastName_Before = oEmpRow["LastName", DataRowVersion.Original];

string sLastName_After = oEmpRow["LastName", DataRowVersion.Current];

The FillConcurrencyValues method uses the row from the DBConcurrencyException and gets a fresh copy of the same row from the database. It then displays the values using the DataRowVersion enumerators to show the original value of the row before the update and the value in the database alongside the current values in the textboxes.

User's Choice

Once the user has been notified of the concurrency issue, you could leave it up to her to decide how to handle it. Another alternative is to code a specific way to deal with concurrency, such as always handling the exception to let the user know (but refreshing the data from the database). In this sample application I let the user decide what to do next. She can either cancel changes, cancel and reload from the database, save changes, or save anyway.

The option to cancel changes simply calls the RejectChanges method of the DataSet and rebinds the DataSet to the controls in the ASP.NET page. The RejectChanges method reverts the changes that the user made back to its original state by setting all of the current field values to the original field values. The option to cancel changes and reload the data from the database also rejects the changes but additionally goes back to the database via the Employee class in order to get a fresh copy of the data before rebinding to the control on the ASP.NET page.

The option to save changes attempts to save the changes but will fail if a concurrency violation is encountered. Finally, I included a "save anyway" option. This option takes the values the user attempted to save and uses the last-in wins technique, overwriting whatever is in the database. It does this by calling a different command object associated with a stored procedure that only uses the primary key field (EmployeeID) in the WHERE clause of the UPDATE statement. This technique should be used with caution as it will overwrite the record.

If you want a more automatic way of dealing with the changes, you could get a fresh copy from the database. Then overwrite just the fields that the current user modified, such as the Extension field. That way, in the example I used the proper LastName would not be overwritten. Use this with caution as well, however, because if the same field was modified by both users, you may want to just back out or ask the user what to do next. What is obvious here is that there are several ways to deal with concurrency violations, each of which must be carefully weighed before you decide on the one you will use in your application.

Wrapping It Up

Setting the SqlDataAdapter's ContinueUpdateOnError property tells the SqlDataAdapter to either throw an exception when a concurrency violation occurs or to skip the row that caused the violation and to continue with the remaining updates. By setting this property to false (its default value), it will throw an exception when it encounters a concurrency violation. This technique is ideal when only saving a single row or when you are attempting to save multiple rows and want them all to commit or all to fail.

I have split the topic of concurrency violation management into two parts. Next time I will focus on what to do when multiple rows could cause concurrency violations. I will also discuss how the DataViewRowState enumerators can be used to show what changes have been made to a DataSet.

How you will set the datarelation between two columns?

ADO.NET provides DataRelation object to set relation between two columns.It helps to enforce the following  constraints,a unique constraint, which guarantees that a column in the table  contains no duplicates and a foreign-key constraint,which can be used to maintain referential  integrity.A unique constraint is implemented either by simply setting the Unique property of a data  column to true, or by adding an instance of the UniqueConstraint class to the DataRelation object's ParentKeyConstraint. As part of the foreign-key constraint, you can specify referential integrity  rules that are applied at three points,when a parent record is updated,when a parent record is deleted and when a change is accepted or rejected.