本系列，或多或少，直接或间接依赖入门系列知识。但，依然追求独立成章。因本文作者水平有限，文中错误难免，敬请读者指出并谅解。本系列将会和入门并存。
案例
某君被邀为一超市设计数据库，用来存储数据。该君根据该超市中实际出现的对象，设计了Customer, Employee，Order, Product等表，用来保存相应的客户，员工，订单，货品等。太好了，该君很有oo的思想吗。
如果，你被要求用类及对象，来描述该关系型数据，你该如何做呢？在linq推出之前，ADO.NET被用来做数据访问层。而后，程序员需要自己去编写事务逻辑层中所出现的类。比如，Customer, Employee，Order, Product等。然后，程序员组装所需的sql语句，通过ADO.NET，将返回的记录，来初始化Customer等类的对象。在这里，你已经自己动手将Customer表和Customer类关联了起来。从Linq To Sql的设计来看，它主要是为了解决data!=objects 的问题而产生的。现在，有了Table和Class之间的映射，数据和对象之间就可以有一个一一对应的关系了。
在Linq To Sql之前，在java领域有Hibernate，在net领域有NHibernate技术，来实现object/relational 持久和查询服务。无论是Hibernate还是NHibernate，其配置复杂，上手时间长，已经不能适应快速开发的需要。而Linq To Sql的推出，恰恰弥补了它们的缺点，彻底降低了程序开发门槛。
Linq
Linq是Language Integrated Query的缩写。Linq To Sql是linq中的一部分，其与ADO.NET Orcas的关系入下。
现在linq分三个部分。Linq To Objects，即以前的linq。其主要是针对CLR-Based Objects的查询。即内存操作。Linq Enabled ADO.NET是针对关系型数据的。这又包含三个部分。Linq To Datasets, Linq To Sql, Linq To Entities. 其中Linq To Sql 是大家所熟悉的部分，即以前的Dlinq. 官方的解释是，Linq To Datasets, support for ADO.NET Datasets; Linq to SQL, support for SQL Server; Linq to Entities, Support fro Entity Data Model. 记得，以前曾有人问及dlinq与ADO.NET Orcas的关系，因为它们隶属于不同的队伍开发，其中有重叠的部分。而现在，其功能归结在一起，其重叠部分已经得到融合。（如果有人是ADO.NET team或熟悉这个的，开辟专栏给我们大家讲一下呀。）最后一部分叫Linq To XML，即以前的Xlinq. 针对xml格式数据的操作。（还有针对ASP.NET的Blinq, 大声问一下，有没有人懂这个呀）
DBML
所谓dbml，即Database Mark Language。数据库描述语言，是一种xml格式的文档，用来描述数据库。上面我们讲了，不是用类来描述数据吗？为什么又有个dbml？是的，dbml只是个中间的产物，其出现的主要原因是，适应c# 和vb.net语言的不同，做中间缓冲。dbml及数据库和code关系如下。
Database ----> DBML ------------> Code.
使用sqlmetal可以产生dbml。键入如下命令：
sqlmetal /server:yourserver /database:northwind /dbml:YourDbml.dbml
最终可以得到dbml文件，如下：
xml version="1.0" encoding="utf-16"?><Database Name="northwind" Class="Northwind" xmlns="http://schemas.microsoft.com/linqtosql/dbml/2007"> <Table Name="dbo.Categories" Member="Categories"> <Type Name="Categories"> <Column Name="CategoryID" Type="System.Int32" DbType="Int NOT NULL IDENTITY" IsPrimaryKey="true" IsDbGenerated="true" CanBeNull="false" /> <Column Name="CategoryName" Type="System.String" DbType="NVarChar(15) NOT NULL" CanBeNull="false" /> <Column Name="Description" Type="System.String" DbType="NText" CanBeNull="true" UpdateCheck="Never" /> <Column Name="Picture" Type="System.Data.Linq.Binary" DbType="Image" CanBeNull="true" UpdateCheck="Never" /> <Association Name="FK_Products_Categories" Member="Products" OtherKey="CategoryID" Type="Products" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.CustomerCustomerDemo" Member="CustomerCustomerDemo"> <Type Name="CustomerCustomerDemo"> <Column Name="CustomerID" Type="System.String" DbType="NChar(5) NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="CustomerTypeID" Type="System.String" DbType="NChar(10) NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Association Name="FK_CustomerCustomerDemo" Member="CustomerDemographics" ThisKey="CustomerTypeID" Type="CustomerDemographics" IsForeignKey="true" /> <Association Name="FK_CustomerCustomerDemo_Customers" Member="Customers" ThisKey="CustomerID" Type="Customers" IsForeignKey="true" /> Type> Table> <Table Name="dbo.CustomerDemographics" Member="CustomerDemographics"> <Type Name="CustomerDemographics"> <Column Name="CustomerTypeID" Type="System.String" DbType="NChar(10) NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="CustomerDesc" Type="System.String" DbType="NText" CanBeNull="true" UpdateCheck="Never" /> <Association Name="FK_CustomerCustomerDemo" Member="CustomerCustomerDemo" OtherKey="CustomerTypeID" Type="CustomerCustomerDemo" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.Customers" Member="Customers"> <Type Name="Customers"> <Column Name="CustomerID" Type="System.String" DbType="NChar(5) NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="CompanyName" Type="System.String" DbType="NVarChar(40) NOT NULL" CanBeNull="false" /> <Column Name="ContactName" Type="System.String" DbType="NVarChar(30)" CanBeNull="true" /> <Column Name="ContactTitle" Type="System.String" DbType="NVarChar(30)" CanBeNull="true" /> <Column Name="Address" Type="System.String" DbType="NVarChar(60)" CanBeNull="true" /> <Column Name="City" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="Region" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="PostalCode" Type="System.String" DbType="NVarChar(10)" CanBeNull="true" /> <Column Name="Country" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="Phone" Type="System.String" DbType="NVarChar(24)" CanBeNull="true" /> <Column Name="Fax" Type="System.String" DbType="NVarChar(24)" CanBeNull="true" /> <Association Name="FK_CustomerCustomerDemo_Customers" Member="CustomerCustomerDemo" OtherKey="CustomerID" Type="CustomerCustomerDemo" DeleteRule="NO ACTION" /> <Association Name="FK_Orders_Customers" Member="Orders" OtherKey="CustomerID" Type="Orders" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.Employees" Member="Employees"> <Type Name="Employees"> <Column Name="EmployeeID" Type="System.Int32" DbType="Int NOT NULL IDENTITY" IsPrimaryKey="true" IsDbGenerated="true" CanBeNull="false" /> <Column Name="LastName" Type="System.String" DbType="NVarChar(20) NOT NULL" CanBeNull="false" /> <Column Name="FirstName" Type="System.String" DbType="NVarChar(10) NOT NULL" CanBeNull="false" /> <Column Name="Title" Type="System.String" DbType="NVarChar(30)" CanBeNull="true" /> <Column Name="TitleOfCourtesy" Type="System.String" DbType="NVarChar(25)" CanBeNull="true" /> <Column Name="BirthDate" Type="System.DateTime" DbType="DateTime" CanBeNull="true" /> <Column Name="HireDate" Type="System.DateTime" DbType="DateTime" CanBeNull="true" /> <Column Name="Address" Type="System.String" DbType="NVarChar(60)" CanBeNull="true" /> <Column Name="City" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="Region" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="PostalCode" Type="System.String" DbType="NVarChar(10)" CanBeNull="true" /> <Column Name="Country" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="HomePhone" Type="System.String" DbType="NVarChar(24)" CanBeNull="true" /> <Column Name="Extension" Type="System.String" DbType="NVarChar(4)" CanBeNull="true" /> <Column Name="Photo" Type="System.Data.Linq.Binary" DbType="Image" CanBeNull="true" UpdateCheck="Never" /> <Column Name="Notes" Type="System.String" DbType="NText" CanBeNull="true" UpdateCheck="Never" /> <Column Name="ReportsTo" Type="System.Int32" DbType="Int" CanBeNull="true" /> <Column Name="PhotoPath" Type="System.String" DbType="NVarChar(255)" CanBeNull="true" /> <Association Name="FK_Employees_Employees" Member="ReportsToEmployees" ThisKey="ReportsTo" Type="Employees" IsForeignKey="true" /> <Association Name="FK_Employees_Employees" Member="Employee" OtherKey="ReportsTo" Type="Employees" DeleteRule="NO ACTION" /> <Association Name="FK_EmployeeTerritories_Employees" Member="EmployeeTerritories" OtherKey="EmployeeID" Type="EmployeeTerritories" DeleteRule="NO ACTION" /> <Association Name="FK_Orders_Employees" Member="Orders" OtherKey="EmployeeID" Type="Orders" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.EmployeeTerritories" Member="EmployeeTerritories"> <Type Name="EmployeeTerritories"> <Column Name="EmployeeID" Type="System.Int32" DbType="Int NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="TerritoryID" Type="System.String" DbType="NVarChar(20) NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Association Name="FK_EmployeeTerritories_Employees" Member="Employees" ThisKey="EmployeeID" Type="Employees" IsForeignKey="true" /> <Association Name="FK_EmployeeTerritories_Territories" Member="Territories" ThisKey="TerritoryID" Type="Territories" IsForeignKey="true" /> Type> Table> <Table Name="dbo.Order Details" Member="OrderDetails"> <Type Name="OrderDetails"> <Column Name="OrderID" Type="System.Int32" DbType="Int NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="ProductID" Type="System.Int32" DbType="Int NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="UnitPrice" Type="System.Decimal" DbType="Money NOT NULL" CanBeNull="false" /> <Column Name="Quantity" Type="System.Int16" DbType="SmallInt NOT NULL" CanBeNull="false" /> <Column Name="Discount" Type="System.Single" DbType="Real NOT NULL" CanBeNull="false" /> <Association Name="FK_Order_Details_Orders" Member="Orders" ThisKey="OrderID" Type="Orders" IsForeignKey="true" /> <Association Name="FK_Order_Details_Products" Member="Products" ThisKey="ProductID" Type="Products" IsForeignKey="true" /> Type> Table> <Table Name="dbo.Orders" Member="Orders"> <Type Name="Orders"> <Column Name="OrderID" Type="System.Int32" DbType="Int NOT NULL IDENTITY" IsPrimaryKey="true" IsDbGenerated="true" CanBeNull="false" /> <Column Name="CustomerID" Type="System.String" DbType="NChar(5)" CanBeNull="true" /> <Column Name="EmployeeID" Type="System.Int32" DbType="Int" CanBeNull="true" /> <Column Name="OrderDate" Type="System.DateTime" DbType="DateTime" CanBeNull="true" /> <Column Name="RequiredDate" Type="System.DateTime" DbType="DateTime" CanBeNull="true" /> <Column Name="ShippedDate" Type="System.DateTime" DbType="DateTime" CanBeNull="true" /> <Column Name="ShipVia" Type="System.Int32" DbType="Int" CanBeNull="true" /> <Column Name="Freight" Type="System.Decimal" DbType="Money" CanBeNull="true" /> <Column Name="ShipName" Type="System.String" DbType="NVarChar(40)" CanBeNull="true" /> <Column Name="ShipAddress" Type="System.String" DbType="NVarChar(60)" CanBeNull="true" /> <Column Name="ShipCity" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="ShipRegion" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="ShipPostalCode" Type="System.String" DbType="NVarChar(10)" CanBeNull="true" /> <Column Name="ShipCountry" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Association Name="FK_Order_Details_Orders" Member="OrderDetails" OtherKey="OrderID" Type="OrderDetails" DeleteRule="NO ACTION" /> <Association Name="FK_Orders_Customers" Member="Customers" ThisKey="CustomerID" Type="Customers" IsForeignKey="true" /> <Association Name="FK_Orders_Employees" Member="Employees" ThisKey="EmployeeID" Type="Employees" IsForeignKey="true" /> <Association Name="FK_Orders_Shippers" Member="Shippers" ThisKey="ShipVia" Type="Shippers" IsForeignKey="true" /> Type> Table> <Table Name="dbo.Products" Member="Products"> <Type Name="Products"> <Column Name="ProductID" Type="System.Int32" DbType="Int NOT NULL IDENTITY" IsPrimaryKey="true" IsDbGenerated="true" CanBeNull="false" /> <Column Name="ProductName" Type="System.String" DbType="NVarChar(40) NOT NULL" CanBeNull="false" /> <Column Name="SupplierID" Type="System.Int32" DbType="Int" CanBeNull="true" /> <Column Name="CategoryID" Type="System.Int32" DbType="Int" CanBeNull="true" /> <Column Name="QuantityPerUnit" Type="System.String" DbType="NVarChar(20)" CanBeNull="true" /> <Column Name="UnitPrice" Type="System.Decimal" DbType="Money" CanBeNull="true" /> <Column Name="UnitsInStock" Type="System.Int16" DbType="SmallInt" CanBeNull="true" /> <Column Name="UnitsOnOrder" Type="System.Int16" DbType="SmallInt" CanBeNull="true" /> <Column Name="ReorderLevel" Type="System.Int16" DbType="SmallInt" CanBeNull="true" /> <Column Name="Discontinued" Type="System.Boolean" DbType="Bit NOT NULL" CanBeNull="false" /> <Association Name="FK_Order_Details_Products" Member="OrderDetails" OtherKey="ProductID" Type="OrderDetails" DeleteRule="NO ACTION" /> <Association Name="FK_Products_Categories" Member="Categories" ThisKey="CategoryID" Type="Categories" IsForeignKey="true" /> <Association Name="FK_Products_Suppliers" Member="Suppliers" ThisKey="SupplierID" Type="Suppliers" IsForeignKey="true" /> Type> Table> <Table Name="dbo.Region" Member="Region"> <Type Name="Region"> <Column Name="RegionID" Type="System.Int32" DbType="Int NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="RegionDescription" Type="System.String" DbType="NChar(50) NOT NULL" CanBeNull="false" /> <Association Name="FK_Territories_Region" Member="Territories" OtherKey="RegionID" Type="Territories" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.Shippers" Member="Shippers"> <Type Name="Shippers"> <Column Name="ShipperID" Type="System.Int32" DbType="Int NOT NULL IDENTITY" IsPrimaryKey="true" IsDbGenerated="true" CanBeNull="false" /> <Column Name="CompanyName" Type="System.String" DbType="NVarChar(40) NOT NULL" CanBeNull="false" /> <Column Name="Phone" Type="System.String" DbType="NVarChar(24)" CanBeNull="true" /> <Association Name="FK_Orders_Shippers" Member="Orders" OtherKey="ShipVia" Type="Orders" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.Suppliers" Member="Suppliers"> <Type Name="Suppliers"> <Column Name="SupplierID" Type="System.Int32" DbType="Int NOT NULL IDENTITY" IsPrimaryKey="true" IsDbGenerated="true" CanBeNull="false" /> <Column Name="CompanyName" Type="System.String" DbType="NVarChar(40) NOT NULL" CanBeNull="false" /> <Column Name="ContactName" Type="System.String" DbType="NVarChar(30)" CanBeNull="true" /> <Column Name="ContactTitle" Type="System.String" DbType="NVarChar(30)" CanBeNull="true" /> <Column Name="Address" Type="System.String" DbType="NVarChar(60)" CanBeNull="true" /> <Column Name="City" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="Region" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="PostalCode" Type="System.String" DbType="NVarChar(10)" CanBeNull="true" /> <Column Name="Country" Type="System.String" DbType="NVarChar(15)" CanBeNull="true" /> <Column Name="Phone" Type="System.String" DbType="NVarChar(24)" CanBeNull="true" /> <Column Name="Fax" Type="System.String" DbType="NVarChar(24)" CanBeNull="true" /> <Column Name="HomePage" Type="System.String" DbType="NText" CanBeNull="true" UpdateCheck="Never" /> <Association Name="FK_Products_Suppliers" Member="Products" OtherKey="SupplierID" Type="Products" DeleteRule="NO ACTION" /> Type> Table> <Table Name="dbo.Territories" Member="Territories"> <Type Name="Territories"> <Column Name="TerritoryID" Type="System.String" DbType="NVarChar(20) NOT NULL" IsPrimaryKey="true" CanBeNull="false" /> <Column Name="TerritoryDescription" Type="System.String" DbType="NChar(50) NOT NULL" CanBeNull="false" /> <Column Name="RegionID" Type="System.Int32" DbType="Int NOT NULL" CanBeNull="false" /> <Association Name="FK_EmployeeTerritories_Territories" Member="EmployeeTerritories" OtherKey="TerritoryID" Type="EmployeeTerritories" DeleteRule="NO ACTION" /> <Association Name="FK_Territories_Region" Member="Region" ThisKey="RegionID" Type="Region" IsForeignKey="true" /> Type> Table>Database>
也可以使用该dbml生成code，命令如下,可以用language选项，控制生成vb.net或c#语言的代码。
sqlmetal YourDbml.dbml /code: nwind.cs
在C#3.0入门系列（七）--之OR工具介绍 一文中，我们介绍了OR Designer工具，它生成的就是dbml，可以使用记事本打开DataClasses1.dbml 文件来看。有些属性，是无法从数据库中抽提出来的，比如继承，等。而我们又想对其映射做继承，就需要我们自己手工去修改dbml。好在OR Designer提供这些功能（以后再介绍）。
关系型数据的映射
数据间的关系，有2种基本关系。1: 1 和1: M。可以参阅C# 3.0入门系列（二）一文，查阅northwind数据库中的关系图。本文用Order 和Order Detail 表，来阐述其关系的映射。Order 为订单，Order Detail 为订单详情。其关系形式为，一条Order记录对应多条Order Detail 记录。
你可以在上面产生的code中，找到相应的order类，其中有这么一段
[Table(Name
=
"
Orders
"
)]
public
partial
class
Order : INotifyPropertyChanging, INotifyPropertyChanged
{
[Association(Name="Order_OrderDetail", Storage="_OrderDetails", OtherKey="OrderID")] public EntitySet<OrderDetail> OrderDetails
{ get { return this._OrderDetails;
} set { this._OrderDetails.Assign(value); } } 
}
在Order类中，有个property，叫OrderDetails，是EntitySet
EntitySet
在OrderDetails类中，也可以找到这么一段。
[Table(Name
=
"
Order Details
"
)]
public
partial
class
OrderDetail : INotifyPropertyChanging, INotifyPropertyChanged
{ private EntityRef<Order> _Order; [Association(Name="Order_OrderDetail", Storage="_Order", ThisKey="OrderID", IsForeignKey=true)] public Order Order { get { return this._Order.Entity; } set { Order previousValue = this._Order.Entity; if (((previousValue != value) || (this._Order.HasLoadedOrAssignedValue == false))) { this.SendPropertyChanging(); if ((previousValue != null)) { this._Order.Entity = null; previousValue.OrderDetails.Remove(this); } this._Order.Entity = value; if ((value != null)) { value.OrderDetails.Add(this); this._OrderID = value.OrderID; } else { this._OrderID = default(int); } this.SendPropertyChanged("Order"); } } } }
也就是说Order 在OrderDetail类中，是以EntityRef出现的。这样，在关系双方的各端，我们使用EntityRef和EntitySet来表示其关系。简言之，One在Many端，以EntityRef出现，而Many在One端，以EntitySet出现。上例中，在property 中，因其返回的是this._Order.Entity，直接返回的是Order。
对于1:1的关系，双双彼此在各自的类中，均以EntityRef出现。大家可以自己试。这样，Order和 OrderDetail
的关系，在各自的类中，都有了体现。体现方式的不同，反映了它们关系主体的不同。
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