C#速成
原文出处：http:
//
www.codeguru.com/cs_syntax/CSharp.html
原作者：Aisha Ikram 在一些术语上我尽量做到与MSDN的中文资料所述术语保持一致 使用环境: .NET, C#, Win XP, Win
2000
绪论 C#是这样的一种语言，具有C
++
的特点，象Java一样的编程风格, 并且象Basic一样的快速开发模型。如果你已经知道了C
++
，本文会在不到一个小时的时间内让你迅速掌握C#的语法。熟悉Java的括会更好，因为Java的程序结构、打包（Packages）和垃圾收集的概念有助于你更快的了解C#。因此在讨论C#的构造时，我会假定你了解C
++
。 本文会讨论C#语言的构造与特点，同时会采取简洁的和你能理解的方式使用些代码示例，我们会尽量让你能稍微看看这些代码就能理解这些概念。 注意：本文不是为C#高手(C# gurus)所写. 这是针对在C#学习上还是初学者的文章。 下面是将要讨论的C#问题的目录: 程序结构 命名空间 数据类型 变量 运算符和表达式 枚举 语句（Statements ） 类（Classes）和结构（Structs） 修饰符（Modifiers） 属性(Properties) 接口(Interfaces) 方法参数(Function Parameters) 数组（Arrays） 索引器（Indexers） 装箱及拆箱操作 委托(Delegates) 继承和多态 下面的内容将不会在被讨论之列： C
++
与C#谁更通用 诸如垃圾回收、线程以及文件处理等概念 数据的类型转换 异常处理 .NET库
-------------------
程序结构
-------------------
这一点象C
++
，C#是一种对大小写字母敏感的语言，分号“;”是语句间的分隔符。与C
++
不同的是，C#当中声明代码文件（头文件）与实现代码文件（cpp文件）不是独立存在的，所有代码（类声明和类实现）都位于一个扩展名为cs的文件内。 让我们瞧瞧C#当中的 Hello world 程序是怎样的。
using
System;
namespace
MyNameSpace 
{ 
class HelloWorld 
{ static void Main(string[] args) { Console.WriteLine ("Hello World"); 
} } 
}
在C#当中的每样东西都被封装到一个类中，C#的类又被封装到一个命名空间当中（就象一个文件夹中的文件）。类似于 C
++
，main方法是你的程序的入口点。C
++
的main函数调用名称是
"
main
"
，而C#的main函数是以大写字母M为起点的名称是
"
Main
"
。 没有必要把分号分隔符放在类语句块或者结构定义语句块后。这在C
++
当中被要求，但在C#当中却不是。
-------------------
命名空间
-------------------
每一个类都被包装进一个命名空间。命名空间的概念与C
++
的完全相同，但在C#当中使用命名空间的频率较C
++
还高。你可以使用点限定符(dot qulifier)访问一个类。在上面的hello world程序当中MyNameSpace就是一个命名空间。 现在思考这样的一个问题，你想从某些别的类的命名空间当中来访问HelloWorld这个类该如何操作。 这有一个例子：
using
System;
namespace
AnotherNameSpace
{ class AnotherClass { public void Func() { Console.WriteLine ("Hello World"); } } }
现在，从你的HelloWorld类里你能象这样去访问上面的这个AnotherNameSpace的命名空间：
using
System;
using
AnotherNameSpace;
//
you will add this using statement
namespace
MyNameSpace
{ class HelloWorld { static void Main(string[] args) { AnotherClass obj = new AnotherClass(); obj.Func(); } } }
在.NET库当中，System是位于顶层的命名空间，别的命名空间都存在这个命名空间之下。默认状态下，存在一个全局的命名空间，因此一个在命名空间外定义的类将直接在这个全局命名空间之下；因此，你能在没有任何点限定符的情况下访问这个类。 你也可以象下面这样定义嵌套的命名空间。 Using C
++
当中的
"
#include
"
指示被C#的
"
using
"
关键字取代，它后面跟着一个命名空间的名字。正如上面的
"
using System
"
。
"
System
"
是别的所有被封装的命名空间和类中最底层的命名空间。所有对象的基类都是System命名空间内的Object类派生的。
-------------------
变量
-------------------
除以下并别外，C#当中的变量几乎与C
++
同： 与C
++
不同，C#变量被访问之前必须被初始化；否则编译时会报错。因此，访问一个未初始化变量是不可能的事。 C#中你不会访问到一个不确定的指针。（译者注：严格说起来C#已经把指针概念异化，限制更严格。所以有些资料上会说C#取消了指针概念） 一个超出数组边界的表达式是不可访问的。 C#中没有全局的的变量或全局函数，全局方式的操作是通过静态函数和静态变量来实现的。
-------------------
数据类型
-------------------
所有C#数据类型都派生自基类Object。这里有两类数据类型： 基本型
/
内置型 用户自定义型 下面一个C#内置类型列表： 类型 字节数 解释
byte
1
无符号字节型
sbyte
1
有符号字节型
short
2
有符号短字节型
ushort
2
无符号短字节型
int
4
有符号整型
uint
4
无符号整型
long
8
有符号长整型
ulong
8
无符号长整型
float
4
浮点数
double
8
双精度数
decimal
8
固定精度数
string
unicode字串型
char
unicode字符型
bool
真假布尔型 注意：C#当中的类型范围与C
++
有所不同；例如，C
++
的long型是4个字节，而在C#当中是8个字节。同样地，bool型和string型都不同于C
++
。bool型只接受true和false两种值。不接受任何整数类型。 用户定义类型包括： 类类型(
class
) 结构类型(
struct
) 接口类型(
interface
) 数据类型的内存分配形式的不同又把它们分成了两种类型： 值类型（value Types） 引用类型（Reference Types） 值类型： 值类型数据在栈中分配。他们包括：所有基本或内置类型（不包括string类型）、结构类型、枚举类型(
enum
type) 引用类型： 引用类型在堆中分配，当它们不再被使用时将被垃圾收集。它们使用new运算符来创建，对这些类型而言，不存在C
++
当中的delete操作符，根本不同于C
++
会显式使用delete这个运算符去释放创建的这个类型。C#中，通过垃圾收集器，这些类型会自动被收集处理。 引用类型包括：类类型、接口类型、象数组这样的集合类型类型、字串类型、枚举类型 枚举类型与C
++
当中的概念非常相似。它们都通过一个enum关键字来定义。 示例：
enum
Weekdays
{ Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday }
类类型与结构类型的比较 除了在内存分配形式上外，类与结构的概念完全与C
++
相同。类的对象被分配在堆中，并且通过new来创建，结构也是被new创建但却被分配在栈当中。C#当中，结构型适于快速访问和拥有少量成员的数据类型。如果涉及量较多，你应该创建一个类来实现他。 （译者注：这与堆和栈内存分配结构的特点有关。简而言之，栈是一种顺序分配的内存；堆是不一定是连续的内存空间。具体内容需要大家参阅相关资料） 示例：
struct
Date
{ int day; int month; int year; }
class
Date
{ int day; int month; int year; string weekday; string monthName; public int GetDay() { return day; } public int GetMonth() { return month; } public int GetYear() { return year; } public void SetDay(int Day) { day = Day ; } public void SetMonth(int Month) { month = Month; } public void SetYear(int Year) { year = Year; } public bool IsLeapYear() { return (year/4 == 0); } public void SetDate (int day, int month, int year) { } }
-------------------
属性
-------------------
如果你熟悉C
++
面象对象的方式，你就一定有一个属性的概念。在上面示例当中，以C
++
的观点来看，Data类的属性就是day、month和year。用C#方式，你可以把它们写成Get和Set方法。C#提供了一个更方便、简单、直接的方式来访问属性。 因此上面的类可以被写成：
using
System;
class
Date
{ public int Day{ get { return day; } set { day = value; } } int day; public int Month{ get { return month; } set { month = value; } } int month; public int Year{ get { return year; } set { year = value; } } int year; public bool IsLeapYear(int year) { return year%4== 0 ? true: false; } public void SetDate (int day, int month, int year) { this.day = day; this.month = month; this.year = year; } }
你可在这里得到并设置这些属性：
class
User
{ public static void Main() { Date date = new Date(); date.Day = 27; date.Month = 6; date.Year = 2003; Console.WriteLine("Date: {0}/{1}/{2}", date.Day, date.Month, date.Year); } }
-------------------
修饰符
-------------------
你必须已经知道public、
private
、protected这些常在C
++
当中使用的修饰符。这里我会讨论一些C#引入的新的修饰符。
readonly
（只读） readonly修饰符仅在类的数据成员中使用。正如这名字所提示的，
readonly
数据成员仅能只读，它们只能在构造函数或是直接初始化操作下赋值一次。readonly与const数据成员不同，
const
要求你在声明中初始化，这是直接进行的。看下面的示例代码：
class
MyClass
{ const int constInt = 100; //直接初始化 readonly int myInt = 5; //直接初始化 readonly int myInt2; //译者注：仅做声明，未做初始化 public MyClass() { myInt2 = 8; //间接的 } public Func() { myInt = 7; //非法操作（译者注：不得赋值两次） Console.WriteLine(myInt2.ToString()); } }
sealed
（密封） 密封类不允许任何类继承，它没有派生类。因此，你可以对你不想被继承的类使用sealed关键字。
sealed
class
CanNotbeTheParent
{ int a = 5; }
unsafe
（不安全） 你可使用unsafe修饰符来定义一个不安全的上下文。在不安全的上下文里，你能写些如C
++
指针这样的不安全的代码。看下面的示例代码：
public
unsafe
MyFunction(
int
*
pInt,
double
*
pDouble)
{ int* pAnotherInt = new int; *pAnotherInt = 10; pInt = pAnotherInt; *pDouble = 8.9; }
-------------------
interface
（接口）
-------------------
如果你有COM方面的概念，你会立亥明白我要谈论的内容。一个接口就是一个抽象的基类，这个基类仅仅包含功能描述，而这些功能的实现则由子类来完成。C#中你要用interface关键字来定义象接口这样的类。.NET就是基于这样的接口上的。C#中你不支持C
++
所允许的类多继承（译者注：即一个派生类可以从两个或两个以上的父类中派生）。但是多继承方式可以通过接口获得。也就是说你的一个子类可以从多个接口中派生实现。
using
System;
interface
myDrawing
{ int originx { get; set; } int originy { get; set; } void Draw(object shape); }
class
Shape: myDrawing
{ int OriX; int OriY; public int originx { get{ return OriX; } set{ OriX = value; } } public int originy { get{ return OriY; } set{ OriY = value; } } public void Draw(object shape) { // do something } // class's own method public void MoveShape(int newX, int newY) { .. } }
-------------------
Arrays（数组）
-------------------
C#中的数组比C
++
的表现更好。数组被分配在堆中，因此是引用类型。你不可能访问超出一个数组边界的元素。因此，C#会防止这样类型的bug。一些辅助方式可以循环依次访问数组元素的功能也被提供了，foreach就是这样的一个语句。与C
++
相比，C#在数组语法上的特点如下： 方括号被置于数据类型之后而不是在变量名之后。 创建数组元素要使用new操作符。 C#支持一维、多维以及交错数组（数组中的数组）。 示例：
int
[] array
=
new
int
[
10
];
//
整型一维数组
for
(
int
i
=
0
; i
<
array.Length; i
++
) array[i]
=
i;
int
[,] array2
=
new
int
[
5
,
10
];
//
整型二维数组
array2[
1
,
2
]
=
5
;
int
[,,] array3
=
new
int
[
5
,
10
,
5
];
//
整型的三维数组
array3[
0
,
2
,
4
]
=
9
;
int
[][] arrayOfarray
=
=
new
int
[
2
];
//
整型交错数组（数组中的数组）
arrayOfarray[
0
]
=
new
int
[
4
]; arrayOfarray[
0
]
=
new
int
[]
{1,2,15}
;
-------------------
索引器
-------------------
索引器被用于写一个访问集合元素的方法，集合使用
"
[]
"
这样的直接方式，类似于数组。你所要做的就是列出访问实例或元素的索引清单。类的属性带的是输入参数，而索引器带的是元素的索引表，除此而外，他们二者的语法相同。 示例： 注意：CollectionBase是一个制作集合的库类。List是一个protected型的CollectionBase成员，储存着集合清单列表。
class
Shapes: CollectionBase
{ public void add(Shape shp) { List.Add(shp); } //indexer public Shape this[int index] { get { return (Shape) List[index]; } set { List[index] = value ; } } }
-------------------
装箱和拆箱操作（Boxing
/
Unboxing）
-------------------
C#的装箱思想是全新的。上面提到过所有的数据类型，不论内置或用户自定义，全都从命名空间System的一个基类object派生出来。因此把基本的或者原始类型转换成object类型被称做装箱，反之，这种方式的逆操作被称为拆箱。 示例：
class
Test
{ static void Main() { int myInt = 12; object obj = myInt ; // 装箱 int myInt2 = (int) obj; // 拆箱 } }
示例展示了装箱和拆箱操作。一个整型值转换成object类型，然后又转换回整型。当一个值类型的变量需要转换成引用类型时，一个object的箱子会被分配容纳这个值的空间，这个值会被复制进这个箱子。拆箱与此相反，一个object箱子中的数据被转换成它的原始值类型时，这个值将被从箱中复制到适当的存储位置。
Environment: .NET, C#, Win XP, Win
2000
Introduction C#
is
a language with the features of C
++
, programming style like Java, and the rapid application model of Basic. If you already know the C
++
language, it will take you less than an hour to quickly go through the syntax of C#. Familiarity with Java will be a plus because the Java program structure, the concept of packages, and garbage collection will definitely help you learn C# more quickly. So
while
discussing C# language constructs, I will assume that you know C
++
. (continued) This article dicusses the C# language constructs and features,
using
code examples
in
a brief and comrehensive way so that you can, just by having a glance at the code, understand the concepts. Note: This article
is
not
for
C# gurus. There must be some other beginner
'
s articles on C#, but this is yet another one.
The following topics of the C# langauge are discussed: Program Structure Namespaces Data Types Variables Operators and Expressions Enumerations Statements Classes and Structs Modifiers Properties Interfaces Function Parameters Arrays Indexers Boxing and Unboxing Delegates Inheritance and Polymorphism The following topics are not discussed: Things that are common
in
C
++
and C# Concepts such
as
garbage collection, threading, file processing, and so forth Data type conversions Exception handling .NET library Program Structure Like C
++
, C#
is
case
-
sensitive. The semicolon, ;,
is
the statement separator. Unlike C
++
, there are no separate declaration (header) and implementation(cpp) files
in
C#. All code (
class
declaration and implementation)
is
placed
in
one file with a cs extention. Have a look at
this
Hello world program
in
C#.
using
System;
namespace
MyNameSpace
{ class HelloWorld { static void Main(string[] args) { Console.WriteLine ("Hello World"); } } }
Everything
in
C#
is
packed into a
class
and classes
in
C# are packed into namespaces (just like files
in
a folder). Like C
++
, a main method
is
the entry point of your program. C
++
'
s main function is called "main", whereas C#
'
s main function starts with a capital M and
is
named
"
Main
"
. There
is
no need to put a semicolon after a
class
block or
struct
definition. It was required
in
C
++
, but not
in
C#. Namespace Every
class
is
packaged into a
namespace
. Namespaces are exactly the same concept
as
in
C
++
, but
in
C# we use namespaces more frequently than
in
C
++
. You can access a
class
in
a
namespace
using
dot . qualifier. MyNameSpace
is
the
namespace
in
the hello world program above. Now, consider that you want to access the HelloWorld
class
from some other
class
in
some other
namespace
.
using
System;
namespace
AnotherNameSpace
{ class AnotherClass { public void Func() { Console.WriteLine ("Hello World"); } } }
Now, from your HelloWorld
class
, you can access it
as
:
using
System;
using
AnotherNameSpace;
//
you will add this using statement
namespace
MyNameSpace
{ class HelloWorld { static void Main(string[] args) { AnotherClass obj = new AnotherClass(); obj.Func(); } } }
In the .NET library, System
is
the top
-
level
namespace
in
which other namespaces exist. By
default
, there exists a global
namespace
, so a
class
defined outside a
namespace
goes directly into
this
global
namespace
; hence, you can access
this
class
without any qualifier. you can also define nested namespaces. Using The #include directive
is
replaced with the
"
using
"
keyword, which
is
followed by a
namespace
name. Just
as
in
"
using System
"
, above.
"
System
"
is
the
base
-
level
namespace
in
which all other namespaces and classes are packed. The
base
class
for
all
object
is
Object
in
the System
namespace
. Variables Variables
in
C# are almost the same
as
in
C
++
, except
for
these differences: Variables
in
C# (unlike C
++
) always need to be initialized before you access them; otherwise, you will
get
a compile time error. Hence, it
'
s impossible to access an uninitialized variable.
You can
'
t access a .dangling. pointer in C#.
An expression that indexes an array beyond its bounds
is
also not accessible. There are no global variables or functions
in
C# and the behavior of globals
is
acheived through
static
functions and
static
variables. Data Types All types of C# are derived from a
base
-
class
object
. There are two types of data types: Basic
/
Built
-
in
types User
-
defined types Following
is
a table that lists built
-
in
C# types: Type Bytes Desc
byte
1
unsigned
byte
sbyte
1
signed
byte
short
2
signed
short
ushort
2
unsigned
short
int
4
signed integer
uint
4
unsigned integer
long
8
signed
long
ulong
8
unsigned
long
float
4
floating point number
double
8
double
precision number
decimal
8
fixed
precision number
string
unicode
string
char
unicode
char
bool
true
,
false
boolean Note: Type range
in
C# and C
++
are different;
for
example,
long
in
C
++
is
4
bytes, and
in
C# it
is
8
bytes. Also, the
bool
type and
string
types are different than those
in
C
++
. Bool accepts only
true
and
false
and not any integer. User
-
defined types include: Classes Structs Interfaces Memory allocation of the data types divides them into two types: Value Types Reference Types Value types Value types are those data types that are allocated
in
the stack. They include: All basic or built
-
in
types except strings Structs Enum types Reference type Reference types are allocated on the heap and are garbage collected when they are no longer being used. They are created
using
the
new
operator
, and there
is
no delete
operator
for
these types, unlike
in
C
++
, where the user has to explicitely delete the types created
using
the delete
operator
. In C#, they are automatically collected by the garbage collector. Reference types include: Classes Interfaces Collection types such
as
arrays String Enumeration Enumerations
in
C# are exactly like C
++
. They are defined through a
enum
keyword. Example:
enum
Weekdays
{ Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday }
Classes and Structs Classes and structs are same
as
in
C
++
, except
in
the difference of their memory allocation. Objects of classes are allocated
in
the heap, and are created
using
new
, where
as
structs are allocated
in
the stack. Structs
in
C# are very light and fast datatypes. For heavy datatypes, you should create classes. Examples:
struct
Date
{ int day; int month; int year; }
class
Date
{ int day; int month; int year; string weekday; string monthName; public int GetDay() { return day; } public int GetMonth() { return month; } public int GetYear() { return year; } public void SetDay(int Day) { day = Day ; } public void SetMonth(int Month) { month = Month; } public void SetYear(int Year) { year = Year; } public bool IsLeapYear() { return (year/4 == 0); } public void SetDate (int day, int month, int year) { } }
Properties If you familiar with the
object
-
oriented way of C
++
, you must have an idea of properties. Properties
in
the above example of the Date
class
are day, month, and year
for
which
in
C
++
, you write Get and Set methods. C# provides a more convinient, simple, and straightforward way of accessing properties. So, the above
class
can be written
as
:
using
System;
class
Date
{ public int Day{ get { return day; } set { day = value; } } int day; public int Month{ get { return month; } set { month = value; } } int month; public int Year{ get { return year; } set { year = value; } } int year; public bool IsLeapYear(int year) { return year%4== 0 ? true: false; } public void SetDate (int day, int month, int year) { this.day = day; this.month = month; this.year = year; } }
Here
is
the way you will
get
and
set
these properties:
class
User
{ public static void Main() { Date date = new Date(); date.Day = 27; date.Month = 6; date.Year = 2003; Console.WriteLine("Date: {0}/{1}/{2}", date.Day, date.Month, date.Year); } }
Modifiers You must be aware of
public
,
private
, and
protected
modifers that are commonly used
in
C
++
. Here, I will discuss some
new
modifiers introduced by C#.
readonly
The
readonly
modifier
is
used only
for
the
class
data members. As the name indicates, the
readonly
data members can only be read once they are written either directly initializing them or assigning values to them
in
constructor. The difference between the
readonly
and
const
data members
is
that
const
requires you to initialize with the declaration, that
is
directly. See
this
example code:
class
MyClass
{ const int constInt = 100; //directly readonly int myInt = 5; //directly readonly int myInt2; public MyClass() { myInt2 = 8; //Indirectly } public Func() { myInt = 7; //Illegal Console.WriteLine(myInt2.ToString()); } }
sealed
the
sealed
modifier with a
class
doesn
'
t let you derive any class from it. So, you use this sealed keyword for the classes that you don
'
t want to be inherited from.
sealed
class
CanNotbeTheParent
{ int a = 5; }
unsafe
You can define an
unsafe
context
in
C# by
using
an
unsafe
modifier. In the
unsafe
context, you can write an
unsafe
code;
for
example, C
++
pointers and so forth. See the following code:
public
unsafe
MyFunction(
int
*
pInt,
double
*
pDouble)
{ int* pAnotherInt = new int; *pAnotherInt = 10; pInt = pAnotherInt; *pDouble = 8.9; }
Interfaces If you have an idea of the COM, you will immediately know what I am talking about. An
interface
is
the
abstract
base
class
containing only the function signatures whose implementation
is
provided by the child
class
. In C#, you define such classes
as
interfaces
using
the
interface
keyword. .NET
is
based on such interfaces. In C#, where you can
'
t use multiple class inheritance, which was previously allowed in C++, the essence of multiple inheritance is acheived through interfaces. That
'
s how your child
class
may implement multiple interfaces.
using
System;
interface
myDrawing
{ int originx { get; set; } int originy { get; set; } void Draw(object shape); }
class
Shape: myDrawing
{ int OriX; int OriY; public int originx { get{ return OriX; } set{ OriX = value; } } public int originy { get{ return OriY; } set{ OriY = value; } } public void Draw(object shape) { // do something } // class's own method public void MoveShape(int newX, int newY) { .. } }
Arrays Arrays
in
C# are much better than
in
C
++
. Arrays are allocated
in
the heap and thus are of the reference type. You can
'
t access an out-of-bound element in an array. So, C# prevents you from that type of bug. Also, some helper functions to iterate array elements are provided. foreach is the statement for such an iteration. The difference between the syntax of the C++ and C# array is:
The square brackets are placed after the type and not after the variable name. You create element locations
using
new
operator
. C# supports single dimensional, multi dimensional, and jagged array (array of an array). Examples:
int
[] array
=
new
int
[
10
];
//
single-dimensional
//
array of int
for
(
int
i
=
0
; i
<
array.Length; i
++
) array[i]
=
i;
int
[,] array2
=
new
int
[
5
,
10
];
//
2-dimensional array
//
of int
array2[
1
,
2
]
=
5
;
int
[,,] array3
=
new
int
[
5
,
10
,
5
];
//
3-dimensional array
//
of int
array3[
0
,
2
,
4
]
=
9
;
int
[][] arrayOfarray
=
=
new
int
[
2
];
//
Jagged array -
//
array of array of
//
int
arrayOfarray[
0
]
=
new
int
[
4
]; arrayOfarray[
0
]
=
new
int
[]
{1,2,15}
; Indexers An indexer
is
used to write a method to access an element from a collection
using
the straight way of
using
[], like an array. All you need
is
to specify the index to access an instance or element. The syntax of an indexer
is
same
as
that of
class
properties, except they take the input parameter, that
is
the index of the element. Example: Note: CollectionBase
is
the library
class
used
for
making collections. List
is
the
protected
member of CollectionBase, which stores the collection list.
class
Shapes: CollectionBase
{ public void add(Shape shp) { List.Add(shp); } //indexer public Shape this[int index] { get { return (Shape) List[index]; } set { List[index] = value ; } } }
Boxing
/
Unboxing The idea of boxing
is
new
in
C#. As mentioned above, all data types, built
-
in
or user defined, are derived from a
base
class
object
in
the System
namespace
. So, the packing of basic or primitive types into an
object
is
called boxing, whereas the reverse of
this
known
as
unboxing. Example:
class
Test
{ static void Main() { int myInt = 12; object obj = myInt ; // boxing int myInt2 = (int) obj; // unboxing } }
The example shows both boxing and unboxing. An
int
value can be converted to an
object
and back again to an
int
. When a variable of a value type needs to be converted to a reference type, an
object
box
is
allocated to hold the value, and the value
is
copied into the box. Unboxing
is
just the opposite. When an
object
box
is
cast back to its original value type, the value
is
copied
out
of the box and into the appropriate storage location. Function Parameters Parameters
in
C# are of three types: By
-
Value
/
In parameters By
-
Reference
/
In
-
Out parameters Out pParameters If you have an idea of the COM
interface
and its parameters types, you will easily understand the C# parameter types. By
-
Value
/
In parameters The concept of value parameters
is
the same
as
in
C
++
. The value of the passed value
is
copied into a location and
is
passed to the function. Example: SetDay(
5
);
void
SetDay(
int
day)
{ . }
By
-
Reference
/
In
-
Out Parameters The reference parameters
in
C
++
are passed either through pointers or a reference
operator
,
&
. In C#, reference parameters are less error prone. Reference parameters are also called In
-
Out parameters because you pass a reference address of the location, so you pass an input value and
get
an output value from that function. You cannot pass an uninitialized reference parameter into a function. C# uses a keyword
ref
for
the reference parameters. You also have to use keyword
ref
with an argument
while
passing it to a function
-
demanding reference parameter. Example:
int
a
=
5
; FunctionA(
ref
a);
//
use ref with argument or you will
//
get a compiler error
Console.WriteLine(a);
//
prints 20
void
FunctionA(
ref
int
Val)
{ int x= Val; Val = x* 4; }
Out parameter The Out parameter
is
the parameter that only returns a value from the function. The input value
is
not required. C# uses the keyword
out
for
the
out
parameters Example:
int
Val; GetNodeValue(Val);
bool
GetNodeValue(
out
int
Val)
{ Val = value; return true; }
Variable number of parameters and arrays Arrays
in
C# are passed through a keyword
params
. An array
-
type parameter should always be the right
-
most argument of the function. Only one parameter can be of the array type. You can pass any number of elements
as
an argument of type of that array. You can better understand it from the following example. Example:
void
Func(
params
int
[] array)
{ Console.WriteLine("number of elements {0}", array.Length); }
Func();
//
prints 0
Func(
5
);
//
prints 1
Func(
7
,
9
);
//
prints 2
Func(
new
int
[]
{3,8,10}
);
//
prints 3
int
[] array
=
new
int
[
8
]
{1,3,4,5,5,6,7,5}
; Func(array);
//
prints 8
Operators and Expressions Operators are exactly the same
as
om C
++
and thus the expression, also. However, some
new
and useful operators have been added. Some of them are discussed here.
is
operator
The
is
operator
is
used to check whether the operand types are equal or convertable. The
is
operator
is
particularly useful
in
the polymorphism scenarios. The
is
operator
takes two operands and the result
is
a boolean. See the example:
void
function(
object
param)
{ if(param is ClassA) //do something else if(param is MyStruct) //do something }
}
as
operator
The
as
operator
checks whether the type of the operands are convertable or equal (
as
is
done by
is
operator
) and
if
it
is
, the result
is
a converted or boxed
object
(
if
the operand can be boxed into the target type, see boxing
/
unboxing). If the objects are not convertable or boxable, the
return
is
a
null
. Have a look at the example below to better understand the concept. Shape shp
=
new
Shape(); Vehicle veh
=
shp
as
Vehicle;
//
result is null, types are not
//
convertable
Circle cir
=
new
Circle(); Shape shp
=
cir; Circle cir2
=
shp
as
Circle;
//
will be converted
object
[] objects
=
new
object
[
2
]; objects[
0
]
=
"
Aisha
"
;
object
[
1
]
=
new
Shape();
string
str;
for
(
int
i
=
0
; i
&<
objects.Length; i
++
)
{ str = objects[i] as string; if(str == null) Console.WriteLine("can not be converted"); else Console.WriteLine("{0}",str); }
Output: Aisha can not be converted Statements Statements
in
C# are just like
in
C
++
except some additions of
new
statements and modifications
in
some statements. The following are
new
statements:
foreach
For iteration of collections, such
as
arrays, and so forth. Example:
foreach
(
string
s
in
array) Console.WriteLine(s);
lock
Used
in
threads
for
locking a block of code, making it a critical section.
checked
/
unchecked
The statements are
for
overflow checking
in
numeric operations. Example:
int
x
=
Int32.MaxValue; x
++
;
//
Overflow checked
{ x++; // Exception }
unchecked
{ x++; // Overflow} }
The following statements are modified: Switch The Switch statement
is
modified
in
C#. Now, after executing a
case
statement, program flow cannot jump to the next
case
, which was previously allowed
in
C
++
. Example:
int
var
=
100
;
switch
(var)
{ case 100: Console.WriteLine("<Value is 100>"); // No break here case 200: Console.WriteLine("<Value is 200>"); break; }
Output
in
C
++
:
<
Value
is
100
><
Value
is
200
>
In C#, you
get
a compile time error: error CS0163: Control cannot fall through from one
case
label (
'
case 100:
'
) to another However, you can
do
this
similarly to the way you
do
it
in
C
++
:
switch
(var)
{ case 100: case 200: Console.WriteLine("100 or 200<VALUE is 200>"); break; }
You also can use constant variables
for
case
values: Example:
const
string
WeekEnd
=
"
Sunday
"
;
const
string
WeekDay1
=
"
Monday
"
; .
string
WeekDay
=
Console.ReadLine();
switch
(WeekDay )
{ case WeekEnd: Console.WriteLine("It's weekend!!"); break; case WeekDay1: Console.WriteLine("It's Monday"); break; }
Delegates Delegates let us store function references into a variable. In C
++
,
this
is
like
using
and storing a function pointer
for
which we usually use typedef. Delegates are declared
using
a keyword
delegate
. Have a look at
this
example, and you will understand what delegates are: Example:
delegate
int
Operation(
int
val1,
int
val2);
public
int
Add(
int
val1,
int
val2)
{ return val1 + val2; }
public
int
Subtract (
int
val1,
int
val2)
{ return val1- val2; }
public
void
Perform()
{ Operation Oper; Console.WriteLine("Enter + or - "); string optor = Console.ReadLine(); Console.WriteLine("Enter 2 operands"); string opnd1 = Console.ReadLine(); string opnd2 = Console.ReadLine(); int val1 = Convert.ToInt32 (opnd1); int val2 = Convert.ToInt32 (opnd2); if (optor == "+") Oper = new Operation(Add); else Oper = new Operation(Subtract); Console.WriteLine(" Result = {0}", Oper(val1, val2)); }
Inheritance and Polymorphism Only single inheritance
is
allowed
in
C#. Mutiple inheritance can be acheived by
using
interfaces. Example:
class
Parent
{ }
class
Child : Parent Virtual Functions Virtual functions implement the concept of polymorphism are the same
as
in
C#, except that you use the
override
keyword with the
virtual
function implementaion
in
the child
class
. The parent
class
uses the same
virtual
keyword. Every
class
that overrides the
virtual
method will use the
override
keyword.
class
Shape
{ public virtual void Draw() { Console.WriteLine("Shape.Draw") ; } }
class
Rectangle : Shape
{ public override void Draw() { Console.WriteLine("Rectangle.Draw"); } }
class
Square : Rectangle
{ public override void Draw() { Console.WriteLine("Square.Draw"); } }
class
MainClass
{ static void Main(string[] args) { Shape[] shp = new Shape[3]; Rectangle rect = new Rectangle(); shp[0] = new Shape(); shp[1] = rect; shp[2] = new Square(); shp[0].Draw(); shp[1].Draw(); shp[2].Draw(); } }
Output: Shape.Draw Rectangle.Draw Square.Draw Hiding parent functions
using
"
new
"
You can define
in
a child
class
a
new
version of a function, hiding the one which
is
in
base
class
. A
new
keyword
is
used to define a
new
version. Consider the example below, which
is
a modified version of the above example and note the output
this
time, when I replace the
override
keyword with a
new
keyword
in
the Rectangle
class
.
class
Shape
{ public virtual void Draw() { Console.WriteLine("Shape.Draw") ; } }
class
Rectangle : Shape
{ public new void Draw() { Console.WriteLine("Rectangle.Draw"); } }
class
Square : Rectangle
{ //wouldn't let you override it here public new void Draw() { Console.WriteLine("Square.Draw"); } }
class
MainClass
{ static void Main(string[] args) { Console.WriteLine("Using Polymorphism:"); Shape[] shp = new Shape[3]; Rectangle rect = new Rectangle(); shp[0] = new Shape(); shp[1] = rect; shp[2] = new Square(); shp[0].Draw(); shp[1].Draw(); shp[2].Draw(); Console.WriteLine("Using without Polymorphism:"); rect.Draw(); Square sqr = new Square(); sqr.Draw(); } }
Output: Using Polymorphism Shape.Draw Shape.Draw Shape.Draw Using without Polymorphism: Rectangle.Draw Square.Draw The polymorphism doesn
'
t take the Rectangle class
'
s Draw method
as
a polymorphic form of the Shape
'
s Draw method. Instead, it considers it a different method. So, to avoid the naming conflict between parent and child, we have used the new modifier.
Note: You cannot use the two version of a method
in
the same
class
, one with
new
modifier and other with
override
or
virtual
. As
in
the above example, I cannot add another method named Draw
in
the Rectangle
class
which
is
a
virtual
or
override
method. Also,
in
the Square
class
, I can
'
t override the virtual Draw method of the Shape class.
Calling
base
class
members If the child
class
has the data members with same name
as
that of
base
class
, to avoid naming conflicts,
base
class
data members and functions are accessed
using
a keyword
base
. See
in
the examples how the
base
class
constructors are called and how the data members are used.
public
Child(
int
val) :
base
(val)
{ myVar = 5; base.myVar; }
OR
public
Child(
int
val)
{ base(val); myVar = 5 ; base.myVar; }
Future Additions: This article
is
just a quick overwiew of the C# language so that you can just become familiar with the langauge features. Although I have tried to discuss almost all the major concepts
in
C#
in
a brief and comprehensive way with code examples, I think there
is
lot much to be added and discussed. In the future, I would like to add more commands and concepts not yet discussed including events and so forth. I would also like to write about Windows programming
using
C#
for
beginners. References our most commonly known MSDN Inside C# by Tom Archer A Programmer
'
s Introduction to C# by Eric Gunnerson
Beginning C# by Karli Watson Programming C# (O
'
Reilly)
About the Author Aisha
is
a Master of Science
in
Computer Science from Quaid
-
i
-
Azam Univeristy. She has worked
in
VC
++
6
, MFC, ATL, COM
/
DCOM, ActiveX, C
++
, SQL, and so forth. These days she
is
working on .NET framework and C#. Inspired with nature, she loves to seek knowledge. She
is
also fond of travelling. She keeps a free source code and articles Web site at http:
//
aishai.netfirms.com.