C#速成

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。
英文原版:

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​​. ​