C#速成

﹏ヽ暗。殇╰゛Y 2021-11-11 23:04 445阅读 0赞

![None.gif][] 原文出处：http: // www.codeguru.com/cs\_syntax/CSharp.html   
![None.gif][] 原作者：Aisha Ikram  
![None.gif][]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  
![None.gif][]  
![None.gif][]使用环境: .NET, C\#, Win XP, Win  2000    
![None.gif][]  
![None.gif][]绪论  
![None.gif][]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 \++ 。  
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]下面是将要讨论的C\#问题的目录:  
![None.gif][]  
![None.gif][]程序结构   
![None.gif][]命名空间  
![None.gif][]数据类型  
![None.gif][]变量  
![None.gif][]运算符和表达式  
![None.gif][]枚举  
![None.gif][]语句（Statements ）  
![None.gif][]类（Classes）和结构（Structs）  
![None.gif][]修饰符（Modifiers）  
![None.gif][]属性(Properties)  
![None.gif][]接口(Interfaces)  
![None.gif][]方法参数(Function Parameters)  
![None.gif][]数组（Arrays）  
![None.gif][]索引器（Indexers）  
![None.gif][]装箱及拆箱操作  
![None.gif][]委托(Delegates)  
![None.gif][]继承和多态  
![None.gif][]  
![None.gif][]下面的内容将不会在被讨论之列：  
![None.gif][]  
![None.gif][]C \++ 与C\#谁更通用  
![None.gif][]诸如垃圾回收、线程以及文件处理等概念  
![None.gif][]数据的类型转换  
![None.gif][]异常处理  
![None.gif][].NET库  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]程序结构  
![None.gif][] \-------------------   
![None.gif][]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。  
![None.gif][]  
![None.gif][]让我们瞧瞧C\#当中的 Hello world 程序是怎样的。  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][]  
![None.gif][] namespace  MyNameSpace  
![None.gif][]  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]class HelloWorld  
![InBlock.gif][None.gif]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]    static void Main(string\[\] args)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine ("Hello World");  
![ExpandedSubBlockEnd.gif][None.gif]     \}  
![ExpandedSubBlockEnd.gif][None.gif]\}  
![InBlock.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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 " 。  
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]命名空间  
![None.gif][] \-------------------   
![None.gif][]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。  
![None.gif][]  
![None.gif][]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。  
![None.gif][]这有一个例子：  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] namespace  AnotherNameSpace  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    class AnotherClass  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        public void Func()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            Console.WriteLine ("Hello World");  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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：  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] using  AnotherNameSpace;     //  you will add this using statement   
![None.gif][] namespace  MyNameSpace  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]class HelloWorld  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]    static void Main(string\[\] args)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        AnotherClass obj = new AnotherClass();  
![InBlock.gif][None.gif]        obj.Func();  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedSubBlockEnd.gif][None.gif]\}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]你也可以象下面这样定义嵌套的命名空间。  
![None.gif][]  
![None.gif][]Using  
![None.gif][]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。  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]变量  
![None.gif][] \-------------------   
![None.gif][]除以下并别外，C\#当中的变量几乎与C \++ 同：  
![None.gif][]  
![None.gif][]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。  
![None.gif][]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）  
![None.gif][]一个超出数组边界的表达式是不可访问的。  
![None.gif][]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。  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]数据类型  
![None.gif][] \-------------------   
![None.gif][]所有C\#数据类型都派生自基类Object。这里有两类数据类型：  
![None.gif][]  
![None.gif][]基本型 / 内置型 用户自定义型  
![None.gif][]  
![None.gif][]下面一个C\#内置类型列表：  
![None.gif][]  
![None.gif][]类型 字节数 解释  
![None.gif][] byte   1  无符号字节型  
![None.gif][] sbyte   1  有符号字节型  
![None.gif][] short   2  有符号短字节型  
![None.gif][] ushort   2  无符号短字节型  
![None.gif][] int   4  有符号整型  
![None.gif][] uint   4  无符号整型  
![None.gif][] long   8  有符号长整型  
![None.gif][] ulong   8  无符号长整型  
![None.gif][] float   4  浮点数  
![None.gif][] double   8  双精度数  
![None.gif][] decimal   8  固定精度数  
![None.gif][] string  unicode字串型  
![None.gif][] char  unicode字符型  
![None.gif][] bool  真假布尔型  
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]用户定义类型包括：  
![None.gif][]  
![None.gif][]类类型( class )  
![None.gif][]结构类型( struct )  
![None.gif][]接口类型( interface )  
![None.gif][]  
![None.gif][]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：  
![None.gif][]  
![None.gif][]值类型（value Types）  
![None.gif][]引用类型（Reference Types）  
![None.gif][]  
![None.gif][]值类型：  
![None.gif][]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)  
![None.gif][]  
![None.gif][]引用类型：  
![None.gif][]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。  
![None.gif][]  
![None.gif][]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  
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]示例：  
![None.gif][]  
![None.gif][] enum  Weekdays  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]  Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]类类型与结构类型的比较  
![None.gif][]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。  
![None.gif][]（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）  
![None.gif][]  
![None.gif][]示例：  
![None.gif][]  
![None.gif][] struct  Date  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int day;  
![InBlock.gif][None.gif]    int month;  
![InBlock.gif][None.gif]    int year;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Date  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int day;  
![InBlock.gif][None.gif]    int month;  
![InBlock.gif][None.gif]    int year;  
![InBlock.gif][None.gif]    string weekday;  
![InBlock.gif][None.gif]    string monthName;  
![InBlock.gif][None.gif]    public int GetDay()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return day;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public int GetMonth()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return month;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public int GetYear()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return year;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetDay(int Day)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        day = Day ;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetMonth(int Month)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        month = Month;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetYear(int Year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        year = Year;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public bool IsLeapYear()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return (year/4 == 0);  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetDate (int day, int month, int year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    ![dot.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]属性  
![None.gif][] \-------------------   
![None.gif][]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。  
![None.gif][]  
![None.gif][]因此上面的类可以被写成：  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] class  Date  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    public int Day![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return day;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            day = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int day;  
![InBlock.gif][None.gif]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    public int Month![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return month;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            month = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int month;  
![InBlock.gif][None.gif]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    public int Year![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return year;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            year = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int year;  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    public bool IsLeapYear(int year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return year%4== 0 ? true: false;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetDate (int day, int month, int year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        this.day   = day;  
![InBlock.gif][None.gif]        this.month = month;  
![InBlock.gif][None.gif]        this.year  = year;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]你可在这里得到并设置这些属性：  
![None.gif][]  
![None.gif][] class  User  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]   public static void Main()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]   ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Date date = new Date();  
![InBlock.gif][None.gif]        date.Day = 27;  
![InBlock.gif][None.gif]        date.Month = 6;  
![InBlock.gif][None.gif]        date.Year = 2003;  
![InBlock.gif][None.gif]        Console.WriteLine("Date: \{0\}/\{1\}/\{2\}", date.Day,  
![InBlock.gif][None.gif]                                               date.Month,  
![InBlock.gif][None.gif]                                               date.Year);  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]修饰符  
![None.gif][] \-------------------   
![None.gif][]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。  
![None.gif][]  
![None.gif][] readonly （只读）  
![None.gif][]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：  
![None.gif][]  
![None.gif][] class  MyClass  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    const int constInt = 100;    //直接初始化  
![InBlock.gif][None.gif]    readonly int myInt = 5;      //直接初始化  
![InBlock.gif][None.gif]    readonly int myInt2;     //译者注：仅做声明，未做初始化  
![InBlock.gif][None.gif]      
![InBlock.gif][None.gif]    public MyClass()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        myInt2 = 8;              //间接的  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public Func()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        myInt = 7;               //非法操作（译者注：不得赋值两次）  
![InBlock.gif][None.gif]        Console.WriteLine(myInt2.ToString());  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]      
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] sealed （密封）  
![None.gif][]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。  
![None.gif][]  
![None.gif][] sealed   class  CanNotbeTheParent  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int a = 5;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] unsafe （不安全）  
![None.gif][]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：  
![None.gif][]  
![None.gif][] public   unsafe  MyFunction(  int   \*  pInt,  double \*  pDouble)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int\* pAnotherInt = new int;  
![InBlock.gif][None.gif]    \*pAnotherInt  = 10;  
![InBlock.gif][None.gif]    pInt = pAnotherInt;  
![InBlock.gif][None.gif]    ![dot.gif][None.gif]  
![InBlock.gif][None.gif]    \*pDouble = 8.9;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][] interface （接口）  
![None.gif][] \-------------------   
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] interface  myDrawing  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int originx  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        get;  
![InBlock.gif][None.gif]        set;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int originy  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        get;  
![InBlock.gif][None.gif]        set;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    void Draw(object shape);  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Shape: myDrawing  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int OriX;  
![InBlock.gif][None.gif]    int OriY;  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    public int originx  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return OriX;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            OriX = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public int originy  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return OriY;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            OriY = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void Draw(object shape)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        ![dot.gif][None.gif]    // do something  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]      
![InBlock.gif][None.gif]    // class's own method  
![InBlock.gif][None.gif]    public void MoveShape(int newX, int newY)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]    ![dot.gif][None.gif]..  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]      
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]Arrays（数组）  
![None.gif][] \-------------------   
![None.gif][]  
![None.gif][]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：  
![None.gif][]  
![None.gif][]方括号被置于数据类型之后而不是在变量名之后。  
![None.gif][]创建数组元素要使用new操作符。  
![None.gif][]C\#支持一维、多维以及交错数组（数组中的数组）。  
![None.gif][]  
![None.gif][]示例：  
![None.gif][]  
![None.gif][]     int \[\] array  =   new   int \[ 10 \];               //  整型一维数组   
![None.gif][]      for  ( int  i  =   0 ; i  <  array.Length; i \++ )  
![None.gif][]        array\[i\]  =  i;   
![None.gif][]  
![None.gif][]     int \[,\] array2  =   new   int \[ 5 , 10 \];           //  整型二维数组   
![None.gif][]     array2\[ 1 , 2 \]  =   5 ;  
![None.gif][]  
![None.gif][]     int \[,,\] array3  =   new   int \[ 5 , 10 , 5 \];        //  整型的三维数组   
![None.gif][]     array3\[ 0 , 2 , 4 \]  =   9 ;  
![None.gif][]  
![None.gif][]     int \[\]\[\] arrayOfarray  =   =   new   int \[ 2 \];     //  整型交错数组（数组中的数组）   
![None.gif][]     arrayOfarray\[ 0 \]  =   new   int \[ 4 \];   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]    arrayOfarray\[ 0 \]  =   new   int \[\]  ![dot.gif][None.gif] \{ 1,2,15\} ;  
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]索引器  
![None.gif][] \-------------------   
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]示例：  
![None.gif][]  
![None.gif][]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。  
![None.gif][] class  Shapes: CollectionBase  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public void add(Shape shp)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        List.Add(shp);  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    //indexer  
![InBlock.gif][None.gif]    public Shape this\[int index\]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return (Shape) List\[index\];  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            List\[index\] = value ;  
![ExpandedSubBlockEnd.gif][None.gif]         \}  
![ExpandedSubBlockEnd.gif][None.gif]     \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][]  
![None.gif][] \-------------------   
![None.gif][]装箱和拆箱操作（Boxing / Unboxing）  
![None.gif][] \-------------------   
![None.gif][]  
![None.gif][]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。  
![None.gif][]  
![None.gif][]示例：  
![None.gif][]  
![None.gif][] class  Test  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]   static void Main()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]   ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]      int myInt = 12;  
![InBlock.gif][None.gif]      object obj = myInt ;       // 装箱  
![InBlock.gif][None.gif]      int myInt2 = (int) obj;    // 拆箱  
![ExpandedSubBlockEnd.gif][None.gif]   \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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:**

![None.gif][]   
![None.gif][]Environment: .NET, C\#, Win XP, Win  2000    
![None.gif][]  
![None.gif][]Introduction  
![None.gif][]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 \++ .  
![None.gif][]![None.gif][]  
![None.gif][](continued)  
![None.gif][]  
![None.gif][]  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Note: This article  is  not  for  C\# gurus. There must be some other beginner ' s articles on C\#, but this is yet another one.   
![None.gif][]   
![None.gif][]The following topics of the C\# langauge are discussed:  
![None.gif][]  
![None.gif][]Program Structure   
![None.gif][]Namespaces   
![None.gif][]Data Types   
![None.gif][]Variables   
![None.gif][]Operators and Expressions   
![None.gif][]Enumerations   
![None.gif][]Statements   
![None.gif][]Classes and Structs   
![None.gif][]Modifiers   
![None.gif][]Properties   
![None.gif][]Interfaces   
![None.gif][]Function Parameters   
![None.gif][]Arrays   
![None.gif][]Indexers   
![None.gif][]Boxing and Unboxing   
![None.gif][]Delegates   
![None.gif][]Inheritance and Polymorphism   
![None.gif][]The following topics are not discussed:  
![None.gif][]  
![None.gif][]Things that are common  in  C \++  and C\#   
![None.gif][]Concepts such  as  garbage collection, threading, file processing, and so forth   
![None.gif][]Data type conversions   
![None.gif][]Exception handling   
![None.gif][].NET library   
![None.gif][]Program Structure  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Have a look at  this  Hello world program  in  C\#.  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][]  
![None.gif][] namespace  MyNameSpace  
![None.gif][]  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]class HelloWorld  
![InBlock.gif][None.gif]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]    static void Main(string\[\] args)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine ("Hello World");  
![ExpandedSubBlockEnd.gif][None.gif]     \}  
![ExpandedSubBlockEnd.gif][None.gif]\}  
![InBlock.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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 " .  
![None.gif][]  
![None.gif][]There  is  no need to put a semicolon after a  class  block or  struct  definition. It was required  in  C \++ , but not  in  C\#.  
![None.gif][]  
![None.gif][]Namespace  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Now, consider that you want to access the HelloWorld  class  from some other  class   in  some other  namespace .  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] namespace  AnotherNameSpace  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    class AnotherClass  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        public void Func()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            Console.WriteLine ("Hello World");  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Now, from your HelloWorld  class , you can access it  as :  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] using  AnotherNameSpace;     //  you will add this using statement   
![None.gif][] namespace  MyNameSpace  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]class HelloWorld  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]    static void Main(string\[\] args)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        AnotherClass obj = new AnotherClass();  
![InBlock.gif][None.gif]        obj.Func();  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedSubBlockEnd.gif][None.gif]\}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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.  
![None.gif][]  
![None.gif][]you can also define nested namespaces.  
![None.gif][]  
![None.gif][]Using  
![None.gif][]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 .  
![None.gif][]  
![None.gif][]Variables  
![None.gif][]Variables  in  C\# are almost the same  as   in  C \++ , except  for  these differences:  
![None.gif][]  
![None.gif][]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.    
![None.gif][] You can ' t access a .dangling. pointer in C\#.    
![None.gif][] An expression that indexes an array beyond its bounds  is  also not accessible.   
![None.gif][]There are no global variables or functions  in  C\# and the behavior of globals  is  acheived through  static  functions and  static  variables.   
![None.gif][]Data Types  
![None.gif][]All types of C\# are derived from a  base \- class   object . There are two types of data types:  
![None.gif][]  
![None.gif][]Basic / Built \- in  types   
![None.gif][]User \- defined types   
![None.gif][]Following  is  a table that lists built \- in  C\# types:  
![None.gif][]  
![None.gif][]Type  
![None.gif][] Bytes  
![None.gif][] Desc  
![None.gif][]   
![None.gif][] byte   1  unsigned  byte    
![None.gif][] sbyte   1  signed  byte    
![None.gif][] short   2  signed  short    
![None.gif][] ushort   2  unsigned  short    
![None.gif][] int   4  signed integer   
![None.gif][] uint   4  unsigned integer   
![None.gif][] long   8  signed  long    
![None.gif][] ulong   8  unsigned  long    
![None.gif][] float   4  floating point number   
![None.gif][] double   8   double  precision number   
![None.gif][] decimal   8   fixed  precision number   
![None.gif][] string   unicode  string    
![None.gif][] char   unicode  char    
![None.gif][] bool   true ,  false  boolean   
![None.gif][]  
![None.gif][]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.  
![None.gif][]User \- defined types include:  
![None.gif][]  
![None.gif][]Classes   
![None.gif][]Structs   
![None.gif][]Interfaces   
![None.gif][]Memory allocation of the data types divides them into two types:  
![None.gif][]  
![None.gif][]Value Types   
![None.gif][]Reference Types   
![None.gif][]Value types  
![None.gif][]Value types are those data types that are allocated  in  the stack. They include:  
![None.gif][]  
![None.gif][]All basic or built \- in  types except strings   
![None.gif][]Structs   
![None.gif][]Enum types   
![None.gif][]Reference type  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Reference types include:  
![None.gif][]  
![None.gif][]Classes   
![None.gif][]Interfaces   
![None.gif][]Collection types such  as  arrays   
![None.gif][]String   
![None.gif][]Enumeration  
![None.gif][]Enumerations  in  C\# are exactly like C \++ . They are defined through a  enum  keyword.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][] enum  Weekdays  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]  Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Classes and Structs  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Examples:  
![None.gif][]  
![None.gif][] struct  Date  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int day;  
![InBlock.gif][None.gif]    int month;  
![InBlock.gif][None.gif]    int year;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Date  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int day;  
![InBlock.gif][None.gif]    int month;  
![InBlock.gif][None.gif]    int year;  
![InBlock.gif][None.gif]    string weekday;  
![InBlock.gif][None.gif]    string monthName;  
![InBlock.gif][None.gif]    public int GetDay()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return day;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public int GetMonth()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return month;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public int GetYear()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return year;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetDay(int Day)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        day = Day ;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetMonth(int Month)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        month = Month;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetYear(int Year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        year = Year;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public bool IsLeapYear()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return (year/4 == 0);  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetDate (int day, int month, int year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    ![dot.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Properties  
![None.gif][]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.  
![None.gif][]  
![None.gif][]So, the above  class  can be written  as :  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] class  Date  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    public int Day![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return day;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            day = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int day;  
![InBlock.gif][None.gif]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    public int Month![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return month;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            month = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int month;  
![InBlock.gif][None.gif]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    public int Year![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return year;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            year = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int year;  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    public bool IsLeapYear(int year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        return year%4== 0 ? true: false;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void SetDate (int day, int month, int year)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        this.day   = day;  
![InBlock.gif][None.gif]        this.month = month;  
![InBlock.gif][None.gif]        this.year  = year;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Here  is  the way you will  get  and  set  these properties:  
![None.gif][]  
![None.gif][] class  User  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]   public static void Main()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]   ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Date date = new Date();  
![InBlock.gif][None.gif]        date.Day = 27;  
![InBlock.gif][None.gif]        date.Month = 6;  
![InBlock.gif][None.gif]        date.Year = 2003;  
![InBlock.gif][None.gif]        Console.WriteLine("Date: \{0\}/\{1\}/\{2\}", date.Day,  
![InBlock.gif][None.gif]                                               date.Month,  
![InBlock.gif][None.gif]                                               date.Year);  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Modifiers  
![None.gif][]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\#.  
![None.gif][]  
![None.gif][] readonly   
![None.gif][]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:  
![None.gif][]  
![None.gif][] class  MyClass  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    const int constInt = 100;    //directly  
![InBlock.gif][None.gif]    readonly int myInt = 5;      //directly  
![InBlock.gif][None.gif]    readonly int myInt2;  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    public MyClass()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        myInt2 = 8;              //Indirectly  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public Func()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        myInt = 7;               //Illegal  
![InBlock.gif][None.gif]        Console.WriteLine(myInt2.ToString());  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] sealed   
![None.gif][]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.  
![None.gif][]  
![None.gif][] sealed   class  CanNotbeTheParent  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int a = 5;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] unsafe   
![None.gif][]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:  
![None.gif][]  
![None.gif][] public   unsafe  MyFunction(  int   \*  pInt,  double \*  pDouble)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int\* pAnotherInt = new int;  
![InBlock.gif][None.gif]    \*pAnotherInt  = 10;  
![InBlock.gif][None.gif]    pInt = pAnotherInt;  
![InBlock.gif][None.gif]    ![dot.gif][None.gif]  
![InBlock.gif][None.gif]    \*pDouble = 8.9;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Interfaces  
![None.gif][]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.  
![None.gif][]  
![None.gif][] using  System;  
![None.gif][] interface  myDrawing  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int originx  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        get;  
![InBlock.gif][None.gif]        set;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    int originy  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        get;  
![InBlock.gif][None.gif]        set;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    void Draw(object shape);  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Shape: myDrawing  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    int OriX;  
![InBlock.gif][None.gif]    int OriY;  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    public int originx  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return OriX;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            OriX = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public int originy  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return OriY;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            OriY = value;  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]    public void Draw(object shape)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        ![dot.gif][None.gif]    // do something  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    // class's own method  
![InBlock.gif][None.gif]    public void MoveShape(int newX, int newY)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]    ![dot.gif][None.gif]..  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Arrays  
![None.gif][]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:   
![None.gif][]   
![None.gif][]The square brackets are placed after the type and not after the variable name.   
![None.gif][]You create element locations  using   new   operator .   
![None.gif][]C\# supports single dimensional, multi dimensional, and jagged array (array of an array).  
![None.gif][]  
![None.gif][]Examples:  
![None.gif][]  
![None.gif][]     int \[\] array  =   new   int \[ 10 \];               //  single-dimensional  
![None.gif][]                                             //  array of int   
![None.gif][]      for  ( int  i  =   0 ; i  <  array.Length; i \++ )  
![None.gif][]        array\[i\]  =  i;  
![None.gif][]  
![None.gif][]     int \[,\] array2  =   new   int \[ 5 , 10 \];           //  2-dimensional array  
![None.gif][]                                             //  of int   
![None.gif][]     array2\[ 1 , 2 \]  =   5 ;  
![None.gif][]  
![None.gif][]     int \[,,\] array3  =   new   int \[ 5 , 10 , 5 \];        //  3-dimensional array  
![None.gif][]                                             //  of int   
![None.gif][]     array3\[ 0 , 2 , 4 \]  =   9 ;  
![None.gif][]  
![None.gif][]     int \[\]\[\] arrayOfarray  =   =   new   int \[ 2 \];     //  Jagged array -  
![None.gif][]                                             //  array of array of  
![None.gif][]                                             //  int   
![None.gif][]     arrayOfarray\[ 0 \]  =   new   int \[ 4 \];  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]    arrayOfarray\[ 0 \]  =   new   int \[\]  ![dot.gif][None.gif] \{ 1,2,15\} ;  
![None.gif][]  
![None.gif][]Indexers  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][]Note: CollectionBase  is  the library  class  used  for  making collections. List  is  the  protected  member of CollectionBase, which stores the collection list.  
![None.gif][] class  Shapes: CollectionBase  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public void add(Shape shp)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        List.Add(shp);  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    //indexer  
![InBlock.gif][None.gif]    public Shape this\[int index\]  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        get ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            return (Shape) List\[index\];  
![ExpandedSubBlockEnd.gif][None.gif]        \}  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]        set ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]            List\[index\] = value ;  
![ExpandedSubBlockEnd.gif][None.gif]         \}  
![ExpandedSubBlockEnd.gif][None.gif]     \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Boxing / Unboxing  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][] class  Test  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]   static void Main()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]   ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]      int myInt = 12;  
![InBlock.gif][None.gif]      object obj = myInt ;       // boxing  
![InBlock.gif][None.gif]      int myInt2 = (int) obj;    // unboxing  
![ExpandedSubBlockEnd.gif][None.gif]   \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Function Parameters  
![None.gif][]Parameters  in  C\# are of three types:  
![None.gif][]  
![None.gif][]By \- Value / In parameters   
![None.gif][]By \- Reference / In \- Out parameters   
![None.gif][]Out pParameters   
![None.gif][]If you have an idea of the COM  interface  and its parameters types, you will easily understand the C\# parameter types.  
![None.gif][]  
![None.gif][]By \- Value / In parameters  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][]SetDay( 5 );  
![None.gif][]![dot.gif][None.gif]  
![None.gif][] void  SetDay( int  day)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    ![dot.gif][None.gif].  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]By \- Reference / In \- Out Parameters  
![None.gif][]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.  
![None.gif][]  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][]     int  a =   5 ;  
![None.gif][]    FunctionA( ref  a);         //  use ref with argument or you will  
![None.gif][]                              //  get a compiler error   
![None.gif][]     Console.WriteLine(a);     //  prints 20   
![None.gif][]   
![None.gif][]     void  FunctionA( ref   int  Val)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]     ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]        int x= Val;  
![InBlock.gif][None.gif]        Val = x\* 4;  
![ExpandedBlockEnd.gif][None.gif]    \}   
![None.gif][]  
![None.gif][]Out parameter  
![None.gif][]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  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][]     int  Val;  
![None.gif][]    GetNodeValue(Val);  
![None.gif][]  
![None.gif][]     bool  GetNodeValue( out   int  Val)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]     ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]        Val = value;  
![InBlock.gif][None.gif]        return true;  
![ExpandedBlockEnd.gif][None.gif]    \}   
![None.gif][]  
![None.gif][]Variable number of parameters and arrays  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][]     void  Func( params   int \[\] array)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]     ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]        Console.WriteLine("number of elements \{0\}",  
![InBlock.gif][None.gif]                           array.Length);  
![ExpandedBlockEnd.gif][None.gif]    \}   
![None.gif][]  
![None.gif][]    Func();                       //  prints 0   
![None.gif][]     Func( 5 );                      //  prints 1   
![None.gif][]     Func( 7 , 9 );                    //  prints 2   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]     Func( new   int \[\]  ![dot.gif][None.gif] \{ 3,8,10\} );     //  prints 3   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif]      int \[\] array  =   new   int \[ 8 \]  ![dot.gif][None.gif] \{ 1,3,4,5,5,6,7,5\} ;  
![None.gif][]    Func(array);                  //  prints 8   
![None.gif][]   
![None.gif][]Operators and Expressions  
![None.gif][]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.  
![None.gif][]  
![None.gif][] is   operator   
![None.gif][]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:  
![None.gif][]  
![None.gif][] void  function( object  param)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    if(param is ClassA)  
![InBlock.gif][None.gif]        //do something  
![InBlock.gif][None.gif]    else if(param is MyStruct)  
![InBlock.gif][None.gif]        //do something  
![ExpandedBlockEnd.gif][None.gif]    \}   
![None.gif][]\}  
![None.gif][]  
![None.gif][] as   operator   
![None.gif][]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.  
![None.gif][]  
![None.gif][]Shape shp  =   new  Shape();  
![None.gif][]Vehicle veh  =  shp  as  Vehicle;    //  result is null, types are not  
![None.gif][]                                 //  convertable   
![None.gif][]   
![None.gif][]Circle cir  =   new  Circle();  
![None.gif][]Shape shp  =  cir;  
![None.gif][]Circle cir2  =  shp  as  Circle;     // will be converted   
![None.gif][]   
![None.gif][] object \[\] objects  =   new   object \[ 2 \];  
![None.gif][]objects\[ 0 \]  =   " Aisha " ;  
![None.gif][] object \[ 1 \]  =   new  Shape();  
![None.gif][]  
![None.gif][] string  str;  
![None.gif][] for ( int  i = 0 ; i &<  objects.Length; i \++ )  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    str = objects\[i\] as string;  
![InBlock.gif][None.gif]    if(str == null)  
![InBlock.gif][None.gif]        Console.WriteLine("can not be converted");  
![InBlock.gif][None.gif]    else  
![InBlock.gif][None.gif]        Console.WriteLine("\{0\}",str);  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Output:  
![None.gif][]Aisha  
![None.gif][]can not be converted  
![None.gif][]  
![None.gif][]Statements  
![None.gif][]Statements  in  C\# are just like  in  C \++  except some additions of  new  statements and modifications  in  some statements.  
![None.gif][]  
![None.gif][]The following are  new  statements:  
![None.gif][]  
![None.gif][] foreach   
![None.gif][]For iteration of collections, such  as  arrays, and so forth.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][]     foreach  ( string  s  in  array)  
![None.gif][]        Console.WriteLine(s);  
![None.gif][]  
![None.gif][] lock   
![None.gif][]Used  in  threads  for  locking a block of code, making it a critical section.  
![None.gif][]  
![None.gif][] checked / unchecked   
![None.gif][]The statements are  for  overflow checking  in  numeric operations.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][] int  x  =  Int32.MaxValue; x \++ ;     //  Overflow checked   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]x\++;                            // Exception  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][] unchecked   
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]x\++;                            // Overflow\}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]The following statements are modified:  
![None.gif][]  
![None.gif][]Switch  
![None.gif][]The Switch statement  is  modified  in  C\#.  
![None.gif][]  
![None.gif][]Now, after executing a  case  statement, program flow cannot jump to the next  case , which was previously allowed  in  C \++ .  
![None.gif][]Example:   
![None.gif][] int  var  =   100 ;  
![None.gif][] switch  (var)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    case 100: Console.WriteLine("<Value is 100>");  
![InBlock.gif][None.gif]        // No break here  
![InBlock.gif][None.gif]    case 200: Console.WriteLine("<Value is 200>"); break;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]Output  in  C \++ :  
![None.gif][]     < Value  is   100 >< Value  is   200 >   
![None.gif][]  
![None.gif][]In C\#, you  get  a compile time error:  
![None.gif][]  
![None.gif][]error CS0163: Control cannot fall through from one  case  label  
![None.gif][]( ' case 100: ' ) to another  
![None.gif][]  
![None.gif][]However, you can  do   this  similarly to the way you  do  it  in  C \++ :   
![None.gif][] switch  (var)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    case 100:  
![InBlock.gif][None.gif]    case 200: Console.WriteLine("100 or 200<VALUE is 200>");  
![InBlock.gif][None.gif]              break;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]You also can use constant variables  for   case  values:   
![None.gif][]Example:  
![None.gif][]  
![None.gif][] const   string  WeekEnd   =   " Sunday " ;  
![None.gif][] const   string  WeekDay1  =   " Monday " ;  
![None.gif][]  
![None.gif][]![dot.gif][None.gif].  
![None.gif][]  
![None.gif][] string  WeekDay  =  Console.ReadLine();  
![None.gif][] switch  (WeekDay )  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]case WeekEnd: Console.WriteLine("It's weekend!!"); break;  
![InBlock.gif][None.gif]case WeekDay1: Console.WriteLine("It's Monday"); break;  
![InBlock.gif][None.gif]  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Delegates  
![None.gif][]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.  
![None.gif][]  
![None.gif][]Delegates are declared  using  a keyword  delegate . Have a look at  this  example, and you will understand what delegates are:  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![None.gif][] delegate   int  Operation( int  val1,  int  val2);  
![None.gif][] public   int  Add( int  val1,  int  val2)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    return val1 \+ val2;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][] public   int  Subtract ( int  val1,  int  val2)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    return val1\- val2;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] public   void  Perform()  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    Operation Oper;  
![InBlock.gif][None.gif]    Console.WriteLine("Enter + or - ");  
![InBlock.gif][None.gif]    string optor = Console.ReadLine();  
![InBlock.gif][None.gif]    Console.WriteLine("Enter 2 operands");  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    string opnd1 = Console.ReadLine();  
![InBlock.gif][None.gif]    string opnd2 = Console.ReadLine();  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    int val1 = Convert.ToInt32 (opnd1);  
![InBlock.gif][None.gif]    int val2 = Convert.ToInt32 (opnd2);  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    if (optor == "\+")  
![InBlock.gif][None.gif]        Oper = new Operation(Add);  
![InBlock.gif][None.gif]    else  
![InBlock.gif][None.gif]        Oper = new Operation(Subtract);  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]    Console.WriteLine(" Result = \{0\}", Oper(val1, val2));  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Inheritance and Polymorphism  
![None.gif][]Only single inheritance  is  allowed  in  C\#. Mutiple inheritance can be acheived by  using  interfaces.  
![None.gif][]  
![None.gif][]Example:  
![None.gif][]  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] class  Parent ![dot.gif][None.gif] \{  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Child : Parent  
![None.gif][]  
![None.gif][]Virtual Functions  
![None.gif][]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.  
![None.gif][]  
![None.gif][] class  Shape  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public virtual void Draw()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Shape.Draw")    ;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Rectangle : Shape  
![None.gif][]  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public override void Draw()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Rectangle.Draw");  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Square : Rectangle  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public override void Draw()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Square.Draw");  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][] class  MainClass  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    static void Main(string\[\] args)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Shape\[\] shp = new Shape\[3\];  
![InBlock.gif][None.gif]        Rectangle rect = new Rectangle();  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]        shp\[0\] = new Shape();  
![InBlock.gif][None.gif]        shp\[1\] = rect;  
![InBlock.gif][None.gif]        shp\[2\] = new Square();  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]        shp\[0\].Draw();  
![InBlock.gif][None.gif]        shp\[1\].Draw();  
![InBlock.gif][None.gif]        shp\[2\].Draw();  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]Output:  
![None.gif][]Shape.Draw  
![None.gif][]Rectangle.Draw  
![None.gif][]Square.Draw  
![None.gif][]  
![None.gif][]Hiding parent functions  using   " new "   
![None.gif][]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 .  
![None.gif][]  
![None.gif][] class  Shape  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public virtual void Draw()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Shape.Draw")    ;  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][] class  Rectangle : Shape  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    public new void Draw()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Rectangle.Draw");  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][] class  Square : Rectangle  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    //wouldn't let you override it here  
![InBlock.gif][None.gif]    public new void Draw()  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Square.Draw");  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][] class  MainClass  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    static void Main(string\[\] args)  
![ExpandedSubBlockStart.gif][None.gif]![ContractedSubBlock.gif][None.gif]    ![dot.gif][None.gif]\{  
![InBlock.gif][None.gif]        Console.WriteLine("Using Polymorphism:");  
![InBlock.gif][None.gif]        Shape\[\] shp = new Shape\[3\];  
![InBlock.gif][None.gif]        Rectangle rect = new Rectangle();  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]        shp\[0\] = new Shape();  
![InBlock.gif][None.gif]        shp\[1\] = rect;  
![InBlock.gif][None.gif]        shp\[2\] = new Square();  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]        shp\[0\].Draw();  
![InBlock.gif][None.gif]        shp\[1\].Draw();  
![InBlock.gif][None.gif]        shp\[2\].Draw();  
![InBlock.gif][None.gif]  
![InBlock.gif][None.gif]        Console.WriteLine("Using without Polymorphism:");  
![InBlock.gif][None.gif]        rect.Draw();  
![InBlock.gif][None.gif]        Square sqr = new Square();  
![InBlock.gif][None.gif]        sqr.Draw();  
![ExpandedSubBlockEnd.gif][None.gif]    \}  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Output:  
![None.gif][]Using Polymorphism  
![None.gif][]Shape.Draw  
![None.gif][]Shape.Draw  
![None.gif][]Shape.Draw  
![None.gif][]Using without Polymorphism:  
![None.gif][]Rectangle.Draw  
![None.gif][]Square.Draw  
![None.gif][]  
![None.gif][]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.   
![None.gif][]   
![None.gif][]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.   
![None.gif][] Calling  base   class  members  
![None.gif][]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.  
![None.gif][]  
![None.gif][] public  Child( int  val) : base (val)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    myVar = 5;  
![InBlock.gif][None.gif]    base.myVar;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]OR  
![None.gif][]  
![None.gif][] public  Child( int  val)  
![ExpandedBlockStart.gif][None.gif]![ContractedBlock.gif][None.gif] ![dot.gif][None.gif] \{  
![InBlock.gif][None.gif]    base(val);  
![InBlock.gif][None.gif]    myVar = 5 ;  
![InBlock.gif][None.gif]    base.myVar;  
![ExpandedBlockEnd.gif][None.gif]\}   
![None.gif][]  
![None.gif][]Future Additions:  
![None.gif][]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.  
![None.gif][]  
![None.gif][]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.  
![None.gif][]  
![None.gif][]References  
![None.gif][]our most commonly known MSDN   
![None.gif][]Inside C\# by Tom Archer   
![None.gif][]A Programmer ' s Introduction to C\# by Eric Gunnerson    
![None.gif][] Beginning C\# by Karli Watson   
![None.gif][]Programming C\# (O ' Reilly)    
![None.gif][] About the Author  
![None.gif][]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.

转载于:https://www.cnblogs.com/Dicky/archive/2005/03/21/122574.html

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