as 运算符类似于强制转换操作。但是，如果无法进行转换，则 as 返回 null 而非引发异常。

as 运算符只执行引用转换和装箱转换。as 运算符无法执行其他转换，如用户定义的转换，这类转换应使用强制转换表达式来执行。

expression

as

type

等效于(但只计算一次 expression)

 

expression

is

type

? (

type)

expression

: (

type)

null

---

as 运算符用于在兼容的引用类型之间执行转换。例如：

---

// cs_keyword_as.cs

// The as operator.

using System;

class Class1

{

}

class Class2

{

}

class MainClass

{

    static void Main()

    {

        object[] objArray = new object[6];

        objArray[0] = new Class1();

        objArray[1] = new Class2();

        objArray[2] = "hello";

        objArray[3] = 123;

        objArray[4] = 123.4;

        objArray[5] = null;

        for (int i = 0; i < objArray.Length; ++i)

        {

            string s = objArray[i] as string;

            Console.Write("{0}:", i);

            if (s != null)

            {

                Console.WriteLine("'" + s + "'");

            }

            else

            {

                Console.WriteLine("not a string");

            }

        }

    }

}

//=============================================================//

0:not a string

1:not a string

2:'hello'

3:not a string

4:not a string

5:not a string

---