as 运算符

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

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as 运算符用于在兼容的引用类型之间执行转换。例如：

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// 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

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