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as (C# Reference)

The as operator is used to perform certain types of conversions between compatible reference types. For example:

   class csrefKeywordsOperators
   {
       class Base
       {
           public override string  ToString()
           {
	             return "Base";
           }
       }
       class Derived : Base 
       { }

       class Program
       {
           static void Main()
           {

               Derived d = new Derived();

               Base b = d as Base;
               if (b != null)
               {
                   Console.WriteLine(b.ToString());
               }

           }
       }
   }

The as operator is like a cast operation. However, if the conversion is not possible, as returns null instead of raising an exception. Consider the following expression:

expression as type

It is equivalent to the following expression except that expression is evaluated only one time.

expression is type ? (type)expression : (type)null

Note that the as operator only performs reference conversions and boxing conversions. The as operator cannot perform other conversions, such as user-defined conversions, which should instead be performed by using cast expressions.

class ClassA { }
class ClassB { }

class MainClass
{
    static void Main()
    {
        object[] objArray = new object[6];
        objArray[0] = new ClassA();
        objArray[1] = new ClassB();
        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");
            }
        }
    }
}
/*
Output:
0:not a string
1:not a string
2:'hello'
3:not a string
4:not a string
5:not a string
*/

For more information, see the following sections in the C# Language Specification:

  • 6 Conversions

  • 7.9.11 The as operator

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