as (C# Reference)

The as operator is used to perform certain types of conversions between compatible reference or nullable 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());
 }
 }
 }
 }

Remarks

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 conversions to reference or nullable types, and boxing conversions. The as operator cannot perform other conversions, such as user-defined conversions, which should instead be performed by using cast expressions.

Example

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
*/

C# Language Specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See Also

Reference

C# Keywords

is (C# Reference)

?: Operator (C# Reference)

Operator Keywords (C# Reference)

Concepts

C# Programming Guide

Other Resources

C# Reference