# as (C# Reference)

You can use the as operator to perform certain types of conversions between compatible reference types or nullable types. The following code shows an example.

class csrefKeywordsOperators
{
class BaseClass
{
public override string ToString()
{
return "BaseClass";
}
}

class DerivedClass : BaseClass
{
public override string ToString()
{
return "DerivedClass";
}
}

class AnotherClass
{
public override string ToString()
{
return "AnotherClass";
}
}

class Program
{
static void Main()
{
object[] array = new object[] { new BaseClass(), new DerivedClass(), new AnotherClass() };

for (int i = 0; i < array.Length; i++)
{
BaseClass b = array[i] as BaseClass;

if (b != null) Console.WriteLine("{0}: {1}", i, b);
else Console.WriteLine("{0}: null", i);
}
}
}

}

/* Output:

0: BaseClass
1: DerivedClass
2: null

*/


As the example shows, you need to compare the result of the as expression with null to check if a conversion is successful. Beginning with C# 7.0, you can use the is expression both to test that a conversion succeeds and conditionally assign a variable when the conversion succeeds. In many scenarios, it's more concise than using the as operator. For more information, see the Type pattern section of the is operator article.

## Remarks

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

expression as type


The code is equivalent to the following expression except that the expression variable is evaluated only one time.

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


Note that the as operator performs only reference conversions, nullable conversions, and boxing conversions. The as operator can't 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 as operator in the C# Language Specification. The language specification is the definitive source for C# syntax and usage.