C# Language Tutorial => as

Example

The as keyword is an operator similar to a cast. If a cast is not possible, using as produces null rather than resulting in an InvalidCastException.

expression as type is equivalent to expression is type ? (type)expression : (type)null with the caveat that as is only valid on reference conversions, nullable conversions, and boxing conversions. User-defined conversions are not supported; a regular cast must be used instead.

For the expansion above, the compiler generates code such that expression will only be evaluated once and use single dynamic type check (unlike the two in the sample above).

as can be useful when expecting an argument to facilitate several types. Specifically it grants the user multiple options - rather than checking every possibility with is before casting, or just casting and catching exceptions. It is best practice to use 'as' when casting/checking an object which will cause only one unboxing penalty. Using is to check, then casting will cause two unboxing penalties.

If an argument is expected to be an instance of a specific type, a regular cast is preferred as its purpose is more clear to the reader.

Because a call to as may produce null, always check the result to avoid a NullReferenceException.

Example usage

object something = "Hello";
Console.WriteLine(something as string);        //Hello
Console.Writeline(something as Nullable<int>); //null
Console.WriteLine(something as int?);          //null

//This does NOT compile:
//destination type must be a reference type (or a nullable value type)
Console.WriteLine(something as int);

Live Demo on .NET Fiddle

Equivalent example without using as:

Console.WriteLine(something is string ? (string)something : (string)null);

This is useful when overriding the Equals function in custom classes.

class MyCustomClass
{

    public override bool Equals(object obj)
    {
        MyCustomClass customObject = obj as MyCustomClass;

        // if it is null it may be really null
        // or it may be of a different type
        if (Object.ReferenceEquals(null, customObject))
        {
            // If it is null then it is not equal to this instance.
            return false;
        }

        // Other equality controls specific to class
    }

}

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