as C# with 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); //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 
} 
}