As Operator in C#

The as operator is used to perform certain types of conversions between compatible reference types. "As" Operator is also known as a safe cast. What is does is it attempts to cast from one type to another and if the cast fails it returns null instead of throwing an InvalidCastException.

As-casting is equivalent to the following expression except that expression is evaluated only one time.

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


Example

using System;
class Program
{
     static void Main()
     {
         Object o = null;
         // Prefix Casting
        String s0 = (String)o; //throw exception InvalidCastException        

        //As Casting        
        String s1 = o as String; // return null , instead of throwing an InvalidCastException
     }
}

Note

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.

There are actually two separate IL instructions to handle the difference between As-casting and Prefix-casting. The Prefix-casting cast uses the castclass IL instruction and the As-casting cast uses the isinst instruction.
Difference between As-casting and Prefix-casting :
Prefix-casting versus as-casting in C#
Who's Afraid of the big-bad cast operator?