What are 'AS' and 'IS' key word in C#

The 'AS' operator is used to perform certain types of conversions between compatible reference types.

 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());
               }

           }
       }
   }


The 'IS' keyword causes a compile-time warning if the expression is known to always be true or to always be false, but typically evaluates type compatibility at run time.
The is operator cannot be overloaded.
Note that the is operator only considers reference conversions, boxing conversions, and unboxing conversions. Other conversions, such as user-defined conversions, are not considered.

class Class1 {}
class Class2 {}
class Class3 : Class2 { }

class IsTest
{
    static void Test(object o)
    {
        Class1 a;
        Class2 b;

        if (o is Class1)
        {
            Console.WriteLine("o is Class1");
            a = (Class1)o;
            // Do something with "a."
        }
        else if (o is Class2)
        {
            Console.WriteLine("o is Class2");
            b = (Class2)o;
            // Do something with "b."
        }

        else
        {
            Console.WriteLine("o is neither Class1 nor Class2.");
        }
    }
    static void Main()
    {
        Class1 c1 = new Class1();
        Class2 c2 = new Class2();
        Class3 c3 = new Class3();
        Test(c1);
        Test(c2);
        Test(c3);
        Test("a string");
    }
}
/*
Output:
o is Class1
o is Class2
o is Class2
o is neither Class1 nor Class2.
*/