Nullable in c#

Nullable types can represent all the values of an underlying type, and an additional  null value. Nullable types are declared in one of two ways:
        System.Nullable variable
        -or-
        T? variable    
Any value type may be used as the basis for a nullable type

 class Program
    {
        static void Main(string[] args)
        {
            int? x = 10;
            if (x.HasValue)
            {
                System.Console.WriteLine(x.Value);
            }
            else
            {
                System.Console.WriteLine("Undefined");
            }

            Console.ReadKey();

            int? y = null;
            if (y != null)
            {
                System.Console.WriteLine(y.Value);
            }
            else
            {
                System.Console.WriteLine("Undefined");
            }

            Console.ReadKey();

            int? n = 5;

            //int m1 = n;      // Will not compile.
            int m2 = (int)n;   // Compiles, but will create an exception if n is null.
            int m3 = n.Value;  // Compiles, but will create an exception if n is null.

            Console.ReadKey();
            int A = 0;
            System.Console.WriteLine("A++ = {0}", A++);
            A = A + 1;
            System.Console.WriteLine("A = A + 1 {0}", A);
            System.Console.WriteLine("++A {0}", ++A);
            Console.ReadKey();

        }
    }