Nullable data type of C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Test1
{
class Program
{
static void Main(string[] args)
{

int? a;
a = null;
if (a == null)
{
Console.WriteLine("a is null");
}
else
Console.WriteLine("a is not null");
Console.ReadLine();
}
}
}