C#中两个问号的双目运算符

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            int? a;
            a = null;//此时输出3
            //a = 6;//此时输出6
            int b = a ?? 3;
            Console.Write(b);
            Console.ReadKey();
        }
    }
}