C# null结合运算符??

using System;

namespace CalculatorApplication {

   class NullablesAtShow {

      static void Main(string[] args) {
         double? num1 = null;
         double? num2 = 3.14157;
         double num3;
         num3 = num1 ?? 5.34;      
         Console.WriteLine( Value of num3: {0}, num3);
         num3 = num2 ?? 5.34;
         Console.WriteLine( Value of num3: {0}, num3);
         Console.ReadLine();
      }
   }
}