virtual 与 override,该怎么处理

virtual 与 override

C# code

   class A     {          public A(){                PrintFields();           }          public virtual void PrintFields(){}      }    class B : A    {        int x = 1;        int y;        public B()        {            y = -1;        }        public override void PrintFields()        {            Console.WriteLine("x={0},y={1}", x, y);        }    }    class Program    {        static void Main(string[] args)        {            B b = new B();            b.PrintFields();            Console.ReadKey();        }    }