虚函数重写

    public class A
        {
            public virtual void Print()
            {}

            int x;
            int y;

            public A()
            {
                Print();
            }

        }

        public class B:A
        {
            public override void Print()
            {
                Console.WriteLine("xyz");
            }

            public B()
            { }
        }

        static void Main()
        {
            B b = new B();//此时输出XYZ
        }
    }