class A {
        virtual public void foo () {
            Console.WriteLine("A:foo()");
        }
    }

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

    class Program {
        static void Work (A a) {
            Console.WriteLine("working...");
            a.foo();
        }

        static void Main(string[] args) {
            A a = new A();
            A b = new B();
            Work(a);
            Work(b);
        }
    }