C#抽象与虚拟动态多态

using System;

namespace polymorphismApplication {

   class Shape {
      protected int width, height;

      public Shape( int a = 0, int b = 0) {
         width = a;
         height = b;
      }

      public virtual int area() {
         Console.WriteLine(Parent class area :);
         return 0;
      }
   }

   class Rectangle: Shape {
      public Rectangle( int a = 0, int b = 0): base(a, b) {

      }

      public override int area () {
         Console.WriteLine(Rectangle class area :);
         return (width * height); 
      }
   }

   class Triangle: Shape {
      public Triangle(int a = 0, int b = 0): base(a, b) {

      }

      public override int area() {
         Console.WriteLine(Triangle class area :);
         return (width * height / 2); 
      }
   }

   class Caller {
      public void CallArea(Shape sh) {
         int a;
         a = sh.area();
         Console.WriteLine(Area: {0}, a);
      }
   }  

   class Tester {

      static void Main(string[] args) {
         Caller c = new Caller();
         Rectangle r = new Rectangle(10, 7);
         Triangle t = new Triangle(10, 5);
         c.CallArea(r);
         c.CallArea(t);
         Console.ReadKey();
      }
   }
}