C#动态多态性

using System;

namespace PolymorphismApplication {

   abstract class Shape {
      public abstract int area();
   }

   class Rectangle:  Shape {
      private int length;
      private int width;

      public Rectangle( int a = 0, int b = 0) {
         length = a;
         width = b;
      }

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

   class RectangleTester {

      static void Main(string[] args) {
         Rectangle r = new Rectangle(10, 7);
         double a = r.area();
         Console.WriteLine("Area: {0}",a);
         Console.ReadKey();
      }
   }
}