C#初始化基类

using System;

namespace RectangleApplication {

   class Rectangle {
      //member variables
      protected double length;
      protected double width;

      public Rectangle(double l, double w) {
         length = l;
         width = w;
      }

      public double GetArea() {
         return length * width;
      }

      public void Display() {
         Console.WriteLine("Length: {0}", length);
         Console.WriteLine("Width: {0}", width);
         Console.WriteLine("Area: {0}", GetArea());
      }
   }//end class Rectangle  

   class Tabletop : Rectangle {
      private double cost;
      public Tabletop(double l, double w) : base(l, w) { }

      public double GetCost() {
         double cost;
         cost = GetArea() * 70;
         return cost;
      }

      public void Display() {
         base.Display();
         Console.WriteLine("Cost: {0}", GetCost());
      }
   }

   class ExecuteRectangle {

      static void Main(string[] args) {
         Tabletop t = new Tabletop(4.5, 7.5);
         t.Display();
         Console.ReadLine();
      }
   }
}