C#中的继承，抽象方法案例，得出圆，矩形面积。

C#中的继承，抽象方法案例，得出圆，矩形面积。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Graph
{
public abstract class Graph
{
protected double x, y;
public const double PI = Math.PI;
public Graph(double x1, double y1)
{
x = x1;
y = y1;
}
public abstract double Area();
}

    public class Circle : Graph
    {
        public Circle(double r)
            : base(r, 0)
        {
        }
        public override double Area()
        {
            Console.WriteLine("该方法计算所得的圆面积为：");
            return PI * x * x;
        }
    }

    public class Rectangle : Graph
    {
        public Rectangle(double length, double width)
            : base(length, width)
        {

        }
        public override double Area()
        {
            Console.WriteLine("该方法计算所得矩形面