一、题目
【5-12】把定义平面直角坐标系上的一个点的类clsPoint作为基类，派生出描述一个直线的类clsLine，再派生出一个矩形类clsRect类。要求方法能求出两点间的距离、矩形的周长和面积等。设计一个测试程序，并构造出完整的程序。

二、我的努力
namespace 书本练习题
{
    class clsPoint
    {
        /// <summary>
        /// 定义属性
        /// </summary>
        int x;
        public int X
        {
            get { return x; }
            set
            {
                if (value >= 60)
                {
                    x = value;
                }
                else
                {
                    x = 60;
                }
            }
        }
        int y;
        public int Y
        {
            get { return y; }
            set
            {
                if (value >= 75)
                {
                    y = value;
                }
                else
                {
                    y = 75;
                }
            }
        }
        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="xx"></param>
        /// <param name="yy"></param>
        public clsPoint(int xx, int yy)
        {
            this.x = xx;
            this.y = yy;
        }
        public clsPoint()
        {
            this.x = 60;
            this.y = 75;
        }
        /// <summary>
        /// 定义方法
        /// </summary>
        public void display()
        {
            Console.WriteLine("x,y的坐标值为({0},{1})", x, y);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            clsPoint Dian = new clsPoint();
            Dian.display();

            clsPoint DianTwo = new clsPoint(78, 79);
            DianTwo.display();

            Console.ReadKey();
        }
    }
}

三、我的困难
    1、我不知道如何派生出 clsLine类 和 clsRect类；  
    2、要有方法能计算两点之间的距离；要有方法能计算矩形的周长和面积；

此题困扰了我整整三天了，希望大侠高手们拔刀相助，小虾在此不胜感谢！

大家好，有哥们百度了这道题目的答案，在此，不再麻烦大家了。
代码如下：
源代码：
ClsPoint.cs
using System;
using System.Collections.Generic;
using System.Text;

namespace __12
{
    //***************************基类clsPoint*****************************
    public class clsPoint
    {
        public double x;
        public double y;
        public clsPoint()
        {
            this.x = 0;
            this.y = 0;
        }
        public void setPoint(double _x,double _y)
        {
            this.x = _x;
            this.y = _y;
        }
        public void display()
        {
            Console.Write(" ({0}，{1})", this.x, this.y);
        }
    }


    //*********************派生类clsLine*********************************
    public class clsLine : clsPoint
    {
        public clsPoint Point_start;
        public clsPoint Point_end;
        public clsLine(clsPoint Point_start, clsPoint Point_end)
        {
            this.Point_start=Point_start;
            this.Point_end = Point_end;
        }
        public double longness()
        {
            double l;
            l = System.Math.Sqrt((this.Point_start.y - this.Point_end.y) * (this.Point_start.y - this.Point_end.y) + (this.Point_start.x - this.Point_end.x) * (this.Point_start.x - this.Point_end.x));
            return l;
        }
    }


    //*********************派生类clsRect*********************************
    public class clsRect : clsPoint
    {
        public clsPoint point1;
        public clsPoint point2=new clsPoint();
        public clsPoint point3;
        public clsPoint point4=new clsPoint();
        double l1,l2;
        public clsRect(clsPoint point1,clsPoint point2)
        {
            this.point1 = point1;
            this.point2.setPoint(point1.x, point2.y);
            this.point3 = point2;
            this.point4.setPoint(point2.x, point1.y);
            this.l1=point2.x-point1.x;
            this.l2=point2.y-point1.y;
        }
        public double girth()        //周长
        {
            return ((this.l1 + this.l2) * 2);
        }
        public double area()        //面积
        {
            return (this.l1 * this.l2);
        }
    }
}
Program.cs
using System;
using System.Collections.Generic;
using System.Text;

namespace __12
{
    class Program
    {
        static void Main(string[] args)
        {
            clsPoint point1 = new clsPoint();
            clsPoint point2 = new clsPoint();
            point1.setPoint(0,0);
            point2.setPoint(10,10);
            clsLine line1 = new clsLine(point1,point2);
            clsRect rect1 = new clsRect(point1,point2);
            Console.Write("由");
            line1.Point_start.display();
            line1.Point_end.display();
            Console.WriteLine("构成的线段的长度为：{0:f}", line1.longness());
            Console.Write("由");
            rect1.point1.display();
            rect1.point2.display();
            rect1.point3.display();
            rect1.point4.display();
            Console.WriteLine("构成的矩形周长为：{0:f}，面积为：{1:f}", rect1.girth(),rect1.area());
        }
    }
}

感谢信
---------------
在这里，感谢这位哥们。

该题增加一个派生圆类clsPark,求以两点为半径的半径、周长和面积。

我照猫画虎，完成了这个派生类的编写：
namespace 书本练习题
{
    public class clsPoint
    {

        //***************************基类clsPoint*****************************
        public double x;
        public double y;
        public clsPoint()
        {
            this.x = 0;
            this.y = 0;
        }
        public void setPoint(double _x, double _y)
        {
            this.x = _x;
            this.y = _y;
        }
        public void display()
        {
            Console.Write("({0}，{1})", this.x, this.y);
        }

        //*********************派生类clsLine*********************************
        public class clsLine : clsPoint
        {
            public clsPoint Point_start;
            public clsPoint Point_end;
            public clsLine(clsPoint Point_start, clsPoint Point_end)
            {
                this.Point_start = Point_start;
                this.Point_end = Point_end;
            }      //此处接收实例对象的传递
            public double longness()
            {
                double l;
                l = System.Math.Sqrt((this.Point_start.y - this.Point_end.y) * (this.Point_start.y - this.Point_end.y) + (this.Point_start.x - this.Point_end.x) * (this.Point_start.x - this.Point_end.x));
                return l;
            }
        }

        //*********************派生类clsRect*********************************
        public class clsRect : clsPoint
        {
            public clsPoint point1;
            public clsPoint point2=new clsPoint();
            public clsPoint point3;
            public clsPoint point4=new clsPoint();
            double l1,l2;
            public clsRect(clsPoint point1,clsPoint point2)
            {
                this.point1 = point1;
                this.point2.setPoint(point1.x, point2.y);
                this.point3 = point2;
                this.point4.setPoint(point2.x, point1.y);
                this.l1=Math.Abs ( point2.x-point1.x);
                this.l2=Math.Abs ( point2.y-point1.y);
            }
           public double girth()        //周长
           {
                return ((this.l1 + this.l2) * 2);
           }
           public double area()        //面积
           {
               return (this.l1 * this.l2);
           }
        }

       //*********************派生类clsPark*********************************
           public class clsPark : clsPoint
           {
               public clsPoint PointOne;
               public clsPoint PointTwo;

               public clsPark(clsPoint _PointOen, clsPoint _PointTwo)
               {
                   this.PointOne = _PointOen;
                   this.PointTwo = _PointTwo;
               }

               double dbradii;

               public double radiiTwo()  // 半径
               {
                   clsLine line1 = new clsLine(this .PointOne ,this .PointTwo );
                   double Radii=line1.longness ();
                   this.dbradii = Radii;
                   return Radii;
               }

               public double girthTwo()  //周长
               {
                   double Cr = Math.PI * this.dbradii * 2;
                   return Cr;
               }

               public double areaTwo()  //面积
               {
                   double Sr = Math.PI * this.dbradii * this.dbradii;
                   return Sr;
               }
               
               
           }

    }

    class Program
    {
        static void Main(string[] args)
        {
            //计算线段的长度
            clsPoint point1 = new clsPoint();
            clsPoint point2 = new clsPoint();
            point1.setPoint(0, 0);
            point2.setPoint(10, 10);
            clsPoint .clsLine line1 = new clsPoint .clsLine(point1, point2); //此处把实例对象作为参数进行传递
            Console.Write("由");
            line1.Point_start.display();
            line1.Point_end.display();
            Console.WriteLine("构成的线段的长度为：{0:f}", line1.longness()); //线段的长度

            //计算矩形周长和面积
            clsPoint.clsRect rect1 = new clsPoint.clsRect(point1, point2); //此处把实例对象作为参数进行传递
            Console.Write("由");
            rect1.point1.display();
            rect1.point2.display();
            rect1.point3.display();
            rect1.point4.display();
            Console.WriteLine("构成的矩形周长为：{0:f}，面积为：{1:f}", rect1 .girth (), rect1.area());

            //计算圆的半径、周长和面积
            clsPoint.clsPark park1= new clsPoint .clsPark (point1,point2);
            Console.Write("由");
            park1.PointOne.display();
            park1.PointTwo.display();
            Console.WriteLine("构成的圆的半径为：{0:f}，周长为：{1:f}，面积为：{2:f}", park1.radiiTwo (),park1 .girthTwo (),park1.areaTwo ());

            Console.ReadKey();
        }
    }
}

感谢信
--------
感谢各位大侠的帮忙！