Define an abstract class named Poly

   1: public abstract class Poly
   2:         {
   3:             private int r = 0;
   4:             public int R
   5:             {
   6:                 get
   7:                 {
   8:                     return r;
   9:                 }
  10:                 set
  11:                 {
  12:                     r = value;
  13:                 }
  14:             }
  15:             public abstract double Area();
  16:         }

CalArea class is derived from Poly calss and also overrides Area() method. 

   1: public class CalArea : Poly
   2: {
   3:     public override double Area()
   4:     {
   5:         return R * R*Math.PI;
   6:     }
   7:  
   8: }

Here is the button-click event:

   1: private void button1_Click(object sender, EventArgs e)
   2:        {
   3:            CalArea calArea = new CalArea(); //實例化衍生類
   4:            Poly poly = calArea;             //使用衍生類物件實例化抽像類
   5:            string str = textBox1.Text.Trim();
   6:            if (!string.IsNullOrEmpty(str))
   7:            {
   8:                try
   9:                {
  10:                    poly.R = int.Parse(str);
  11:                    textBox2.Text = poly.Area().ToString();
  12:                }
  13:                catch (Exception exc)
  14:                {
  15:                    MessageBox.Show(exc.Message);
  16:                }
  17:            }
  18:        }

Add an interface to Poly (abstract class), in which a string value should be return when calling QueryArea method

   1: interface IArea
   2:       {
   3:           string QueryArea();
   4:       }
   5:       public abstract class Poly : IArea
   6:       {
   7:           private int r = 0;
   8:           public int R
   9:           {
  10:               get
  11:               {
  12:                   return r;
  13:               }
  14:               set
  15:               {
  16:                   r = value;
  17:               }
  18:           }
  19:           public abstract double Area();
  20:           public string QueryArea()          // 實作IArea
  21:           {
  22:               return Area().ToString();
  23:           }
  24:  
  25:       }

And button click event is shown below

   1: private void button1_Click(object sender, EventArgs e)
   2: {
   3:     CalArea calArea = new CalArea(); //實例化衍生類
   4:     Poly poly = calArea;             //使用衍生類物件(等號右邊)實例化抽像類(等號左邊)
   5:     string str = textBox1.Text.Trim();
   6:     if (!string.IsNullOrEmpty(str))
   7:     {
   8:         try
   9:         {
  10:             poly.R = int.Parse(str);               // 相當於執行 calArea.R
  11:             textBox2.Text = poly.QueryArea();      // 取得面積字串, 與下一行程式碼等效 
  12:             //textBox2.Text = calArea.QueryArea();   
  13:         }
  14:         catch (Exception exc)
  15:         {
  16:             MessageBox.Show(exc.Message);
  17:         }
  18:     }
  19: }
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