程序代码：

 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Square s = new Square();
            s.Length = 2;
            Pentagon p = new Pentagon();
            p.Length = 5;
            MessageBox.Show(s.GetPeri() + "\n" + p.GetPeri());
        }
    }

    public class Polygen
    {
        private int length = 0;
        private int sides = 0;
       
        public int Length
        {
            get { return length; }
            set { length = value; }
        }

        public int Sides
        {
            get { return sides; }
            set { sides = value; }
        }

        public Polygen()
        {
            Length = 1;
        }

        public virtual string GetPeri()
        {
            return "This is a virtual method.";
        }
    }

    public class Square : Polygen
    {
        public Square()
        {
            Sides = 4;
        }

        public override string GetPeri()
        {
            return "The perimeter of the Polygon is " + Sides * Length;
        }
    }

    public class Pentagon : Polygen
    {
        public Pentagon()
        {
            Sides = 5;
        }

        public override string GetPeri()
        {
            return "The perimeter of the Polygon is " + Sides * Length;
        }
    }