using System;

public class Shape

{

    // A few example properties

    public int X { get; private set; }

    public int Y { get; private set; }

    public double Height { get; set; }

    public double Width { get; set; }

    public double Area

    {

        get

        {

            return CalculateArea();

        }

    }

    protected virtual double CalculateArea()

    {

        return 2;

    }

    public virtual int GetNumberOfSides()

    {

        return 1;

    }

    // Virtual method (on the base class)

    public virtual void Draw()

    {

        Console.WriteLine("Performing base class drawing tasks");

    }

}

​

public class Circle : Shape             //Circle extends Shape

{

    public double Radius{get; set;}

    public Circle(double r)

    {

        r = Radius;

    }

    public override void Draw() //keyword OVERRIDE make sure the compiler knows this is the new implementation for Circle

    {

        // Code to draw a circle...

        Console.WriteLine("Drawing a circle");

        base.Draw();            //keyword BASE lets you call the original virtual method        

    }

    protected override double CalculateArea()

    {

        return Math.PI*Math.Pow(Radius,2);

    }

}

public class Rectangle : Shape

{

    public override void Draw()

    {

        // Code to draw a rectangle...

        Console.WriteLine("Drawing a rectangle");

        base.Draw();

    }

    protected override double CalculateArea()

    {

        return Height*Width;

    }

}

public class Triangle : Shape

{

    public override void Draw()

    {

        // Code to draw a triangle...

        Console.WriteLine("Drawing a triangle");

        base.Draw();

    }

}

​

public class Program

{

    public static void Main()

    {

        // Polymorphism at work #1: a Rectangle, Triangle and Circle

        // can all be used whereever a Shape is expected. No cast is

        // required because an implicit conversion exists from a derived 

        // class to its base class.

        System.Collections.Generic.List<Shape> shapes = new System.Collections.Generic.List<Shape>();

        shapes.Add(new Rectangle());

        shapes.Add(new Triangle());

        shapes.Add(new Circle());   

        // Polymorphism at work #2: the virtual method Draw is

        // invoked on each of the derived classes, not the base class.

        foreach (Shape s in shapes)

        {

            s.Draw();

            Console.WriteLine("Area =" + s.Area.ToString());

        }

    }

}