using System;

​

// parent class

public class Shape

{

    // fields and properties:

    // A few example properties

    public int X { get; set; }

    public int Y { get; set; }

    public int Height { get; set; }

    public int Width { get; set; }

    // methods:

    // Virtual method (on the base class) -- virtual makes it possible to overwrite this method

    public virtual void Draw()

    {

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

    }

}

​

// child class

class Circle : Shape            //Circle extends Shape

{

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

    {

        // Code to draw a circle...

        Console.WriteLine("Drawing a circle");

        base.Draw(); // (NEW) keyword BASE lets you call the original virtual method; brings you back up to parent class

    }

}

​

// child class #2

class Rectangle : Shape

{

    public override void Draw()

    {

        // Code to draw a rectangle...

        Console.WriteLine("Drawing a rectangle");

        base.Draw();

    }

}

​

// child class #3

class Triangle : Shape

{

    public override void Draw()

    {

        // Code to draw a triangle...

        Console.WriteLine("Drawing a triangle");

        base.Draw();

    }

}

​

// sort of NEW part: 

public class Program

{

    public static void Main()

    {

        // Polymorphism at work #1: a Rectangle, Triangle and Circle can all be used wherenever 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 = list; Shape = class created at the start

        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("\n");

        // Polymormpishm at work #3: Trying to call the virtual method directly

        shapes[0].Draw();  // recall the first element [0] is a rectangle

        // Vs.

        ((Rectangle)shapes[0]).Draw();

        ((Shape)shapes[0]).Draw();

​

        // This also works outside of the Shapes List!

        Console.WriteLine("\n");

        Shape s1 = new Rectangle();

        s1.Draw();

    }

}