using System;

//class #1 (abstract)

public abstract class ShapesClass

{

    //no fields or properties in this class

    // abstract method (notice no curly brackets here with code)

    abstract public int Area();

    //nonabstract method (notice that here there are curly brackets and there is code)

    // why is it called virtual? (it can be either abstract or virtual)

    public virtual int Perimeter()

    {

        return 1;

    }

}

​

//class #2 (ShapesClass is a base/parent class, IRectange is a interface)

public class Square : ShapesClass, IRectangle // use the "," to inherit from and implement interfaces!

{

    // field and properties

    int side = 0;

    //constructor

    public Square(int n)

    {

        side = n;

    }

    // Area method is required to avoid

    // a compile-time error.

    public override int Area()

    {

        return side * side;

    }

    public override int Perimeter()

    {

        return 2;

    }

    // Because the Interface requires it!

    public void SpecialProperty()

    {       

        Console.WriteLine("parallel sides, opposite sides are congruent, and diagonals bisect each other");

    }

    public static void Main()

    {

        Square sq = new Square(12);

        Console.WriteLine("Area of the square = {0}", sq.Area());

        Console.WriteLine("Perimeter = {0}", sq.Perimeter());

        sq.SpecialProperty();

​

    }

}

​

// here is the interface

public interface IRectangle

{

    void SpecialProperty();

}

​

// Output: Area of the square = 144