using System;

// base / parent class

public abstract class ShapesClass  // abstract class meaning it doesn't exist in reality

{

    // fields and properties

    // methods

    abstract public int Area(); // abstract method within an abstract class - > LIKE an interface

    public virtual int Perimeter()  // non abstract methood inside abstract class -> LIKE inheritance

    {

        return 1;

    }

}

​

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

{

    // fields 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()  // fullfil contract that looked like an interface in an abstract class

    {

        return side * side;

    }

    public override int Perimeter()  // even though inherited perimiter, you can override it

    {

        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();

​

    }

}

​

// interface delegates

public interface IRectangle

{

    // fields and properties

    // method

    void SpecialProperty();

}

​

// Output: Area of the square = 144