<<< Abstraction: Interface

using System;

namespace CsharpInterface {

  interface IPolygon {

    // method without body

    void calculateArea(int l, int b);

  }

  class Rectangle : IPolygon {

    // implementation of methods inside interface

    public void calculateArea(int l, int b) {

      int area = l * b;

      Console.WriteLine("Area of Rectangle: " + area);

    }

  }

  class Program {

    static void Main (string [] args) {

      Rectangle r1 = new Rectangle();

      r1.calculateArea(100, 200);

    }

  }

}

Area of Rectangle: 20000