using System;

​

public class Program

{

  public interface IShape {

    void Draw();

    double Area();

  }

​

  class Circle : IShape {

    public double Radius { get; set; }

​

    public Circle(double radius) {

      Radius = radius;

    }

​

    public void Draw() {

      // draw a circle!

    }

    public double Area() {

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

    }

  }

​

  public static void Main() {

    IShape c = new Circle(5);

    Console.WriteLine(c.Area());

    // 78.5398163397448

  }

}