using System;

public class Program

{

    public static void Main()

    {

        IFoo smth = new SomeFooClass();

        Console.WriteLine(smth.Square(5));

    }

}

​

public interface IFoo

{

    public int Square(int x)

    {

        return x*x;

    }

}

​

public class SomeFooClass: IFoo

{

}