using System;

​

public class Program

{

    public static void Main()

    {

        var echoMachine1 = new EchoMachine();

        echoMachine1.Echo();

        IEcho echoMachine2 = echoMachine1;

        echoMachine2.Echo();

        Echo(echoMachine1);

        Echo(echoMachine2);

    }

​

    public static void Echo(IEcho echoMachine)

    {

        echoMachine.Echo();

    }

}

​

public interface IEcho

{

    void Echo(string source = "interface");

}

​

public class EchoMachine : IEcho

{

    public void Echo(string source = "implementation")

    {

        Console.WriteLine($"Hello from the {source}!");

    }

}