cleancode.cs

using System;
using Xunit;

public class HelloWorldTests {
  [Fact]
  public void ShouldSayHello {
    var listener = new Listener();
    var helloWorld = new HelloWorld();
    helloWorld.SayHello(listener.Listen);
    Asser.Equal("Hello world", listener.ReceivedMessage)
  }
  
  class Listener {
    public void Listen(string message) {
      ReceivedMessage = message;
    }
    
    public string ReceivedMessage { get; set; }
  }
}

public class HelloWorld {
  public void SayHello(Action<string> listener) {
    listener("Hello World");
  }
}

static class Program {
  public static void Main() {
    var helloWorld = new HelloWorld();
    helloWorld.SayHello(Console.WriteLine);
  }
}

cleancode.fsx

printf "HelloWorld";;