Colleagues.cs

public class Colleague1 : Colleague
{
  public Colleague1(Mediator mediator) : base(mediator) { }
  
  public override void HandleNotification(string message){
    Console.WriteLine($"Colleague1 receives message:{message}");
  }
}

public class Colleague2 : Colleague
{
  public Colleague1(Mediator mediator) : base(mediator) { }
  
  public override void HandleNotification(string message){
    Console.WriteLine($"Colleague2 receives message:{message}");
  }
}