ConcreteMediator.cs

public class ConcreteMediator : Mediator
{
  public Colleague1 Colleague1 { get; set; }
  public Colleague2 Colleague2 { get; set; }
  
  public override void Send(string message, Colleague colleague){
    if(colleague == Colleague1){
      Colleague2.HandleNotification(message);
    }
    else{
      Colleague1.HandleNotification(message);
    }
  }
}