MainClass.cs

interface IMediatorMessage
{

}

interface IMessageHandler<T> where T : IMediatorMessage
{
  void Handle(T message);
}

StepMessage : IMediatorMessage
{

}

Step : IMessageHandler<IMediatorMessage>
{

   public Step(IMediator mediator)
   {
     mediator.Subscribe(this);
   }

   Handle(IMediatorMessage message)
   {
     // This is the handler for the message type T
   }
}

class SimpleUseCase : UseCase
{
  List<Step> steps = new List<Step>();
  
  public UseCase(IMediator mediator)
  {
    this.mediator = mediator;
    steps.Add(new Step(mediator);
  }

  public void Publish()
  {
    mediator.Publish(new StepMessage());
  }
}

class Session
{
  private IMediator mediator;
  private List<UseCase> useCaseList = new List<UseCase>();

  public Session()
  {
    mediator = new Mediator();
    useCaseList.Add(new SimpleUseCase (mediator));
    useCaseList.Add(new SimpleUseCase (mediator));
  }
}