Subscribe event to event in C#?

Question

Is it possible in C# to connect one event to another so emitting first event will emit second one? The only way i can see is to create a stub function that will emit second event and connect the first event to this function. Compiler do not want to connect event to event or event to anonymouse function / lambda that calls another event:

class Ui { public event EventHandler OnClick; }
class Logic { public event EventHandler OnExit; }
var ui = new Ui();
var logic = new Logic();
ui.OnClick += logic.OnExit; // Not working.
ui.OnClick += ( a, b ) => logic.OnExit; // Not working either :(.

Maybe it's some decorator available or some black magic that allows to chain events without stub functions?

Answer 1

You cannot do this, because you generally cannot do anything to an event from outside the object which owns it except for adding and removing handlers. In particular, you cannot list the existing registered handlers, and you cannot raise it. In your case, "copying" the event is essentially the same thing in disguise, and would allow you to circumvent this restriction; therefore, it's not allowed.

See this recent answer of mine for a more in-depth explanation of why things are the way they are - I just don't feel like retyping it all here.

For your particular case, if you own both classes, the workaround is to make them cooperate specifically - make Ui be aware of the associated Logic instance, and add event handlers to Logic.OnClick in Ui.OnClick.add implementation. Of course, this introduces coupling; you can reduce it to some extent by using more generic interfaces, but you can't get rid of it entirely.

As a side note, OnClick is not a good name for a .NET event. Common naming guide says that it should be simply Click (and OnClick should be the name of a protected virtual method that raises it).

Answer 2

You could do this by hiding the underlying event for Logic and then controlling calls to Add/Remove which require a UI parameter.

public class UI {
  public EventHandler OnClick;
}
public class Logic {
  private event EventHandler _onExit;
  public void AddOnExit(UI ui, EventHandler e) {
    ui.OnClick += e;
    _onExit += e;
  }
  public void RemoveOnExit(UI ui, EventHandler e) {
    ui.OnClick -= e;
    _onExit -= e;
  }
}