Can an event listener be limited to only having one subscriber?

Question

Is it possible to stop multiple subscribers from subscribing to an event?

I have created a quick example snippet to give my question some context but unfortunately I can't test it right now because I'm not at my VS machine.

The goal is to:

• Return an empty list if there are no subscribers.
• Return the return value of a single subscriber.
• Throw an exception if more than one subscriber tries to subscribe to the event (this is the crux of the matter).

Is this possible?

public delegate List<IBaseWindow> GetWindowListDelegate();
public static event GetWindowListDelegate GetWindowListEvent;

public List<IBaseWindow> GetWindowList() {

    if (GetWindowListEvent == null) {
        return new List<IBaseWindow>();
    }

    return GetWindowListEvent();
 }

---

Note: I'm using .NET 3.5 sp1.

Answer 1

You could use event accessors to accomplish this. Something like the following:

  private EventHandler _h;
  public event EventHandler H {
      add {
         if (...) { // Your conditions here.
                    // Warning (as per comments): clients may not
                    // expect problems to occur when adding listeners!
           _h += value;
         }
      }
      remove {
         _h -= value;
      }
  }

As Andrew points out, you don't really need events to accomplish this. Is there some particular reason you need them?