C# language: why WeakReference or Weak Event Patte

问题:

I'm reading "The C# Language", 4th edition, it talks about WeakReference and Weak Event Pattern:

CHRISTIAN NAGEL: Memory leaks often result from wrong usage of events. If client objects attach to events but do not detach from them, and the reference to the client object is no longer used, the client object still cannot be reclaimed by the garbage collector because the reference by the publisher remains. This can be avoided by (1) detaching of events when the client object is no longer used, (2) a custom implementation of the add and remove accessors using the WeakReference class holding the delegate, or (3) the Weak Event pattern that is used by WPF with the IWeakEventListener interface.

I have doubts here: Option "(2) WeakReference" brings NO convenience at all, comparing to "option (1) detaching of events explictly", because using WeakReference still need explicitly calls both add and remove.

Otherwise, even if one of the event handler's object was assigned to null, the "orphan" object will still respond to the event - this will cause unexpected behavior.

Note: WeakReference only helps Garbage collection in the way that event handlers' objects will not become affected by event publisher objects; WeakReference does NOT force event handler objects get garbage collected.

Similar issue applies to Weak Event pattern, too.

Maybe this is a bit abstract, take Josh Smith's Mediator pattern (http://joshsmithonwpf.wordpress.com/2009/04/06/a-mediator-prototype-for-wpf-apps/) as example.

public class Mediator //...
{
  public void Register(object message, Action<object> callback)
  {
    // notice: Mediator has no Unregister method
  }
  public void NotifyColleagues(object message, object parameter)
  {
    // ...
  }
}
public class ObjectA //...
{
  public string ObjectAText
  {
    get { return _objectAText; }
    set
    {
      //...
      _mediator.NotifyColleagues(MediatorMessages.ObjectASaidSomething, _objectAText);
    }
  }
}
public class ObjectB //...
{
  //...
  public ObjectB(Mediator mediator)
  {
    //...
    _mediator.Register(
      MediatorMessages.ObjectASaidSomething,
      param =>
      {
        // handling event ObjectASaidSomething
      });
  }
}

If we have

ObjectA objectA = new ObjectA();
ObjectB objectB1st = new objectB();
objectA.ObjectAText = "John"; // objectB1st will respond to this event.

objectB1st = null; // due to delay of garbage collection, the object is actually still in memory

ObjectB objectB2nd = new objectB();
objectA.ObjectAText = "Jane"; // both objectB1st and objectB2nd will respond to this event!

Wouldn't the last line caused an unexpected behavior, due to the WeakReference?

But if the Mediator class provides "Unregister" method (actually I implemented one), "option (2) WeakReference" will be no difference to "option (1) detaching of events explictly". (Mediator itself is still a useful pattern, that can penetrate hierarchy of WPF or MVVM component layers )

回答1:

If I understand what you are asking, then there is a need for some clarification.

Otherwise, even if one of the event handler's object was assigned to null, the "orphan" object will still respond to the event - this will cause unexpected behavior.

Not really. This is not unexpected behavior. It is totally expected for the object to be called, if you do not unregister it explicitly.

The whole idea of the weak events is a safety net for not keeping objects in memory only because they are subscribed to an event. It has nothing to do with unregistering the object from the event when it goes out of scope.

If you need to do the later, either use IDisposable pattern and "using" construct for the subscribers, or do explicit unsubscribe.

I.e. weak events are solution for a very specific problem - to allow garbage collection of objects, which were subscribed to a long living object (like GUI or some static class).

Weak events are not about automatic unsibscribing from an even in the moment the object goes out of scope.

回答2:

If the event subscriber and publisher both cooperate, it is possible to implement a reasonable weak-event pattern in .net without need for Reflection or other CLR tricks. It would be possible for an event subscriber to implement a weak-event pattern unilaterally if events' unsubscribe methods were required to function correctly if called by the finalizer thread, but unfortunately such an expectation is not reasonable when subscribing to events from an unknown class (e.g. an INotifyPropertyChanged). The trick would be for anyone who was really "interested" in an object to hold a strong reference to a wrapper, and for event handlers and other things to hold a reference to the object's "guts". The wrapper could hold a reference to both the guts and to an object with a Finalize method that would unsubscribe the event.