C# Explicitly Removing Event Handlers

13

I was wondering if setting an object to null will clean up any eventhandlers that are attached to the objects events...

e.g.

Button button = new Button();
button.Click += new EventHandler(Button_Click);
button = null;

button = new Button();
button.Click += new EventHandler(Button_Click);
button = null;

etc...

Will this cause a memory leak?

Damien

출처 c# events memory-leaks garbage-collection

3 답변

16

If there are no other references to button anywhere, then there is no need to remove the event handler here to avoid a memory leak. Event handlers are one-way references, so removing them is only needed when the object with events is long-lived, and you want to avoid the handlers (i.e. objects with handler methods) from living longer than they should. In your example, this isn't the case.

  • Does this mean event handlers for application bars doesnt need explicit removing aswell (statechanged for example)? - Hans Petter Naumann
  • This is a reverse situation. AppBar is a long-lived object, and an event handler would, at the minimum, keep the object the delegates refers to alive for as long as AppBar is alive, and you might not want that to happen. On the other hand, if the handler is a method on the window that owns the bar, it doesn't matter. - Pavel Minaev
11

Summary: You need to explicitly unsubscribe when the event source/publisher is long-lived and the subscribers are not. If the event source out-lives the subscribers, all registered subscribers are kept "alive" by the event source (not collected by the GC) unless they unsubscribe (and remove the reference to themselves from the event publisher's notification list)

Also this is a duplicate of Is it necessary to explicitly remove event handlers in C# and has a good title n answer. So voting to close.

  • I saw that Q&A. My question is far more straight forward and requires a much simpler answer. - Damien
8

See the discussion here under "The final question: do we have to remove event handlers?"

Conclusion: you should remove delegates from events when they reach outside the class itself; i.e. when you subscribe to external events, you should end your subscription when you are done. Failing to do so will keep your object alive longer than necessary.

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