Does `Control.Dispose()` remove all event registrations?

Question

When I call Dispose() on a System.Windows.Forms.Control, does it automatically remove all event registrations?

So for example, is the following sufficient:

var button = new Button();
button.Click += someObject.clickHandler;
// ...
button.Dispose();

Or do I need to unregister the event handler explicitly, like this:

var button = new Button();
button.Click += someObject.clickHandler;
// ...
button.Click -= someObject.clickHandler;
button.Dispose();

Annoyingly, the model that `Component` (and therefore `Control`) use for most of their events (via `EventHandlerList`) would actually have made this very achievable. It is interesting that they chose not to do this; all they would have to do is set the `events` field to `null` at the end of `Dispose(bool disposing)` — Marc Gravell, Sep 19 '14 at 12:56
Yeah I saw the `EventHandlerList` field in `Component` and was therefore kind of expecting that they use it in `Dispose(bool)` to unsubscribe all event handlers - but it isn't mentioned in the documentation, that's why I had to ask here... Annoying indeed! — Robert Hegner, Sep 19 '14 at 13:06

score 4 · Answer 1 · answered Sep 19 '14 at 12:49

Calling Dispose() does not explicitly remove event handlers, but it does result in those events no longer being sent. When the garbage collector reclaims the memory from button, the even registrations will be finally removed.

The more important situation to consider is when button is listing for events from other objects:

var button = new Button();
otherObject.SomeEvent += button.HandleSomeEvent;
// ...
button.Dispose();

In this case, the registered event handler can actually prevent button from getting garbage collected. If this situation applies to your code, you'll need to either remove the relevant event listeners in the Dispose() method, or use a weak event listener pattern to allow the garbage collector to clean up those handlers.

score 2 · Accepted Answer · edited May 23 '17 at 12:05

Nope, It doesn't. You ought to remove the handlers yourself. Simple test can prove that.

protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);

    var button = new Button();
    button.Click += button_Click;

    //Some code
    button.Dispose();
    button.PerformClick();//Simulate a click event
}

void button_Click(object sender, EventArgs e)
{
    MessageBox.Show("Oops! Still alive?");
}

Run the above code, You'll see MessageBox popping up saying "Oops! Still alive?" which means handlers are not removed even when we Dispose the Control.

Refer Marc's answer here to see how this can affect Garbage collection.

Does `Control.Dispose()` remove all event registrations?

2 Answers2

Linked