In what cases are detaching from events necessary?

Question

I\'m not sure if I\'m entirely clear on the implications of attaching to events in objects.

This is my current understanding, correct or elaborate:

1

Answer 1

The relevant case where you have to unsubscribe from an event is like this:

public class A
{
    // ...
    public event EventHandler SomethingHappened;
}

public class B
{
    private void DoSomething() { /* ... */ } // instance method

    private void Attach(A obj)
    {
       obj.SomethingHappened += DoSomething();
    }
}

In this scenario, when you dispose of a B, there will still be a dangling reference to it from obj's event handler. If you want to reclaim the B's memory, then you need to detach B.DoSomething() from the relevant event handler first.

You could run into the same thing if the event subscription line looked like this, of course:

obj.SomethingHappened += someOtherObject.Whatever.DoSomething();

Now it's someOtherObject that's on the hook and can't be garbage collected.