How do I Unregister 'anonymous' event handler [duplicate]

Say if I listen for an event:

Subject.NewEvent += delegate(object sender, NewEventArgs e)
{
    //some code
}); 

Now how do I un-register this event? Or just allow the memory to leak?

Solution 1:

Give your instance of the anonymous delegate a name:

EventHandler<NewEventArg> handler = delegate(object sender, NewEventArgs e)
{
    //some code
};

Subject.NewEvent += handler;
Subject.NewEvent -= handler;

Solution 2:

If you need to unregister an event, I recommend avoiding anonymous delegates for the event handler.

This is one case where assigning this to a local method is better - you can unsubscribe from the event cleanly.

Solution 3:

To remove the handler on first invocation:

//SubjectType Subject = ..... already defined if using (2)

EventHandler handler = null;
handler = delegate(object sender, EventArgs e)
{
    // (1)
    (sender as SubjectType).NewEvent -= handler;
    // or
    // (2) Subject.NewEvent -= handler;

    // do stuff here
};

Subject.NewEvent += handler;

Solution 4:

You can create method for unregistering from all listeners of event. This not exactly what you whant, but sometimes it can be helpfull. For example (this really works =)) :

    class Program {
    static void Main(string[] args) {
        A someClass = new A();
        someClass.SomeEvent += delegate(object sender, EventArgs e) {
            throw new NotImplementedException();
        };

        someClass.ClearEventHandlers();
        someClass.FireEvent();

        Console.WriteLine("No error.");
    }

    public class A {
        public event EventHandler SomeEvent;

        public void ClearEventHandlers() {
            Delegate[] delegates = SomeEvent.GetInvocationList();
            foreach (Delegate delegate in delegates) {
                SomeEvent -= (EventHandler) delegate;
            }
        }

        public void FireEvent() {
            if (SomeEvent != null) {
                SomeEvent(null, null);
            }
        }
    }
}