Unsubscribe anonymous method in C#

Question

Is it possible to unsubscribe an anonymous method from an event?

If I subscribe to an event like this:

void MyMethod()
{
    Console.WriteLine("I did it!");
}

MyEvent += MyMethod;

I can un-subscribe like this:

MyEvent -= MyMethod;

But if I subscribe using an anonymous method:

MyEvent += delegate(){Console.WriteLine("I did it!");};

is it possible to unsubscribe this anonymous method? If so, how?

Answer 1

From memory, the specification explicitly doesn't guarantee the behaviour either way when it comes to equivalence of delegates created with anonymous methods.

If you need to unsubscribe, you should either use a "normal" method or retain the delegate somewhere else so you can unsubscribe with exactly the same delegate you used to subscribe.

Answer 2

Kind of lame approach:

public class SomeClass
{
  private readonly IList<Action> _eventList = new List<Action>();

  ...

  public event Action OnDoSomething
  {
    add {
      _eventList.Add(value);
    }
    remove {
      _eventList.Remove(value);
    }
  }
}

1. Override the event add/remove methods.
2. Keep a list of those event handlers.
3. When needed, clear them all and re-add the others.

This may not work or be the most efficient method, but should get the job done.

Answer 3

var myDelegate = delegate(){Console.WriteLine("I did it!");};

MyEvent += myDelegate;


// .... later

MyEvent -= myDelegate;

Just keep a reference to the delegate around.

Answer 4

One technique is to declare a variable to hold the anonymous method which would then be available inside the anonymous method itself. This worked for me because the desired behavior was to unsubscribe after the event was handled.

Example:

MyEventHandler foo = null;
foo = delegate(object s, MyEventArgs ev)
    {
        Console.WriteLine("I did it!");
        MyEvent -= foo;
    };
MyEvent += foo;

Answer 5

In 3.0 can be shortened to:

MyHandler myDelegate = ()=>Console.WriteLine("I did it!");
MyEvent += myDelegate;
...
MyEvent -= myDelegate;

Answer 6

If you want to be able to control unsubscription then you need to go the route indicated in your accepted answer. However, if you are just concerned about clearing up references when your subscribing class goes out of scope, then there is another (slightly convoluted) solution which involves using weak references. I've just posted a question and answer on this topic.