Unsubscribe anonymous method in C#

Question

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

MyEvent += myDelegate;


// .... later

MyEvent -= myDelegate;

Just keep a reference to the delegate around.

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;

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.

Since C# 7.0 local functions feature has been released, the approach suggested by J c becomes really neat.

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

So, honestly, you do not have an anonymous function as a variable here. But I suppose the motivation to use it in your case can be applied to local functions.

In 3.0 can be shortened to:

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

Instead of keeping a reference to any delegate you can instrument your class in order to give the event's invocation list back to the caller. Basically you can write something like this (assuming that MyEvent is declared inside MyClass):

public class MyClass 
{
  public event EventHandler MyEvent;

  public IEnumerable<EventHandler> GetMyEventHandlers()  
  {  
      return from d in MyEvent.GetInvocationList()  
             select (EventHandler)d;  
  }  
}

So you can access the whole invocation list from outside MyClass and unsubscribe any handler you want. For instance:

myClass.MyEvent -= myClass.GetMyEventHandlers().Last();

I've written a full post about this tecnique here.