Unsubscribe anonymous method in C#

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?

Answers


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.


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.


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.


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.


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.


One simple solution:

just pass the eventhandle variable as parameter to itself. Event if you have the case that you cannot access the original created variable because of multithreading, you can use this:

MyEventHandler foo = null;
foo = (s, ev, mehi) => MyMethod(s, ev, foo);
MyEvent += foo;

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

if you want refer to some object with this delegate, may be you can use Delegate.CreateDelegate(Type, Object target, MethodInfo methodInfo) .net consider the delegate equals by target and methodInfo


If the best way is to keep a reference on the subscribed eventHandler, this can be achieved using a Dictionary.

In this example, I have to use a anonymous method to include the mergeColumn parameter for a set of DataGridViews.

Using the MergeColumn method with the enable parameter set to true enables the event while using it with false disables it.

static Dictionary<DataGridView, PaintEventHandler> subscriptions = new Dictionary<DataGridView, PaintEventHandler>();

public static void MergeColumns(this DataGridView dg, bool enable, params ColumnGroup[] mergedColumns) {

    if(enable) {
        subscriptions[dg] = (s, e) => Dg_Paint(s, e, mergedColumns);
        dg.Paint += subscriptions[dg];
    }
    else {
        if(subscriptions.ContainsKey(dg)) {
            dg.Paint -= subscriptions[dg];
            subscriptions.Remove(dg);
        }
    }
}

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