How can I clear event subscriptions in C#?

问题:

Take the following C# class:

c1 {
 event EventHandler someEvent;
}

If there are a lot of subscriptions to c1's someEvent event and I want to clear them all, what is the best way to achieve this? Also consider that subscriptions to this event could be/are lambdas/anonymous delegates.

Currently my solution is to add a ResetSubscriptions() method to c1 that sets someEvent to null. I don't know if this has any unseen consequences.

回答1:

From within the class, you can set the (hidden) variable to null. A null reference is the canonical way of representing an empty invocation list, effectively.

From outside the class, you can't do this - events basically expose "subscribe" and "unsubscribe" and that's it.

It's worth being aware of what field-like events are actually doing - they're creating a variable and an event at the same time. Within the class, you end up referencing the variable. From outside, you reference the event.

See my article on events and delegates for more information.

回答2:

Add a method to c1 that will set 'someEvent' to null...

class c1
{
    event EventHandler someEvent;
    ResetSubscriptions() {someEvent = null;}
}

回答3:

class c1
{
    event EventHandler someEvent;
    ResetSubscriptions() {someEvent = delegate{};}
}

It is better to use delegate{} than null

回答4:

Setting the event to null inside the class works. When you dispose a class you should always set the event to null, the GC has problems with events and may not clean up the disposed class if it has dangling events.

回答5:

The best practice to clear all subscribers is to set the someEvent to null by adding another public method if you want to expose this functionality to outside. This has no unseen consequences. The precondition is to remember to declare SomeEvent with the keyword 'event'.

Please see the book - C# 4.0 in the nutshell, page 125.

Some one here proposed to use Delegate.RemoveAll method. If you use it, the sample code could follow the below form. But it is really stupid. Why not just SomeEvent=null inside the ClearSubscribers() function?

   public void ClearSubscribers ()
    {
          SomeEvent = (EventHandler) Delegate.RemoveAll(SomeEvent, SomeEvent);// Then you will find SomeEvent is set to null.
    }

回答6:

You can achieve this by using the Delegate.Remove or Delegate.RemoveAll methods.

回答7:

Conceptual extended boring comment.

I rather use the word "event handler" instead of "event" or "delegate". And used the word "event" for other stuff. In some programming languages (VB.NET, Object Pascal, Objective-C), "event" is called a "message" or "signal", and even have a "message" keyword, and specific sugar syntax.

const
  WM_Paint = 998;  // <-- "question" can be done by several talkers
  WM_Clear = 546;

type
  MyWindowClass = class(Window)
    procedure NotEventHandlerMethod_1;
    procedure NotEventHandlerMethod_17;

    procedure DoPaintEventHandler; message WM_Paint; // <-- "answer" by this listener
    procedure DoClearEventHandler; message WM_Clear;
  end;

And, in order to respond to that "message", a "event handler" respond, whether is a single delegate or multiple delegates.

Summary: "Event" is the "question", "event handler (s)" are the answer (s).

回答8:

This is my solution:

public class Foo : IDisposable
{
    private event EventHandler _statusChanged;
    public event EventHandler StatusChanged
    {
        add
        {
            _statusChanged += value;
        }
        remove
        {
            _statusChanged -= value;
        }
    }

    public void Dispose()
    {
        _statusChanged = null;
    }
}

You need to call Dispose() or use using(new Foo()){/*...*/} pattern to unsubscribe all members of invocation list.

回答9:

Remove all events, assume the event is an "Action" type:

Delegate[] dary = TermCheckScore.GetInvocationList();

if ( dary != null )
{
    foreach ( Delegate del in dary )
    {
        TermCheckScore -= ( Action ) del;
    }
}