Correct usage of Action and Events

Question

I am a bit new to c# so please overlook if you find it trivial. I saw the following "weird" code.

Can anybody shed a bit of light on it.

public event Action _action;

if (_action != null)            
{
    foreach (Action c in _action.GetInvocationList())
    {
         _action -= c;
    }
}

Specially the _action -= c; part.

Take a look at this SO question: http://stackoverflow.com/questions/447821/how-do-i-unsubscribe-all-handlers-from-an-event-for-a-particular-class-in-c — Ian Gilroy, Mar 13 '13 at 17:51
Check out http://stackoverflow.com/questions/1431359/event-action-vs-event-eventhandler for an overview of Action vs EventHandler — PHeiberg, Mar 13 '13 at 18:04

score 23 · Accepted Answer · answered Mar 13 '13 at 17:52

A delegate can be a delegate to more than one function. If you have a delegate alpha that delegates to Alpha() and a delegate beta that delegates to Beta() then gamma = alpha + beta; is a delegate that calls Alpha() then Beta(). gamma - beta produces a delegate that calls Alpha(). It's a bit of a weird feature, to be perfectly frank.

The code you've posted is bizarre. It says "go through the list of functions in action, produce a whole pile of delegates that invoke fewer and fewer functions, and then finally assign a delegate that does nothing to action. Why on earth would anyone do this? Just assign null to action and be done with it.

score 6 · Answer 2 · answered Mar 13 '13 at 17:51

6

public event Action _action; //an event


if (_action != null) // are there any subscribers?

{
        foreach (Action c in _action.GetInvocationList()) //get each subscriber
        {
            _action -= c; //remove its subscription to the event
        }
}

answered Mar 13 '13 at 17:51

PHeiberg

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Thanks for the reply. `public event Action _action;` is this bit ok? Meaning shouldnt Action take parameters from 1 to 16. Shouldnt it be like `public event Action<..> _action;` – saam Mar 13 '13 at 18:03
If you need some kind of argument to the action, yes. But there are two versions of the Action class in the BCL. The non-generic you see here (Action) and the generic (Action<>). I'd however recommend to go with the standard event convention of using event EventHandler<>. See my comment on your question. – PHeiberg Mar 13 '13 at 18:07

score 3 · Answer 3 · answered Mar 13 '13 at 17:49

3

It's removing the handlers for the action.

answered Mar 13 '13 at 17:49

msarchet

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that's an unhelpful answer – Sam I am says Reinstate Monica Mar 13 '13 at 17:50
5

@SamIam: Is it? It is exactly what the OP asked for. – Ed S. Mar 13 '13 at 17:51
2

@SamIam - yup - exactly what it does. – Mark Segal Mar 13 '13 at 17:52
1

@SamIam: Really? OP can always follow up if clarification is required. – Pieter Geerkens Mar 13 '13 at 17:52
Just because it's true doesn't mean it's helpful. – D Stanley Mar 13 '13 at 17:53
5

Your answer is about as valuable as the answer `"yes"` in response to a question of the form `"is there a way to do x"` – Sam I am says Reinstate Monica Mar 13 '13 at 18:01
1

@SamIam: Nope, it certainly is not. A correct analogy would be answering "use named pipes" to the question "how do I communicate between processes in *nix?" Terse, but correct. OP asked "what does this line of code do?". This guy answered "it is detaching event handlers." Terse, and also correct. – Ed S. Mar 13 '13 at 18:59
1

@SamIam I answered the question exactly as asked. That is the entire point of SO. – msarchet Mar 16 '13 at 21:28
6

@msarchet The point of SO is to help people fix their programming problems. It's not trivia quiz meant for you to pad your epenis numbers. In order to understand the answer you've given, someone would already have to know the answer in the first place. – Sam I am says Reinstate Monica Mar 16 '13 at 21:45

score 2 · Answer 4 · answered Mar 13 '13 at 17:52

An event is actually a MultiCastDelegate. When you "attach" an event handler, it adds a reference to it's InvocationList.

The code above is detaching each event handler in the InvocationList from the event - essentially "clearing" the event, which could also be done by saying _action = null.

Correct usage of Action and Events

4 Answers4