CODEWITHSUNDEEP
c#eventsevent-handling
Tvd
Tvd

Reputation: 4601

C# Know how many EventHandlers are set?

As we all know, we can create an EventHandler and add methods to it N number of times. Like:

   // Declare and EventHandler     
   public event EventHandler InternetConnectionAvailableEvent;
    private void OnInternetConnectionAvailableEvent()
    {
        if (InternetConnectionAvailableEvent != null)
        {
            EventHandler handle = InternetConnectionAvailableEvent;

            EventArgs e = EventArgs.Empty;
            handle(this, e);
        }
    }


   // IN OTHER CLASS WHERE I USE THE EVENT
   // Set the method name to handle the event    
   monitorInternet.InternetConnectionAvailableEvent += HandleInternetConnectionEvent;

    void HandleInternetConnectionEvent(object sender, EventArgs e)
    {
        if (syncContext != null)
        {
            MonitorInternetConnection mic = (MonitorInternetConnection)sender;
            if (mic != null)
            {
                syncContext.Post(o => InternetConnected(), null);
            }
        }
    }

   // To remove
   monitorInternet.InternetConnectionAvailableEvent -= null;

UPDATE :

   // To remove it should be 
   monitorInternet.InternetConnectionAvailableEvent -= HandleInternetConnectionEvent;  // CORRECT

Same method can be called multiple times without removing it.

If I make monitorInternet.InternetConnectionAvailableEvent -= null;, all the event handlers be removed. I mean if it is set 2-3 times, and removed only once, by making it null, also all the other methods will be removed automatically.

I believe it will, but I just wanted to confirm with you experts. While googling I didn't get my required satisfactory answer.

Please correct me if am wrong.

Upvotes: 18

Views: 15910

Answers (3)

agent-j
agent-j

Reputation: 27923

To find the number of event handlers, you can use this code:

InternetConnectionAvailableEvent.GetInvocationList().Length;

The following code demonstrates that MyEvent -= null does not clear the list of handlers.

public static event EventHandler MyEvent;

[STAThread]
static void Main()
{
   MyEvent += (s,dea) => 1.ToString();
   MyEvent -= null;

   Console.WriteLine(MyEvent.GetInvocationList().Length);
   // Prints 1
   MyEvent = null;
   Console.WriteLine(MyEvent == null);
   // Prints true
}

To clear the list (which is probably never a good idea), you can set the event to null (as long as you are in the class that declared the event).

Upvotes: 19

user195488
user195488

Reputation:

What you are describing is a field-like event. It is the same as the longhand event declaration, except no body.

From inside the class, you can set the event to null. From outside the class you cannot do this. Events follow a subscribe and unsubscribe methodology. Within the class you reference the variable, outside the class you reference the event.

See this answer by Jon Skeet on events.

Upvotes: 4

Grant Thomas
Grant Thomas

Reputation: 45083

Delegates are removed by equality, so you're not removing anything from the invocation list because nothing in the invocation list would be null.

Upvotes: 4

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