CODEWITHSUNDEEP
vb.neteventsevent-handling
SqlRyan
SqlRyan

Reputation: 33914

Determine if an event has been attached to yet

I have two objects - one that contains some code with will fire an event, and one that contains the handler for that event. I can't "AddHandler" in the Load of the first object, because an instance of the second object doesn't exist yet. When I raise my event, I want to check to see if a copy of object2 has been instantiated (easy to do), and if a handler has been attached to the event yet (not sure how to do this).

I'm also open to another recommendation about how to do this instead. If I do my AddHandler in Object1.Load, and Object2 doesn't exist yet, then it will never handle my event, even if I create it later. Right now, in the code that fires the event, I've just resorted to doing a RemoveHandler and then an AddHandler every single time the event is raised, and then I know I'll attach when the object finally exists, but I know this is a crappy method.

I saw an article about something similar (Determine list of event handlers bound to event), and maybe I'm missing something in the translation, but I can't get the code to work on my custom event in VB.NET.

Upvotes: 9

Views: 16925

Answers (4)

Joshua Whitley
Joshua Whitley

Reputation: 1186

According to the responses here: http://social.msdn.microsoft.com/Forums/en-US/netfxbcl/thread/9ec8ff1c-eb9b-4cb3-8960-9cd4b25434f2 (which seem to work according to my testing), a check for existing event handlers is done upon calling RaiseEvent. If you do not want to raise an event and just need to check to see if any handlers are attached, you can check the value of a hidden variable called <your_event_name>Event like:

Public Event Foo As ActionFoo

If FooEvent IsNot Nothing Then...

Upvotes: 1

Jon
Jon

Reputation: 2509

VB.Net creates a special private member variable in the pattern of <YourEvent>Event that you can then use to test against Nothing.

Public Event MyClick As EventHandler

Private Sub OnMyClick()
    If MyClickEvent IsNot Nothing Then
        RaiseEvent MyClick(Me, New EventArgs())
    Else
        ' No event handler has been set.
        MsgBox("There is no event handler. That makes me sad.")
    End If
End Sub

http://blogs.msdn.com/b/vbteam/archive/2009/09/25/testing-events-for-nothing-null-doug-rothaus.aspx

Upvotes: 25

NastyNateDoggy
NastyNateDoggy

Reputation: 292

You could also just have a bool field that you check before hooking the event.

if not eventHooked then
 addhandler
 eventHooked = true
end if

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Upvotes: 2

StriplingWarrior
StriplingWarrior

Reputation: 156524

If you just want to know whether any handler has been attached, you should be able to check whether the event is null.

if (MyButton.Click == null)
{
    MyButton.Click += myEventHandler;
}

(I'll let you translate that into VB)

Upvotes: 0

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