Determine if an event has been attached to yet

Question

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

Answer 1

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)

Answer 2

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...

Answer 3

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

Answer 4

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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