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Tip of the Day
Language: Visual Basic
Expertise: Beginner
Apr 8, 1999



Building the Right Environment to Support AI, Machine Learning and Deep Learning

byref variables in ActiveX component events

I am creating an ActiveX DLL and I want to be able to fire events that can be cancelled similiarly to cancelling the Unload event for a form. however, I notice that even though an argument (passed byRef) can be included in the event declaration, there is not an explicit way to reference the argument in the ActiveX component after the event has fired—to then process the Cancel argument. See the included code:

ActiveX component code
Option Explicit

Public Event WillFire(Cancel As Boolean)
Public Sub myMethod(Cancel As Boolean)
    RaiseEvent WillFire(False)
    MsgBox "Event has Fired"
End Sub

Client code
Option Explicit
Private WithEvents myObject As CObject

Private Sub btnExample_Click()
    Set myObject = New CObject
    myObject.myMethod False
End Sub

Private Sub myObject_MethodHasFired()
    MsgBox "Object Event has Fired"
End Sub

Private Sub myObject_MethodWillFire(Cancel As Boolean)
    'if some condition exists then
        ' set cancel flag for the object method
        Cancel = True
End Sub

My question is: How can I include a byRef argument in my Event Declaration and evaluate that same argument in the ActiveX component once the event has fired?

You must pass a ByRef variable when you raise the event, because that is the variable that is passed to the client's event procedure:

Public Event WillFire(Cancel As Boolean)
Public Sub myMethod()
Dim b as boolean
    RaiseEvent WillFire(b)
	If b=true then
	    MsgBox "Event has not Fired"
	    MsgBox "Event has Fired"
	End If
End Sub
Try enclosing the b variable in two sets of parenthesis, ((b)), this passes the variable ByVal.
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