Does WithEvents in Visual Basic keep its EventHandlers when changing the reference to the object?

Question

Does WithEvents in Visual Basic keep its EventHandlers when changing the reference to the object?

Say I have declared a button, firing events:

Private WithEvents _MyButton

Now I am subscribing to an event handler:

Private Sub _MyButton_Click() Handles _MyButton.Click 
  ' Here I DoClick()
End Sub

Will the DoClick() function be still called when I change the instance of the button object like shown below?

_MyButton = New Button()

Yes, it will. When the reference is set, the handlers are automatically added and/or removed as necessary. — Steven Doggart, Mar 11 '13 at 12:12

score 3 · Accepted Answer · answered Mar 11 '13 at 12:06

This got me curious so I wrote a small console application to visualize what happens if you do this experiment using a timer:

Private WithEvents _t As New Timers.Timer With {.Enabled = True}
Private Sub _t_Elapsed(sender As Object, e As Timers.ElapsedEventArgs) Handles _t.Elapsed
    Console.WriteLine("tick")
End Sub

Sub Main()
    ' let it tick for 5 seconds
    Task.Delay(5000).Wait()

    ' destroy the current timer
    Console.WriteLine("destroying this timer")
    _t.Dispose()
    _t = Nothing

    ' add a little pause
    Task.Delay(1000).Wait()

    ' create a new timer
    Console.WriteLine("creating a new timer")
    _t = New Timers.Timer With {.Enabled = True}

    ' let it tick for 5 seconds
    Task.Delay(5000).Wait()

End Sub

If you run this code, you will find that it does indeed attach the event handler when the instance of _t is replaced. I don't know how it does that, but the magic probably lies in the Handles keyword. Anyhow, the answer is yes.

Does WithEvents in Visual Basic keep its EventHandlers when changing the reference to the object?

1 Answers1