How to Removehandler event? - vb.net

Question

I had difficulty deleting an event
, I only call contextmenu temporarily (assigning the event and after the event has finished I no longer use it), and so on for each call.
sub register()
Dim f_Cm As Windows.Forms.ContextMenuStrip = New System.Windows.Forms.ContextMenuStrip(Me.components)
AddHandler f_Cm.Closed, Sub() f_Cm_Closed(f_Cm)
end sub
'mycode1
Private Sub f_Cm_Closed(f_Cm As Windows.Forms.ContextMenuStrip)
'....mycode
RemoveHandler f_Cm.Closed, Sub() f_Cm_Closed(f_Cm)
End Sub
'mycode2
Private Sub f_Cm_Closed(f_Cm As Windows.Forms.ContextMenuStrip)
'....mycode
Dim e1 As ToolStripDropDownClosedEventArgs = address of f_Cm_Closed(f_Cm)
RemoveHandler f_Cm.Closed, e1
End Sub
Do I need to delete them in this case? and how to do this?
Thanks you!

Answer 1

Sub() f_Cm_Closed(f_Cm) is what's called a lambda expression. Lambda expression are basically methods without a name; they're useful shortcuts in some situations. What you are doing in the code Sub() f_Cm_Closed(f_Cm) is creating a new, nameless method, which then calls f_Cm_Closed(f_Cm).
This isn't what you want, you want to pass a reference directly to your handler so you can remove it later. For that, you use AddressOf.
Before you can do that, the method signatures will have to match. So
Private Sub f_Cm_Closed(f_Cm As Windows.Forms.ContextMenuStrip)
will have to become
Private Sub f_Cm_Closed(sender As Object, e As ToolStripDropDownClosedEventArgs)
sender will always be f_Cm, so you can cast like so:
Dim f_Cm As Windows.Forms.ContextMenuStrip = sender
To pull everything together, your AddHandler call now becomes:
AddHandler f_Cm.Closed, AddressOf f_Cm_Closed
And your method f_Cm_Closed becomes:
Private Sub f_Cm_Closed(sender As Object, e As ToolStripDropDownClosedEventArgs)
Dim f_Cm As Windows.Forms.ContextMenuStrip = sender
RemoveHandler f_Cm.Closed, AddressOf f_Cm_Closed
End Sub
As a final thought, I have no idea why you would want to remove the handler for the Closed event after the menu is closed. But this is how you would restructure your code to do it.