why is this worker/ thread not stopping? - vb.net

Question

I have a panel which has a several comboboxes in it and a button to close the panel. I set up the comboboxes to get populated with another thread because it has many entries. So the background worker starts and everything goes as expected until you close the panel. If I close the panel before the background worker is finished, I get an error. Let me explain:
I start the process of by starting the background worker:
get_info.RunWorkerAsync()
The close button sub looks like this:
Private Sub close_everything()
' dispose of the panel that holds the comboboxes
info_panel.dispose()
' tell the background worker to stop working
get_info.CancelAsync()
End Sub
I set up the background worker like this:
Private Sub getInfo_doWork()
'populate the combo boxes
populate_last_names()
End Sub
Then the subroutine to populate the last name looks like this:
Private Sub populate_last_names()
' get the names
get_names()
' since the combobox control is in another thread, I need to use the Invoke method to access it
' put them in the combobox
for a as integer = 0 to last_names.count - 1
last_name_box.Invoke(Sub() last_name_box.Items.Add(last_names(a)))
next
End Sub
When the close button is pressed, I get an error stating that the combobox has been disposed. So I added a check for the background worker first. So my code changed to:
' put them in the combobox
for a as integer = 0 to last_names.count - 1
if not(get_info.CancellationPending) then
last_name_box.Invoke(Sub() last_name_box.Items.Add(last_names(a)))
end if
next
but I still got the same error, so I added another check and changed the code to this:
' put them in the combobox
for a as integer = 0 to last_names.count - 1
if not(get_info.CancellationPending) then
if not(last_name_box.IsDisposed) then
last_name_box.Invoke(Sub() last_name_box.Items.Add(last_names(a)))
end if
end if
next
and I STILL get an error stating that the last_name box has been disposed, if I close the panel before the loop is finished.
The error even states that the value of last_name_box.IsDisposed is TRUE and the value of CancellationPending is TRUE. So why is it executing the next line if it is TRUE?
How can I stop this from happening?

Answer 1

Your code is badly constructed. What you should be doing is getting the data in the DoWork event handler and then passing it to the RunWorkerCompleted event handler via the e.Result property. You then populate the controls with the data in the RunWorkerCompleted event handler, which is executed on the UI thread.
You should also not be hiding the Panel anywhere but in the RunWorkerCompleted event handler. You request cancellation from the UI thread and then you actually perform the cancellation in the DoWork event handler. In the RunWorkerCompleted event handler, you check whether the operation was cancelled or not. If it was then you simply hide the Panel, otherwise you populated the list(s).

Answer 2

Try stopping the background worker first:
Private Sub close_everything()
' tell the background worker to stop working
get_info.CancelAsync()
' dispose of the panel that holds the comboboxes
info_panel.dispose()
End Sub

Answer 3

Since a viable answer to my problem did not seem possible, I ended up switching to a textbox with autofill instead of a combobox with autofill. I was just using the combo box because of the autofill capability and didn't realize the textbox could do the same thing. The data is gathered in the background worker and the source for the autofill is assigned after the data is collected in the main UI. I couldn't accomplish exactly what I wanted so I settled for this. There is still some delay but it is manageable.