Control background worker exception when program closes

Hi Folks,

I have created a windows form control derived from the ComboBox to allow visual feedback from the control to confirm if the required change is complete or not after the SelectedIndex is changed. This is because the change actually sends data to an external device by writing to the serial port and then reading back the same item to confirm the change. If the data on the device has not been changed as intended the user needs to know about that.

The first part of this visual feedback is undoing the SelectedIndex change. So the derived control always saves the current index. It then overrides the SelectedIndexChanging event, raising a SelectedIndexChanging event in its place. This allows the system to cancel the event if the change was not confirmed which then replaces the selected index with the previously saved, old, index, undoing the change.

This works fine, but with lots of controls on a form it's hard to notice that one in particular hasn't changed as a user thought it should.

Thus I added in a color change and fade-out to the BackColor. Now when a change is made and is not cancelled the background of the control changes to green, then fades back to white. If it is cancelled then the background changes to red before also fading back to white.

The fading is achieved by adding a BackgroundWorker to the control which should complete when the color has faded to white. So far, so good. The problem comes in however when a user closes the application and the BackgroundWorker is still running, then the ReportProgress() method call causes an InvalidOperationException with the message "Invoke or BeginInvoke cannot be called on a control until the window handle has been created.". I assume this is because the BackgroundWorker is trying to call an Invoke type method on the control that, at this point, has been disposed.

I tried several avenues of attack to overcome the issue: an Applications.DoEvents() in the form's FormClosing handler (this seemed to help somewhat but not completely), adding multiple checks for CancellationPending in the DoWork event of the BackgroundWorker, but nothing solve the issue completely.

I imagine I can add a Dispose() override to the derived control to wait for the BackgroundWorker to complete before calling MyBase.Dispose() might prevent the exception but what is the correct and reliable way to do that? And how can I avoid the BackgroundWorker taking ages to actually complete and thus noticeably affecting my applications close time?
Or is there a better way to have the BackgroundWorker work?

(For this minor BackgroundWorker work I wasn't really interested in switching to using threads directly unless achieving this using a BackgroundWorker would prove prohibitive.)

The derived control class code:

vb Code:

Public Class BetterComboBox
    Inherits ComboBox
 
    Public Event SelectedIndexChanging As System.ComponentModel.CancelEventHandler 'Create a new event
 
    Private WithEvents FadingWorker As New System.ComponentModel.BackgroundWorker
    Private _lastIndex As Integer = -1
    Private Const _fade As Double = 17
    Private Const _fadeInterval As Double = 150
 
    <System.ComponentModel.Browsable(False)>
    Public ReadOnly Property LastIndex As Integer
        Get
            Return _lastIndex
        End Get
    End Property
 
    Public Sub New()
        MyBase.New()
        _lastIndex = -1 'Initialise the last index as the default first index.
        FadingWorker.WorkerReportsProgress = True
        FadingWorker.WorkerSupportsCancellation = True
    End Sub
 
    Protected Overridable Sub OnSelectedIndexChanging(e As System.ComponentModel.CancelEventArgs)
        RaiseEvent SelectedIndexChanging(Me, e) 'Raise the new event
    End Sub
 
    Protected Overrides Sub OnSelectedIndexChanged(e As System.EventArgs)
        If _lastIndex <> SelectedIndex Then
            Dim cancelEventArgs As New System.ComponentModel.CancelEventArgs()
            OnSelectedIndexChanging(cancelEventArgs) 'Raise the new event
 
            If cancelEventArgs.Cancel = False Then 'Check the response from the event
                _lastIndex = SelectedIndex 'Save the index change
                BackColor = Color.FromArgb(0, &HFF, 0) 'Change BG color to green
                If Not FadingWorker.IsBusy Then 'Check the worker isnt already started
                    FadingWorker.RunWorkerAsync(Me) 'Start the worker
                End If
                MyBase.OnSelectedIndexChanged(e) 'Raise the normal event
            Else
                BackColor = Color.FromArgb(&HFF, 0, 0) 'Change BG color to red
                If Not FadingWorker.IsBusy Then 'Check the worker isnt already started
                    FadingWorker.RunWorkerAsync(Me) 'Start the worker
                End If
                SelectedIndex = _lastIndex 'undo the index change
            End If
        End If
    End Sub
 
    Private Sub FadingWorker_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles FadingWorker.DoWork
        Dim worker As System.ComponentModel.BackgroundWorker = CType(sender, System.ComponentModel.BackgroundWorker)
        While Not worker.CancellationPending 'Check the worker wasn't cancelled during progress reporting
            System.Threading.Thread.Sleep(_fadeInterval) ' Wait N milliseconds
            If Not worker.CancellationPending Then 'Check the worker wasn't cancelled during the delay
                FadingWorker.ReportProgress(0, e.Argument) ' Signal the main thread to adjust the color
            End If
        End While
    End Sub
 
    Private Sub FadingWorker_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles FadingWorker.ProgressChanged
        Dim cb As BetterComboBox = CType(e.UserState, BetterComboBox)
        
        ' Code that fades the BackgroundColor RGB value here
 
        If r = g = b = &HFF Then 'Check if the BackColor has reached white
            FadingWorker.CancelAsync() 'Cancel the BackgroundWorker
        End If
    End Sub
 
    Protected Overrides Sub Dispose(ByVal disposing As Boolean)
        'Use this to wait for the background worker to be die before disposing
        If Not Me.IsDisposed Then
            If disposing Then
                'Free managed resources here
            End If
            'Free unmanaged resources here
        End If
        MyBase.Dispose(disposing)
    End Sub
End Class

PS: Also, is a BackgroundWorker considered managed or unmanaged?

Many thanks!! T

As my BackgroundWroker takes the form of:

wait n msec
do some work
repeat

Maybe I should switch to using a System.Timer instead?
Would I see problems (such as UI sometimes becoming unresponsive) with using a timer when there are multiple instances of this control on a form?

You need to cancel the BGW when the form closes... the only call to cancel is in the progress changed event... but the problem is that when the form closes, you're not cancelling the BGW, so it's still running.

-tg

D'oh! So I don't!! :blush:
Just added that to my Dispose()