Closing form with Gif throws InvalidOperationException - vb.net

Question

This is clearly a problem of me not understanding how to properly setup a UI thread, but I can't figure out how to fix it.
I have a datagridview where I click a button, get the information from the network, and then display it on the datagridview with the new data. While it is on the network I have a form I show with an updating gif, a form I called "loading". Within that form I have the gif updating using the typical OnFrameChanged and m_isAnimating code that is on the internet.
However, no matter what format I use, I always get this exception caught here:
Public loader As New Loading
Private Sub OnFrameChanged(ByVal o As Object, ByVal e As EventArgs)
Try ' If animation is allowed call the ImageAnimator UpdateFrames method
' to show the next frame in the animation.
Me.Invalidate()
If m_IsAnimating Then
ImageAnimator.UpdateFrames()
Me.Refresh()
'Draw the next frame in the animation.
Dim aGraphics As Graphics = PictureBox1.CreateGraphics
aGraphics.DrawImage(_AnimatedGif, New Point(0, 0))
aGraphics.Dispose()
End If
Catch ex As InvalidOperationException
End Try
End Sub
And it usually says something along the lines of "was accessed from a thread it wasn't created on" or "Cannot access a disposed object. Object name: 'PictureBox'."
But I don't know why that is, since I am creating a new instance here every time. Here's the button's code:
Private Sub btnSlowSearch_Click(sender As Object, e As EventArgs) Handles btnSlowSearch.Click
Me.Cursor = Cursors.WaitCursor
'get datatable
loader.Show()
BWorkerLoadProp.RunWorkerAsync() 'go get data on network
'bworker will update datagridview with new data
'wait for worker to finish
If BWorkerLoadProp.IsBusy Then
Threading.Thread.Sleep(1)
End If
loader.Close()
End Sub
I realize it isn't very good code, but I have tried putting the loader inside the background worker, I have tried whatever. But no matter what the exception is called.
What's the proper way to show another updating form as I do background work?

Answer 1

The behavior documented is difficult to reproduce.
Probably something between the thread switching causes a call to OnFrameChanged after the call to close in the btnSlowSearch_Click.
In any case logic seems to suggest to call the ImageAnimator.StopAnimate in the close event of the form that shows the animation
So looking at your comment above I would add the following to your animator form
// Not needed
// Public loader As New Loading
Private Sub OnFrameChanged(ByVal o As Object, ByVal e As EventArgs)
Try
Me.Invalidate()
If m_IsAnimating Then
ImageAnimator.UpdateFrames()
Me.Refresh()
'Draw the next frame in the animation.
Dim aGraphics As Graphics = PictureBox1.CreateGraphics
aGraphics.DrawImage(_AnimatedGif, New Point(0, 0))
aGraphics.Dispose()
End If
Catch ex As InvalidOperationException
.. do not leave this empty or remove altogether
End Try
End Sub
Private Sub Form_Closing(sender As Object, e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
... if you need to stop the closing you should do it here without stopping the animation
If m_IsAnimating Then
ImageAnimator.StopAnimate(AnimatedGif, _
New EventHandler(AddressOf Me.OnFrameChanged))
m_isAnimating = False
End If
End Sub

Answer 2

This is certainly not the only way to do this but I will provide you the simplest working example in hopes that it will help you to correct your own application.
1) Create a new vb.net windows forms application and add a button (Button1) onto the form.
2) Change the Form1 code to this:
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
If fLoading Is Nothing Then ' can only show one loading screen at a time
Dim oLoadingThread As clsLoadingThread = New clsLoadingThread ' creat new thread
oLoadingThread.ShowWaitScreen() ' show the loading screen
'-----------------------------------------
' your real processing would go here
'-----------------------------------------
For i As Int32 = 0 To 999999
Application.DoEvents()
Next
'-----------------------------------------
oLoadingThread.CloseLoadingScreen() ' we are done processing so close the loading form
oLoadingThread = Nothing ' clear thread variable
End If
End Sub
End Class
Public Class clsLoadingThread
Dim oThread As System.Threading.Thread
Private Delegate Sub CloseLoadingScreenDelegate()
Public Sub ShowWaitScreen()
' create new thread that will open the loading form to ensure animation doesn't pause or stop
oThread = New System.Threading.Thread(AddressOf ShowLoadingForm)
oThread.Start()
End Sub
Private Sub ShowLoadingForm()
Dim fLoading As New frmLoading
fLoading.ShowDialog() ' Show loading form
If fLoading IsNot Nothing Then fLoading.Dispose() : fLoading = Nothing ' loading form should be closed by this point but dispose of it just in case
End Sub
Public Sub CloseLoadingScreen()
If fLoading.InvokeRequired Then
' Since the loading form was created on a seperate thread we need to invoke the thread that created it
fLoading.Invoke(New CloseLoadingScreenDelegate(AddressOf CloseLoadingScreen))
Else
' Now we can close the form
fLoading.Close()
End If
End Sub
End Class
Module Module1
Public fLoading As frmLoading
End Module
3) Add a new form and call it frmLoading. Add a picturebox to the form and set the image to your updating gif.
4) Change the frmLoading code to this:
Public Class frmLoading
Private Sub frmLoading_Load(sender As Object, e As EventArgs) Handles Me.Load
fLoading = Me ' ensure that the global loading form variable is set here so we can use it later
End Sub
Private Sub frmLoading_FormClosed(sender As Object, e As FormClosedEventArgs) Handles Me.FormClosed
fLoading = Nothing ' clear the global loading form since the form is being disposed
End Sub
End Class
Normally I would add the clsLoadingThread Class and Module1 Module to their own files but it's easier to show the code to you this way.