[RESOLVED] PictureBox Image - Random Errors

[dbasnett]

I have a picturebox that I have assigned a bitmap to its image property.

I have this code that throws exceptions periodically. Typically when it is called there are 2500 points in the queue.
Periodically means I have to push the button that adds points(2500) to the queue as fast as possible for a period of time.

        Dim pt As Point
        Try
            Do While newPTS.Count > 0
                If newPTS.TryDequeue(pt) Then
                    PBbm.SetPixel(pt.X, pt.Y, PBbmDotColor)
                End If
            Loop
        Catch ex As Exception
            Dim s As String = String.Format("X: {0}  Y:{1}  {2}", pt.X, pt.Y, ex.Message)
            AddToError(s)
        End Try

I know that the points being set are within the bounds of the bitmap.

PBbm is the bitmap
newPTS is a queue of points

The errors
X: 446 Y:170 Object is currently in use elsewhere.
X: 34 Y:549 Object is currently in use elsewhere.

[boops boops]

Don't use System.Drawing.Bitmap.SetPixel, which is notoriously slow. It has to recreate the bitmap each time a pixel changes.

Instead try the FastPix method in my sig below. Treat the queue as an array of 32-bit integers, and the same for ARGB colors. The result can be as much as 100 times as fast.

BB

[dday9]

You typically only see that error when doing multithreaded tasks. In this situation, is it possible that those points are being referenced in another thread?

[dbasnett]

@boops - it is not slow though I bet it could be faster. Didn't see the code you were talking about though I did see you said it had error...

@dday - I'm guessing it is related to being multi threaded. That is the only place I change the bitmap. The stack is not in my code when it errors,

at System.Drawing.Image.get_Width()
at System.Drawing.Image.get_Size()
at System.Windows.Forms.PictureBox.ImageRectangleFromSizeMode(PictureBoxSizeMode mode)
at System.Windows.Forms.PictureBox.OnPaint(PaintEventArgs pe)
at System.Windows.Forms.Control.PaintWithErrorHandling(PaintEventArgs e, Int16 layer)
at System.Windows.Forms.Control.WmPaint(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

[dday9]

That makes me think, although I'm not positive, that perhaps the UI thread is using the object when you go to call SetPixel.

I'm wondering if you call SuspendLayout and then ResumeLayout before/after that call is made will stop throwing the exception. If so, it wouldn't solve your underlying issue rather it would only confirm my suspicion.

[dbasnett]

Added a lock and made certain that I was only Invalidating the picture box in one location.

    Private newPTS As New Concurrent.ConcurrentQueue(Of Point)
    Private Sub AddPixelsToPict(count As Integer) 'can be called from any thread
        If TRNG IsNot Nothing Then
            For ct As Integer = 1 To count
                Dim x As Integer = TRNG.Next(0, PBW) 'get random x and y
                Dim y As Integer = TRNG.Next(0, PBH)
                Dim pt As New Point(x, y)
                newPTS.Enqueue(pt)
            Next
            SetPBbm()
        End If
    End Sub

    Private Sub SetPBbm()
        PBbmLock.WaitOne() 'lock for bitmap - Threading.AutoResetEvent
        Dim pt As Point
        Try
            Do While newPTS.Count > 0
                If newPTS.TryDequeue(pt) Then 'get a point
                    PBbm.SetPixel(pt.X, pt.Y, PBbmDotColor)
                End If
            Loop
        Catch ex As Exception
            Dim s As String = String.Format("X: {0}  Y:{1}  {2}", pt.X, pt.Y, ex.Message)
            AddToError(s)
        End Try
        PBbmLock.Set()
    End Sub

Every 100 ms this runs on the UI,

                                   If PBbmLock.WaitOne(0) Then
                                       Try
                                           PictDots.Invalidate()
                                       Catch ex As Exception
                                           Dim s As String = "Invalidate"
                                           AddToError(s)
                                       End Try
                                       PBbmLock.Set()
                                   End If

Still getting the error...

[dbasnett]

To create the error I was clicking a button that added several points and then holding down the enter key. This seems to have fixed it because I can't press enter continuously and I guess I can't click the button fast enough to cause the error.

    Private addPtsTask As Task
    Private Async Sub btnAddPts_Click(sender As Object, e As EventArgs) Handles btnAddPts.Click
        If Monitoring.WaitOne(0) Then
            If addPtsTask Is Nothing OrElse addPtsTask.Status = TaskStatus.RanToCompletion Then
                btnAddPts.Enabled = False
                addPtsTask = Task.Run(Sub() AddPixelsToPict(1000))
                Await addPtsTask
                addPtsTask = Nothing
                btnAddPts.Enabled = True
            End If
        End If
    End Sub