NotifyCollectionChangedEventHandler is not workling with background worker

Question

NotifyCollectionChangedEventHandler command does not work with background worker.Event is as :

void MainWindowsViewModel_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    StrokesEllipse = ((StrokeCollection)sender);
    using (var memoryStream = new MemoryStream())
    {
        StrokesEllipse.Save(memoryStream);
        //convert memory stream to  array
        EllipseDrawing = memoryStream.ToArray();
        //save the above array to say - database
        }
    }

And we declared event on constructor as below

_strokesEllipse = new StrokeCollection();
(_strokesEllipse as INotifyCollectionChanged).CollectionChanged += new NotifyCollectionChangedEventHandler(MainWindowsViewModel_CollectionChanged);

And we are binding stoke collection on the background worker completed event. as Below

string s = GetMechanicSignature();
if (s != "")
{
    EllipseDrawing = Convert.FromBase64String(s);
}
if (EllipseDrawing != null)
{
    try
    {
        using (var memoryStream = new MemoryStream(EllipseDrawing))
        {
            _strokesEllipse = new StrokeCollection(memoryStream);
        }
    }
    catch (Exception)
    {

}

inkcanvas control does not showing loaded data. why? When we try without background worker then inkcanvas control loading the data very well? inkcanvas xml is as below

<InkCanvas x:Name="inkCanVas" Grid.Row="0" IsEnabled="{Binding VCRSignatureModel.IsEnable,Mode=TwoWay}" Background="White"  Width="700" Height="90" Margin="40,0,0,0" Strokes="{Binding StrokesEllipse,Mode=TwoWay}">
    <InkCanvas.DefaultDrawingAttributes>
        <DrawingAttributes Color = "Black" Width = "6" />
    </InkCanvas.DefaultDrawingAttributes>
</InkCanvas>

Cross-thread operations are not allowed. I guess that is the issue. — Patrick Hofman, Jun 07 '16 at 08:11
You need to use Dispatcher.Invoke to run code on the UI thread. — Thomas Levesque, Jun 07 '16 at 08:14
can you provide exact solution how to call by Dispatcher.Invoke — naina, Jun 07 '16 at 08:33
@PatrickHofman Shouldn't be cross threaded, OP said it was fired on the completed event code which should be marshalled back to the UI thread. — Bob Vale, Jun 07 '16 at 09:00
@PatrickHofman I Quote "And we are binding stoke collection on the background worker completed event. as Below" — Bob Vale, Jun 07 '16 at 09:02
That doesn't mean `CollectionChanged` is called on the UI thread @BobVale — Patrick Hofman, Jun 07 '16 at 09:03
@PatrickHofman Why not, the collection would be modified on the UI thread as so the changed event should also fire there? OP is not getting an exception, which they would if it was a cross thread, they are getting nothing happening. the real problem is they are not modifying the collection, simply replacing the field with a new collection. — Bob Vale, Jun 07 '16 at 09:07

Bob Vale · Accepted Answer · 2016-06-07T09:04:27.597

You are not modifying the collection, you are replacing it. As your background work completed event should be firing in the UI thread it is not a threading issue.

The quickest way to fix this would be to add the following line after the line _strokesEllipse = new StrokeCollection(memoryStream); in your worker completed code.

MainWindowsViewModel_CollectionChanged(
   _strokesEllipse,
   new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Replace);

Alternatively you could change the code to read:

try
{
    using (var memoryStream = new MemoryStream(EllipseDrawing))
    {
        var newCollection = new StrokeCollection(memoryStream);
        _strokesEllipse.Clear();
        _strokesEllipse.Add(newCollection);
    }
}
catch (Exception)
{

NotifyCollectionChangedEventHandler is not workling with background worker

1 Answers1