Cannot update UI from another thread in WPF

Question

I have a simple method which call a function that returns a bitmap image.

private void ImageToGrayScale(object sender, RoutedEventArgs e)
{
    pbStatus.Visibility = Visibility.Visible;
    if (loadedImage != null)
    {
        new Thread(() =>
        {
             BitmapImage bitmapImage= ThreadProcedure();
             this.Dispatcher.Invoke(new Action(()=> {
                 pbStatus.Visibility = Visibility.Hidden;
                 EditedImage.Source = bitmapImage;
             }));
        }).Start();
    }
    else
    {
        MessageBox.Show("Please select the image first!");
    }
}

Everytime at line: EditedImage.Source = bitmapImage; a get an error:

The calling thread cannot access this object because a different thread owns it.

Also, here is the method which returns a bitmap image.

private BitmapImage ThreadProcedure()
{
     Bitmap editedImage = new Bitmap(loadedImage);
     for (int x = 0; x < editedImage.Width; x++)
     {
         for (int y = 0; y < editedImage.Height; y++)
         {
             Color pixelColor = editedImage.GetPixel(x, y);
             int rgb = (int)((pixelColor.R + pixelColor.G + pixelColor.B) / 3);
             editedImage.SetPixel(x, y, Color.FromArgb(rgb, rgb, rgb)); // Now greyscale
         }
     }
     return Converter.ConvertBitmapToBitmapImage(editedImage, extension);
 }

What is loadedImage, is that the problem? You create new Bitmap from it in the thread process. Or maybe you should pass the bitmapImage as a parameter to the dispatcher delegate... — japesu, Mar 03 '17 at 11:00
The issue here is most likely with the `bitmapImage` object - it's created on a different thread to the Dispatcher sync context. So reading from it, and copying it to the Source is probably failing with this error. To test this, move the call to `ThreadProcedure` inside the dispatcher invoke and see if it works. — Baldrick, Mar 03 '17 at 11:02
@Baldrick, it works, but it blocks the UI thread and the window it is not responsive. — Mihai Alexandru-Ionut, Mar 03 '17 at 11:07
So create delegate for the dispatcher process and pass the bitmapImage as a parameter when it has been created in you thread. — japesu, Mar 03 '17 at 11:10
@Alexandru-IonutMihai, check the link in the answer below, is it possible that `EditedImage.Dispatcher` and `this.Dispatcher` are different? If so, you'll have to make another `Invoke`. Like `this.EditedImage.Dispatcher.Invoke(() => this.EditedImage.Source = blabla)` — Nerlog, Mar 03 '17 at 11:30

score 1 · Accepted Answer · answered Mar 03 '17 at 21:36

You should freeze the BitmapImage by calling its Freeze() method in order to be able to use it on another thread than the one on which it was created:

new Thread(() =>
{
        BitmapImage bitmapImage = ThreadProcedure();
        bitmapImage.Freeze(); //<--

        this.Dispatcher.Invoke(new Action(() => {
            pbStatus.Visibility = Visibility.Hidden;
            EditedImage.Source = bitmapImage;
        }));
}).Start();

score 0 · Answer 2 · edited May 23 '17 at 12:17

0

There is a difference between Dispatcher and Application.Current.Dispatcher:

Dispatcher.CurrentDispatcher vs. Application.Current.Dispatcher

edited May 23 '17 at 12:17

Community

1
1

answered Mar 03 '17 at 11:03

Nawed Nabi Zada

2,819
5
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Nerlog · Answer 3 · 2017-03-03T11:22:05.743

0

Make sure the dispatcher you use is the same one for the EditedImage object. You could use the latter dispatcher directly (see https://msdn.microsoft.com/en-us/library/system.windows.threading.dispatcherobject.dispatcher(v=vs.110).aspx )

edited Mar 03 '17 at 11:22

answered Mar 03 '17 at 11:06

Nerlog

261
1
5

score -1 · Answer 4 · answered Mar 03 '17 at 11:03

Creating a thread for updating an image is not the best thing to do use Dispatcher.BeginInvoke instead this will execute the action asynchronous.

"Control.BeginInvoke: Executes on the UI thread, and calling thread doesn't wait for completion."

Remove the thread part and use this code the problem should be solved.

         BitmapImage bitmapImage= ThreadProcedure();

         Dispatcher.BeginInvoke(DispatcherPriority.Input, new Action(() =>
         {
             pbStatus.Visibility = Visibility.Hidden;
             EditedImage.Source = bitmapImage;
         }));

Cannot update UI from another thread in WPF

4 Answers4