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I want to use a simple routine: if user clicks a button, then the button disappears and progress bar shows up. After processing is done, the button becomes visible.
string script = viewModel.LilyScript;
ThreadStart x = delegate
    Thread.Sleep(1000); // simulate long processing
    Dispatcher.Invoke(new Action(delegate
        viewModel.LilyRenderOutput = lily.ImageSource;
    }), DispatcherPriority.Normal);
Thread thr = new Thread(x);
thr.Name = "Renderowanie";

The viewModel has dep properties which are bound to controls' properties.
class MainWindowVM
// (obvious code omitted)
        public Visibility RenderingProgressBarVisibility...
        public static readonly DependencyProperty ...
        public Visibility PaintNotesButtonVisibility...
        public static readonly DependencyProperty ...
        public ImageSource LilyRenderOutput
        internal void StartRendering()
            RenderingProgressBarVisibility = Visibility.Visible;
            PaintNotesButtonVisibility = Visibility.Hidden;

        internal void EndRendering()
            RenderingProgressBarVisibility = Visibility.Hidden;
            PaintNotesButtonVisibility = Visibility.Visible;


I got an exception The calling thread cannot access this object because a different thread owns it. on line
viewModel.LilyRenderOutput = lily.ImageSource;

In xaml:
<Image Source="{Binding LilyRenderOutput}" Stretch="None" />

Interesting fact: the code
Dispatcher.Invoke(new Action(delegate
}), DispatcherPriority.Normal);

does work. Do I use a wrong dispatcher or what? I tried viewModel.Dispather with no success.

Any help appreciated.
Updated 20-Nov-11 5:15am
Sergey Alexandrovich Kryukov 19-Nov-11 17:31pm    
How did you get a dispatcher used in the last code sample?
Lutosław 19-Nov-11 17:57pm    
In the same way and in the same place as the first one. It is a MainWindow's Dispather property. The only difference is in the body of the anymonous method -- it accesses a Title property directly, not through a binding.

1 solution

Root cause is due to the architecture of Windows Presentation Framework.

When working with WPF be aware that if you create a UI object in one thread you can't access it from another thread. Your UI objects should (typically) be created the UI thread, and then you need the UI thread to access them later. No other thread will be able to access objects created on the UI thread.

If you need to access a UI object from another thread you need the UI thread Dispatcher, and then you can use this to invoke calls on the UI thread.

Hope, it clarifies your query.
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Lutosław 21-Nov-11 17:37pm    
As you have noticed, I am aware of all these UI-related threading issues. However, there is one thing I didn't know: the ImageSource cannot be used from another thread.

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