WPF多级绑定线程问题

Question

just found an interesting WPF problem I could really use your help with: I've got a multi-level binding Binding="{Binding PreviewItem.Status} which produces strange behavior if and only if the properties are updated from a background thread. I used diagnostics:PresentationTraceSources.TraceLevel=High and what seems to happen is that if Status is updated, it prevents the binding from realizing the whole PreviewItem changes just a split second after it!

I managed to reproduce the behavior in an new, empty C# test project (VS 2012, .NET 4.5). This is the main code: Both channel and TimelineItem are empty classes that just have a property with all the proper notifypropertychanged event notifications, StatusType is an Enum, the control's DataContext is set to _channel :

Task.Run(() =>
        {
            if ( _channel.PreviewItem != null )
            {
                _channel.PreviewItem.Status = TimelineItem.StatusType.Default;
            }
            _channel.PreviewItem = new TimelineItem();
            _channel.PreviewItem.Status = TimelineItem.StatusType.Preview;
        });
}

Let's say we do this three times to create three items:

1. A is created and assigned to PreviewItem, A's status goes to "Preview" (ok)
2. A's status goes to Default, B is created and assigned to PreviewItem (fails, binding still uses A when the status changes, updates to the new state and never re-binds to B!)
3. B's status goes to Default, C is created and assigned to PreviewItem (ok as the status of A - where the binding is still resolved to - does not change, therefore it notices that PreviewItem has changed to C).

If I a) move the exact same code to the main thread or b) assign the new PreviewItem BEFORE changing the old items status, the code works! Unfortunately, I cannot use either approach in my project.

Anyone got an idea why this does not work? As far as I understood it, WPF is supposed to take care of dispatching the change event to the proper dispatcher. Any help is highly appreciated...

Alex

UPDATE Aug 4, 2014 #

For some weird reason, the bug behavior only happens on Windows 7. Whole project can be found here .

UPDATE Sep 2, 2014

The problems was resolved in a Hotfix: http://support.microsoft.com/kb/2783808/en-us

Answer 1

In WPF, only the thread that created the UI (controls) may access\\modify that object. For example, a background thread that is started from the main UI thread cannot update the contents of any control that was created on the UI thread.

You will need to invoke back onto the UI thread, there are multiple ways to do this, the following being one of the simplest:

Application.Current.Dispatcher.Invoke(() => 
{
    // your GUI related code here
});

Answer 2

Can you update your .Net version to 4.5.1/4.5.2 and recheck? This might be a bug which might have been fixed in a recent update. I checked your code on a Windows 7 machine only and was unable to reproduce it, so it is definitively framework version related.