Binding ComboBox.SelectedItem in Silverlight (more)

Question

Related to my previous question: Binding ComboBox.SelectedItem in Silverlight

I have a ComboBox bound like so:

<ComboBox x:Name="PART_CommentaryList" 
    HorizontalAlignment="Left" 
    Margin="3" 
    ItemsSource="{Binding Path=CurrentVideo.Commentaries}" 
    SelectedItem="{Binding Path=CurrentCommentary, Mode=TwoWay}">

Both the CurrentVideo and CurrentCommentary property change regularly. After a few times, I get this error:

Category: ManagedRuntimeError       
Message: System.ArgumentException: Value does not fall within the expected
   range.
   at MS.Internal.XcpImports.MethodEx(IntPtr ptr, String name, 
       CValue[] cvData)
   at MS.Internal.XcpImports.MethodPack(IntPtr objectPtr, String methodName, 
       Object[] rawData)
   at MS.Internal.XcpImports.UIElement_TransformToVisual(UIElement element, 
       UIElement visual)
   at System.Windows.UIElement.TransformToVisual(UIElement visual)
   at System.Windows.Controls.Primitives.Selector.IsOnCurrentPage(
       Int32 index, Rect& itemsHostRect, Rect& listBoxItemRect)
   at System.Windows.Controls.Primitives.Selector.ScrollIntoView(
       Int32 index)
   at System.Windows.Controls.Primitives.Selector.SetFocusedItem(
       Int32 index, Boolean scrollIntoView)
   at System.Windows.Controls.ComboBox.PrepareContainerForItemOverride(
       DependencyObject element, Object item)
   at System.Windows.Controls.ItemsControl.UpdateContainerForItem(
       Int32 index)
   at System.Windows.Controls.ItemsControl.RecreateVisualChildren()
   at System.Windows.Controls.ItemsControl.RecreateVisualChildren(
       IntPtr unmanagedObj)

This seems like a ComboBox bug to me. I can verify that CurrentVideo changes before CurrentCommentary, so the selected item should always be an item which is in the list.

Related, I really don't want the Mode=TwoWay, because when the ItemsSource is changed, the SelectedItem is temporarily null, which gets set back in my model, which I don't actually want. But the binding doesn't work at all otherwise (which seems like another bug).

Answer 1

This is a bug in the ComboBox control that has to do with the changing pointer of the ItemsSource's binding. The solution that I have found is to:

1) Always bind the ItemsSource to an observable collection and never reset the pointer of the OC.

<ComboBox ItemsSource="{Binding MyList}" SelectedItem="{Binding MyItem}" />

Bad:

MyList = new ObservableCollection();

Good:

MyList.Clear();
MyList.AddRange(...);

2) Set MyItem = null, before Clearing MyList

In your case you are changing the reference of the List whenever you change CurrentView. Therefore, if SelectedItem is not null, there is a brief moment in time where the ItemsSource is being reset, the internals of the ComboBox are attempting to locate the SelectedItem object in the new ItemsSource but the old object is not there.