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.

Answer 2

Thanks for the suggestions above. In my situation I am able to go for the "nuclear option", which is -- whenever the selected item needs to change, I completely destroy the ComboBox, make a new one, and set its SelectedItem appropriately.

Ridiculous, but it works.

Answer 3

Combobox is a quite buggy SL control :-(.

In my case I gave up with the selected item declarativa binding and use the nasty coding approach... ugly but works:

http://blogs.msdn.com/mikehillberg/archive/2009/03/26/implementing-selectedvalue-with-the-silverlight-combobox.aspx

HTH Braulio

Answer 4

I was getting the same issue a while ago and from what I can tell it's a bug in ComboBox when the ItemSource is changed it has an issue with the layout and scrolls badly.

There is a work around by calling ComboBox.UpdateLayout between setting the ItemSource and SelectedItem.

I blogged about the problem a while ago at Gotcha when databinding a ComboBox in Silverlight.

I've yet to verify whether the problem still exists in the Silverlight 3 Beta