Gotcha when databinding a ComboBox in Silverlight

Posted Thursday, March 5, 2009 by

Occasionally we all run into bugs in systems not built by ourselves. Some are minor, some however really make you wonder how much QA was really done. Sadly this was the latter.

I found this one when building a system that bound a collection of objects from a service to a ComboBox, at one point this collection changes and the list is refreshed from the service. I had code in this refresh to set the selected item (bound using a two way data binding) to the previously selected item. It was failing with the falling exception when I attempted to set the SelectedItem.

Error: Unhandled Error in Silverlight 2 Application
Code: 4004    
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)    

Assuming a page with a ComboBox and a Button this is the simplest piece of code that recretes the error. Just change the ComboBox to any item other than the first and then hit the Refresh button.

public partial class Page

{

    public Page()

    {

        InitializeComponent();

        Loaded += OnLoaded;

        RefreshButton.Click += OnRefresh;

    }

 

    private void OnLoaded(object sender, RoutedEventArgs e)

    {

        BindVideos();

    }

 

    private void BindVideos()

    {

        var videos = GetVideos();

        VideoList.ItemsSource = videos;

        VideoList.SelectedItem = videos[0];

    }

 

    private void OnRefresh(object sender, RoutedEventArgs e)

    {

        BindVideos();

    }

 

    private static ObservableCollection<Video> GetVideos()

    {

        return new ObservableCollection<Video>

        {

            new Video(1, "test.wmv"),

            new Video(2, "tv-ad.wmv"),

            new Video(3, "porn.wmv")

        };

    }

}

 

There's a few possible solutions out there, the one that seemed to work for me was calling ComboBox.UpdateLayout like so.

private void BindVideos()

{

    var videos = GetVideos();

    VideoList.ItemsSource = videos;

    VideoList.UpdateLayout();

    VideoList.SelectedItem = videos[0];

}

 

While this is a workable solution for the example I'm trying to build my solution using the Model View ViewModel (MVVM) pattern. This makes the solution useless since my ViewModel does the interaction. I tried a solution where my View listened for PropertyChanges on the ViewModel but since you can't guarantee in which order these events occur it wasn't viable. In the end I created a new event on my ViewModel that the View listens to and calls UpdateLayout. It's not a nice solution and I feel dirty for having to use it.

This needs to be fixed. It's hours of my life I won't be getting back.

Edit : I'm glad to say it's fixed in Silverlight 3.

Professional Windows App Development