“BindingExpression path error” using ItemsControl

Question

I'm using Silverlight on Windows Phone 7.

Is it normal to get loads of "BindingExpression path error" debug messages when using a VirtualizingStackPanel? I think it is happening because the visual items are temporarily unbound from the data items collection as they are recycled ...

I have an ItemsControl whose ItemsPanel's ItemsPanelTemplate is a VirtualizingStackPanel. It binds to a "Notes" ObservableCollection on my ViewModel.

<ItemsControl x:Name="ListView" ItemsSource="{Binding Notes}" 
              ItemTemplate="{StaticResource ListDataTemplate}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <VirtualizingStackPanel/>
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.Template>
        <ControlTemplate>
            <ScrollViewer x:Name="ScrollViewer" Padding="{TemplateBinding Padding}">
                <ItemsPresenter/>
            </ScrollViewer>
        </ControlTemplate>
    </ItemsControl.Template>
</ItemsControl>

This is within a UserControl whose DataContext is set to a MainViewModel ViewModel.

The ListDataTemplate contains a Button, which itself contains some text, data-bound to a property of the items in the Notes ObservableCollection:

<DataTemplate x:Key="ListDataTemplate">
        <Button>
        <Grid>
            <TextBlock TextWrapping="Wrap" Text="{Binding Title}">
        </Grid>
    </Button>
</DataTemplate>

When I run my program it all works fine - only a small number of items in my Notes collection are bound to at any one time. However I get lots of debug errors as I scroll up and down:

System.Windows.Data Error: BindingExpression path error: 'Title' property not found on 'EnWp7.ViewModels.MainViewModel' 'EnWp7.ViewModels.MainViewModel' (HashCode=119211466). BindingExpression: Path='Title' DataItem='EnWp7.ViewModels.MainViewModel' (HashCode=119211466); target element is 'System.Windows.Controls.TextBlock' (Name=''); target property is 'Text' (type 'System.String')..
System.Windows.Data Error: BindingExpression path error: 'Title' property not found on 'EnWp7.ViewModels.MainViewModel' 'EnWp7.ViewModels.MainViewModel' (HashCode=119211466). BindingExpression: Path='Title' DataItem='EnWp7.ViewModels.MainViewModel' (HashCode=119211466); target element is 'System.Windows.Controls.TextBlock' (Name=''); target property is 'Text' (type 'System.String')..
System.Windows.Data Error: BindingExpression path error: 'Title' property not found on 'EnWp7.ViewModels.MainViewModel' 'EnWp7.ViewModels.MainViewModel' (HashCode=119211466). BindingExpression: Path='Title' DataItem='EnWp7.ViewModels.MainViewModel' (HashCode=119211466); target element is 'System.Windows.Controls.TextBlock' (Name=''); target property is 'Text' (type 'System.String')..
System.Windows.Data Error: BindingExpression path error: 'Title' property not found on 'EnWp7.ViewModels.MainViewModel' 'EnWp7.ViewModels.MainViewModel' (HashCode=119211466). BindingExpression: Path='Title' DataItem='EnWp7.ViewModels.MainViewModel' (HashCode=119211466); target element is 'System.Windows.Controls.TextBlock' (Name=''); target property is 'Text' (type 'System.String')..

It is looking for the "Title" property on the main view model (whose Notes property is the ItemsSource) - I think this is normal - it is a transient situation as the visual items are unbound from items in the Notes collection in order to be recycled by assigning them to another item in the collection.

Has anyone else seen this? Am I right that it is normal?

If I create a dummy Title property on my MainViewModel and set a debug breakpoint then I see this stack trace. Note the "UnlinkContainerFromItem":

...!EnWp7.ViewModels.MainViewModel.Title.get() Line 54  C#
mscorlib.dll!System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo rtmi = {System.Reflection.RuntimeMethodInfo}, object obj = {EnWp7.ViewModels.MainViewModel}, System.Reflection.BindingFlags invokeAttr = Default, System.Reflection.Binder binder = null, object parameters = null, System.Globalization.CultureInfo culture = null, bool isBinderDefault = false, System.Reflection.Assembly caller = null, bool verifyAccess = true, ref System.Threading.StackCrawlMark stackMark = LookForMyCaller) 
mscorlib.dll!System.Reflection.RuntimeMethodInfo.InternalInvoke(object obj = {EnWp7.ViewModels.MainViewModel}, System.Reflection.BindingFlags invokeAttr = Default, System.Reflection.Binder binder = null, object[] parameters = null, System.Globalization.CultureInfo culture = null, ref System.Threading.StackCrawlMark stackMark = LookForMyCaller) + 0x14e bytes 
mscorlib.dll!System.Reflection.RuntimePropertyInfo.InternalGetValue(System.Reflection.PropertyInfo thisProperty = {System.Reflection.RuntimePropertyInfo}, object obj = {EnWp7.ViewModels.MainViewModel}, object[] index = null, ref System.Threading.StackCrawlMark stackMark = LookForMyCaller) + 0x4e bytes  
mscorlib.dll!System.Reflection.RuntimePropertyInfo.GetValue(object obj = {EnWp7.ViewModels.MainViewModel}, object[] index = null) + 0x2 bytes   
System.Windows.dll!System.Windows.CLRPropertyListener.Value.get() + 0x1b bytes  
System.Windows.dll!System.Windows.PropertyAccessPathStep.ConnectToPropertyInSource() + 0x148 bytes  
System.Windows.dll!System.Windows.PropertyAccessPathStep.ConnectToProperty() + 0x16 bytes   
System.Windows.dll!System.Windows.PropertyAccessPathStep.ReConnect(object newSource = {EnWp7.ViewModels.MainViewModel}) + 0x13 bytes    
System.Windows.dll!System.Windows.PropertyPathListener.ReConnect(object source = {EnWp7.ViewModels.MainViewModel}) + 0x10 bytes 
System.Windows.dll!System.Windows.Data.BindingExpression.SourceAquired() + 0x11 bytes   
System.Windows.dll!System.Windows.Data.BindingExpression.DataContextChanged(object o = {System.Windows.Controls.TextBlock}, System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x26 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x15 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x11 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x18 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x18 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x18 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x18 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x18 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnAncestorDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x18 bytes   
System.Windows.dll!System.Windows.FrameworkElement.NotifyDataContextChanged(System.Windows.DataContextChangedEventArgs e = {System.Windows.DataContextChangedEventArgs}) + 0x59 bytes   
System.Windows.dll!System.Windows.FrameworkElement.OnPropertyChanged(System.Windows.DependencyProperty dp = {System.Windows.CoreDependencyProperty}) + 0x1d bytes   
System.Windows.dll!System.Windows.DependencyObject.RaisePropertyChangeNotifications(System.Windows.DependencyProperty dp = {System.Windows.CoreDependencyProperty}, object newValue = {EnWp7.ViewModels.MainViewModel}, object oldValue = {EnWp7.Store.NoteLocal}) + 0x38 bytes 
System.Windows.dll!System.Windows.DependencyObject.ClearValueInternal(System.Windows.DependencyProperty dp = {System.Windows.CoreDependencyProperty}) + 0x138 bytes 
System.Windows.dll!System.Windows.DependencyObject.ClearValue(System.Windows.DependencyProperty dp = {System.Windows.CoreDependencyProperty}) + 0x7 bytes   
System.Windows.dll!System.Windows.Controls.ItemContainerGenerator.UnlinkContainerFromItem(System.Windows.DependencyObject container = {System.Windows.Controls.ContentPresenter}, object item = {EnWp7.Store.NoteLocal}, bool isRecycling = true) + 0x1f bytes  
System.Windows.dll!System.Windows.Controls.ItemContainerGenerator.Remove(System.Windows.Controls.Primitives.GeneratorPosition position = {System.Windows.Controls.Primitives.GeneratorPosition}, int count = 2, bool isRecycling = true) + 0x14a bytes  
System.Windows.dll!System.Windows.Controls.ItemContainerGenerator.System.Windows.Controls.Primitives.IRecyclingItemContainerGenerator.Recycle(System.Windows.Controls.Primitives.GeneratorPosition position = {System.Windows.Controls.Primitives.GeneratorPosition}, int count = 2) + 0x9 bytes    
System.Windows.dll!System.Windows.Controls.VirtualizingStackPanel.CleanupRange(int startIndex = 0, int count = 2) + 0x20 bytes  
System.Windows.dll!System.Windows.Controls.VirtualizingStackPanel.CleanupContainers(System.Windows.Controls.ItemsControl itemsControl = {System.Windows.Controls.ItemsControl}) + 0x73 bytes    
System.Windows.dll!System.Windows.Controls.VirtualizingStackPanel.MeasureOverride(System.Windows.Size constraint = {System.Windows.Size}) + 0xe4 bytes  
System.Windows.dll!System.Windows.FrameworkElement.MeasureOverride(System.IntPtr nativeTarget = 102509504, float inWidth = 480.0, float inHeight = 499.0, out float outWidth = 0.0, out float outHeight = 0.0) + 0x45 bytes 

Thanks, Damian

Answer 1

I know this post is pretty old, but a solution without creating the dummy properties is making sure the DataContext of the items can only be of the type you expect.

I do this with the following converter on the root of the data template:

public class SpecificTypeConverter : IValueConverter
{
    public string Type { get; set; }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null || value.GetType().FullName == this.Type)
        {
            return value;
        }

        return DependencyProperty.UnsetValue;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Add a static resource like this:

<p:PhoneApplicationPage.Resources>
    <converters:SpecificTypeConverter x:Key="MustBeItem" Type="My.Namespace.MyListItemViewModel" /> 
</p:PhoneApplicationPage.Resources>

And in the data template, use the converter:

<DataTemplate>
    <StackPanel DataContext="{Binding Converter={StaticResource MustBeItem}}" >
        <TextBlock Tekst="{Binding Path=Name}" />
    </StackPanel>
</DataTemplate>

Now the Name property of the parent will never be checked.

Answer 2

I think your assessment of the situation is correct, but it sounds like a bug in the way the VirtualizingStackPanel works. It's not affecting the execution or bindings of your application at the points where you need it, but raising exceptions is a relatively expensive operation and so it would make sense to update the appropriate DataContext accordingly when unlinking the containers.

I think the best place to raise it as a potential bug would be on the App Hub forums