WPF: How do I debug binding errors?

Question

I'm getting this in my output Window:

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.ItemsControl', AncestorLevel='1''. BindingExpression:Path=VerticalContentAlignment; DataItem=null; target element is 'ListBoxItem' (Name=''); target property is 'VerticalContentAlignment' (type 'VerticalAlignment')

This is my XAML, which when run looks correct

        <GroupBox Header="Grant/Deny Report">
            <ListBox ItemsSource="{Binding  Converter={StaticResource MethodBinder}, ConverterParameter=GrantDeny, Mode=OneWay}">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal">
                            <Label Content="{Binding Entity}"/>
                            <Label Content="{Binding HasPermission}"/>
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </GroupBox>

Answer 1

I was also going to recommend Bea Stollnitz's article but Jonathan Allen got his post in while I was still typing this one. I also recommend the links in this blog entry.

In this particular case you can see that somewhere a ListBoxItem has a FindAncestor binding to an ItemsControl that is failing. That tells you right away there is a ListBoxItem somewhere that is either:

1. Not in the visual tree, or
2. Not under an ItemsControl (a ListBox is an ItemsControl)

In addition, you know that someone, somewhere, is binding a ListBoxItem's VerticalContentAlignment property to FindAncestor.

Looking at the system themes (shipped with Expression Blend and also available through NET Reflector's BAMLViewer Add-in), we see this:

<Style x:Key="{x:Type ListBoxItem}">
  <Setter Property="VerticalContentAlignment"
          Value="{Binding Path=VerticalContentAlignment,RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}" />

This explains where the binding comes from. The next question is, how is a ListBoxItem being created that is not under a ListBox (or other ItemsControl)?

Some things to look for:

• Are you constructing ListBoxItems in code anywhere?
• Is there any ListBoxItem explicitly specified in your XAML?
• Do you have any code that manually manipulates the items in ListBox?

Hopefully this will head you in the right direction.

Answer 2

I ran into a similar problem with the TreeView (though my data binding errors showed up as being Informational).

I solved the problem by defining an implicit style in the TreeView resource for TreeViewItem. Within that style I defined the missing vertical and horizontal content alignment properties.

<TreeView.Resources>
     <Style TargetType="{x:Type TreeViewItem}" >
          <Setter Property="VerticalContentAlignment" Value="Stretch"/>
          <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
     </Style>
</TreeView.Resources>