ListBox with Grid as ItemsPanelTemplate produces weird binding errors

Question

I've got a ListBox control and I'm presenting a fixed number of ListBoxItem objects in a grid layout. So I've set my ItemsPanelTemplate to be a Grid.

I'm accessing the Grid from code behind to configure the RowDefinitions and ColumnDefinitions.

So far it's all working as I expect. I've got some custom IValueConverter implementations for returning the Grid.Row and Grid.Column that each ListBoxItem should appear in.

However I get weird binding errors sometimes, and I can't figure out exactly why they're happening, or even if they're in my code.

Here's the error I get:

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

Can anybody explain what's going on?

Oh, and, here's my XAML:

<UserControl.Resources>
    <!-- Value Converters -->
    <v:GridRowConverter x:Key="GridRowConverter" />
    <v:GridColumnConverter x:Key="GridColumnConverter" />
    <v:DevicePositionConverter x:Key="DevicePositionConverter" />
    <v:DeviceBackgroundConverter x:Key="DeviceBackgroundConverter" />

    <Style x:Key="DeviceContainerStyle" TargetType="{x:Type ListBoxItem}">
        <Setter Property="FocusVisualStyle" Value="{x:Null}" />
        <Setter Property="Background" Value="Transparent" />

        <Setter Property="Grid.Row" Value="{Binding Path=DeviceId, Converter={StaticResource GridRowConverter}}" />
        <Setter Property="Grid.Column" Value="{Binding Path=DeviceId, Converter={StaticResource GridColumnConverter}}" />

        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ListBoxItem}">
                    <Border CornerRadius="2" BorderThickness="1" BorderBrush="White" Margin="2" Name="Bd"
                            Background="{Binding Converter={StaticResource DeviceBackgroundConverter}}">
                        <TextBlock FontSize="12" HorizontalAlignment="Center" VerticalAlignment="Center" 
                                Text="{Binding Path=DeviceId, Converter={StaticResource DevicePositionConverter}}" >
                            <TextBlock.LayoutTransform>
                                <RotateTransform Angle="270" />
                            </TextBlock.LayoutTransform>
                        </TextBlock>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsSelected" Value="true">
                            <Setter TargetName="Bd" Property="BorderThickness" Value="2" />
                            <Setter TargetName="Bd" Property="Margin" Value="1" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>            
    </Style>

</UserControl.Resources>

<Border CornerRadius="3" BorderThickness="3" Background="#FF333333" BorderBrush="#FF333333" >
    <Grid ShowGridLines="False">
        <Grid.RowDefinitions>
            <RowDefinition Height="15" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <StackPanel Grid.Row="0" Orientation="Horizontal">
            <Image Margin="20,3,3,3" Source="Barcode.GIF" Width="60" Stretch="Fill" />
        </StackPanel>

        <ListBox ItemsSource="{Binding}" x:Name="lstDevices" Grid.Row="1" 
                 ItemContainerStyle="{StaticResource DeviceContainerStyle}"
                 Background="#FF333333"
                 SelectedItem="{Binding SelectedDeviceResult, ElementName=root, Mode=TwoWay}" 
        >
            <ListBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <Grid>
                        <Grid.LayoutTransform>
                            <RotateTransform Angle="90" />
                        </Grid.LayoutTransform>                            
                    </Grid>
                </ItemsPanelTemplate>
            </ListBox.ItemsPanel>
        </ListBox>
    </Grid>
</Border>

Answer 1

According to the Data Templating Overview on MSDN, DataTemplates should be used as the ItemTemplate to define how the data is presented, while a Style would be used as the ItemContainerStyle to style just the generated container, such as ListBoxItem.

However, it appears that you are trying to use the latter to do the job of the former. I can't recreate your situation without much more code, but I suspect that doing databinding in the container style could be throwing a wrench in the assumed visual/logical tree.

I also can't help but think that a custom layout of items based on the item's information calls for creating a custom Panel. It's probably better for the custom Panel to layout the items than for the items to lay themselves out with a Rube Goldberg assortment of IValueConverters.

Answer 2

The binding problem comes from the default style for ListBoxItem. By default when applying styles to elements WPF looks for the default styles and applies each property that is not specifically set in the custom style from the default style. Refer to this great blog post By Ian Griffiths for more details on this behavior.

Back to our problem. Here is the default style for ListBoxItem:

<Style
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:s="clr-namespace:System;assembly=mscorlib"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    TargetType="{x:Type ListBoxItem}">
    <Style.Resources>
       <ResourceDictionary/>
    </Style.Resources>
    <Setter Property="Panel.Background">
       <Setter.Value>
          <SolidColorBrush>
        #00FFFFFF
          </SolidColorBrush>
       </Setter.Value>
    </Setter>
    <Setter Property="Control.HorizontalContentAlignment">
       <Setter.Value>
          <Binding Path="HorizontalContentAlignment" RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType=ItemsControl, AncestorLevel=1}"/>
       </Setter.Value>
    </Setter>
    <Setter Property="Control.VerticalContentAlignment">
       <Setter.Value>
          <Binding Path="VerticalContentAlignment" RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType=ItemsControl, AncestorLevel=1}"/>
       </Setter.Value>
    </Setter>
    <Setter Property="Control.Padding">
       <Setter.Value>
          <Thickness>
        2,0,0,0
          </Thickness>
       </Setter.Value>
    </Setter>
    <Setter Property="Control.Template">
       <Setter.Value>
          <ControlTemplate TargetType="{x:Type ListBoxItem}">
             ...
          </ControlTemplate>
       </Setter.Value>
    </Setter>
 </Style>

Note that I have removed the ControlTemplate to make it compact (I have used StyleSnooper to retrieve the style). You can see that there is a binding with a relative source set to ancestor with type ItemsControl. So in your case the ListBoxItems that are created when binding did not find their ItemsControl. Can you provide more info with what is the ItemsSource for your ListBox?

P.S.: One way to remove the errors is to create new setters for HorizontalContentAlignment and VerticalContentAlignment in your custom Style.

Answer 3

This is a common problem with ListBoxItems and other ephemeral *Item containers. They are created asynchronously/on the fly, while the ItemsControl is loaded/rendered. You have to attach to ListBox.ItemContainerGenerator's StatusChanged event and wait for the Status to become ItemsGenerated before trying to access them.

Answer 4

Setting OverridesDefaultStyle to True in your ItemContainerStyle will also fix these problems.

<Style TargetType="ListBoxItem">
    <Setter Property="OverridesDefaultStyle" Value="True"/>
    <!-- set the rest of your setters, including Template, here -->
</Style>

Answer 5

If you want to completely replace the ListBoxItem template such that no selection is visible (perhaps you want the look of ItemsControl with the grouping/etc behaviour of ListBox) then you can use this style:

<Style TargetType="ListBoxItem">
  <Setter Property="Margin" Value="2" />
  <Setter Property="FocusVisualStyle" Value="{x:Null}" />
  <Setter Property="OverridesDefaultStyle" Value="True" />
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="{x:Type ListBoxItem}">
        <ContentPresenter Content="{TemplateBinding ContentControl.Content}" 
                          HorizontalAlignment="Stretch" 
                          VerticalAlignment="{TemplateBinding Control.VerticalContentAlignment}" 
                          SnapsToDevicePixels="{TemplateBinding UIElement.SnapsToDevicePixels}" />
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

This template also excludes the standard Border wrapper. If you need that, you can use replace the template with this:

<Border BorderThickness="{TemplateBinding Border.BorderThickness}" 
        Padding="{TemplateBinding Control.Padding}" 
        BorderBrush="{TemplateBinding Border.BorderBrush}" 
        Background="{TemplateBinding Panel.Background}" 
        SnapsToDevicePixels="True">
  <ContentPresenter Content="{TemplateBinding ContentControl.Content}" 
                    ContentTemplate="{TemplateBinding ContentControl.ContentTemplate}" 
                    HorizontalAlignment="{TemplateBinding Control.HorizontalContentAlignment}" 
                    VerticalAlignment="{TemplateBinding Control.VerticalContentAlignment}" 
                    SnapsToDevicePixels="{TemplateBinding UIElement.SnapsToDevicePixels}" />
</Border>

If you don't need all these TemplateBinding values then you can remove some for performance.