I am using Expression Blend 4 and Visual Studio 2010 to create a Sketchflow prototype.

I have a Sample Data collection and a ListBox that is bound to it. This displays as I would expect both at design time and at run time. However, the ListBoxItem template it just complex enough that I wanted to pull it out into its own XAML file. Even though the items still render as expected in the main ListBox where the template is used, when I open the template itself, all of the databound controls are empty.

If I add a DataContext to the template, I can see and work with the populated objects while in the template, but then that local DataContext overrides the DataContext set on the listbox.

A bit of code will illustrate. Start by creating a Sketchflow project (I am using Silverlight, but it should work the same for WPF), then add a project data source called SampleDataSource. Add a collection called ListData, with a single String property called Title.

Here is the (scaled down) code for the main Sketchflow screen, which we'll call Main.xaml:

<UserControl
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:local="clr-namespace:DemoScreens"
 mc:Ignorable="d"
 x:Class="DemoScreens.Main"
 Width="800" Height="600">
    <UserControl.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="ProjectDataSources.xaml"/>
            </ResourceDictionary.MergedDictionaries>
            <DataTemplate x:Key="ListBoxItemTemplate">
             <local:DemoListBoxItemTemplate d:IsPrototypingComposition="True"     Margin="0,0,5,0" Width="748"/>
            </DataTemplate>
        </ResourceDictionary>
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" Background="#5c87b2" DataContext="{Binding Source={StaticResource SampleDataSource}}">
          <ListBox Background="White" x:Name="DemoList" Style="{StaticResource ListBox-Sketch}" Margin="20,100,20,20" ItemTemplate="{StaticResource ListBoxItemTemplate}" ItemsSource="{Binding ListData}" ScrollViewer.HorizontalScrollBarVisibility="Disabled"/>
    </Grid>
</UserControl>

You can see that it references the DemoListBoxItemTemplate, which is defined in its own DemoListBoxItemTemplate.xaml:

<UserControl
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:local="clr-namespace:DemoScreens"
 mc:Ignorable="d"
 x:Class="DemoScreens.DemoListBoxItemTemplate">
 <Grid x:Name="LayoutRoot">
  <TextBlock Text="{Binding Title}" Style="{StaticResource BasicTextBlock-Sketch}" Width="150"/>
    </Grid>
</UserControl>

Obviously, this is way simpler than my actual listbox, but it should be enough to illustrate my problem. When you open Main.xaml in the Expression designer, the list box is populated with sample data. But when you open DemoListBoxItemTemplate.xaml, there is no data context and therefore no data to display—which makes it more difficult to identify controls visually.

How can I have sample data displayed when I am working with the template, while still allowing the larger set of sample data to be used for the ListBox itself?