title: How can I use predefined headers inside a Silverlight treeview for each item?
body: <p>For my Silverlight <code>TreeView</code>'s items source I have a collection of <code>Order</code> items, and each <code>Order</code> has two collections, <code>OrderItems</code> and <code>Commissions</code>. So I want a treeview that looks kind of like</p>
  
  <pre><code>Order #1
      - Order Items
          - Order Item #1
          - Order Item #2
          - Order Item #3
      - Commissions
          - Commission #1
          - Commission #2
          - Commission #3
          - Commission #4
  </code></pre>
  
  <p>etc. So <em>ever</em> Order will have a <strong>Order Items</strong> and <strong>Commissions</strong> header, and the contents of these are databound. I'm kind of stumped by this, even though it seems kind of simple.</p>
  
  <p>This is the XAML I have so far. Obviously creating the <code>HierarchicalDataTemplate</code>s for the <code>OrderItems</code> and <code>CommissionsItems</code> collections is simple, but how do I set the <code>ItemsSource</code> of the HDT above? In other words, what would [what goes here?] look like?</p>
  
  <pre><code>    &lt;Grid&gt;
          &lt;Grid.Resources&gt;                
              &lt;sdk:HierarchicalDataTemplate 
                  x:Key="OrdersTreeLevel0" 
                  ItemsSource="{StaticResource [what goes here?]}"
                  ItemTemplate={StaticResource OrdersTreeLevel1}"&gt;
  
                      &lt;TextBlock 
                          FontWeight="{Binding IsUnread}" 
                          Text="{Binding Id, StringFormat='Order #{0}'}" /&gt;
  
              &lt;/sdk:HierarchicalDataTemplate&gt;
          &lt;/Grid.Resources&gt;
  
          &lt;sdk:TreeView 
              ItemsSource="{Binding Items}" 
              ItemTemplate="{StaticResource OrdersTreeLevel0}"&gt;
          &lt;/sdk:TreeView&gt;
  
      &lt;/Grid&gt;
  </code></pre>