How can I use predefined headers inside a Silverlight treeview for each item?

For my Silverlight TreeView's items source I have a collection of Order items, and each Order has two collections, OrderItems and Commissions. So I want a treeview that looks kind of like

Order #1
    - Order Items
        - Order Item #1
        - Order Item #2
        - Order Item #3
    - Commissions
        - Commission #1
        - Commission #2
        - Commission #3
        - Commission #4

etc. So ever Order will have a Order Items and Commissions header, and the contents of these are databound. I'm kind of stumped by this, even though it seems kind of simple.

This is the XAML I have 开发者_开发技巧so far. Obviously creating the HierarchicalDataTemplates for the OrderItems and CommissionsItems collections is simple, but how do I set the ItemsSource of the HDT above? In other words, what would [what goes here?] look like?

    <Grid>
        <Grid.Resources>                
            <sdk:HierarchicalDataTemplate 
                x:Key="OrdersTreeLevel0" 
                ItemsSource="{StaticResource [what goes here?]}"
                ItemTemplate={StaticResource OrdersTreeLevel1}">

                    <TextBlock 
                        FontWeight="{Binding IsUnread}" 
                        Text="{Binding Id, StringFormat='Order #{0}'}" />

            </sdk:HierarchicalDataTemplate>
        </Grid.Resources>

        <sdk:TreeView 
            ItemsSource="{Binding Items}" 
            ItemTemplate="{StaticResource OrdersTreeLevel0}">
        </sdk:TreeView>

    </Grid>

You'll need a value converter for this to take an Order object and convert it to an array of a type that carries Name and Items properties:-

public class ItemWrapper
{
     public string Name {get; set; }
     public IEnumerable Items {get; set;}
}

public class OrderConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        Order order = value as Order;
        if (order != null)
        {
            List<object> result = new List<object>();
            result.Add(new ItemWrapper() {Name = "Order Items", Items = order.OrderItems} );
            result.Add(new ItemWrapper() {Name = "Commission Items", Items = order.CommissionItems} );
        }
    }
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
         throw new NotImplementedException();
    }
}

Now you add this converter to your resources and reference it in your template:-

    <Grid.Resources>
        <local:OrderConverter x:Key="OrderConverter" />                    
        <sdk:HierarchicalDataTemplate     
            x:Key="OrdersTreeLevel0"     
            ItemsSource="{Binding Converter=OrderConverter}"    
            ItemTemplate={StaticResource OrdersTreeLevel1}">    

                <TextBlock     
                    FontWeight="{Binding IsUnread}"     
                    Text="{Binding Id, StringFormat='Order #{0}'}" />    

        </sdk:HierarchicalDataTemplate>    
    </Grid.Resources>  

Of course thats the easy bit, the really tricky bit is if you need a different template for Commission and OrderItem. If these both have simple properties in common such as Name then a single template for level2 will suffice. Otherwise things get more complicated.