I have TreeView control and I want to bind tree nodes' IsExpanded property to my DataSource items!

But I have an exception: 

System.Windows.Markup.XamlParseException occurred
  Message=Set property '' threw an exception.

  StackTrace:
       at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
       at SilverlightTree.BSTreeView.InitializeComponent()
       at SilverlightTree.BSTreeView..ctor()
  InnerException: System.NotSupportedException
       Message=Cannot set read-only property ''.
       StackTrace:
            at MS.Internal.XamlMemberInfo.SetValue(Object target, Object value)
            at MS.Internal.XamlManagedRuntimeRPInvokes.SetValue(XamlTypeToken inType, XamlQualifiedObject& inObj, XamlPropertyToken inProperty, XamlQualifiedObject& inValue)
       InnerException:

inner exception:

{System.NotSupportedException: Cannot set read-only property ''.

XAML:

<Grid x:Name="LayoutRoot">
    <controls:TreeView Name="treeView" SelectedItemChanged="treeView_SelectedItemChanged"
                       Style="{Binding  TreeViewConnectingLines}" BorderBrush="{x:Null}">
        <controls:TreeView.ItemTemplate>
            <toolkit:HierarchicalDataTemplate ItemsSource="{Binding Children}">
                <StackPanel Orientation="Horizontal"  Background="Transparent">
                    <toolkitDrag:ContextMenuService.ContextMenu>
                        <toolkitDrag:ContextMenu Loaded="ContextMenu_Loaded"
                                                 Opened="ContextMenu_Opened"/>
                    </toolkitDrag:ContextMenuService.ContextMenu>
                    <Image Source="{Binding Path=Type.Icon}"  Width="20" Height="20" />
                    <TextBlock Text="{Binding Path=FullDescription}"   Height="20"
wpf silverlight data-binding treeview
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