TreeView 做类似导航的功能

我在项目中做了个TreeView 的导航
代码如下
<control:TreeView x:Name="treeView" SelectedItemChanged="treeView_SelectedItemChanged" >
<control:TreeViewItem Header="教师管理系统 (Control)">
<control:TreeViewItem Header="Scores" Tag="PageDemo"/>
</control:TreeViewItem>
</control:TreeView>
<Grid x:Name="pnl" Grid.Column="2" Grid.Row="1">


</Grid>
----------------------------
我想当我点击了Scores时,在 pnl 中新增加Scores容器.

我cs代码
private void treeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
var treeView = sender as System.Windows.Controls.TreeView;
string tag = ((System.Windows.Controls.TreeViewItem)treeView.SelectedItem).Tag.ToString();

if (tag == null)
{
return;
}


System.Type type = typeof(Page).Assembly.GetType(tag,true);
UIElement element = Activator.CreateInstance(type) as UIElement;

this.pnl.Children.Clear();
this.pnl.Children.Add(element);

pageScroll.ScrollToVerticalOffset(0);
}

----------------------------------
问题出来了.type=null 不能获取到tag 的类型.

还请高手指点.
(SL3)