WPF Treeview height

In my program i have a Treeview.
If i click on a treeviewitem i want the properties of the textblock.

I want to know the top, width of the textblock.

Private Sub TreeGroupFolders_SelectedItemChanged(sender As Object, e As RoutedPropertyChangedEventArgs(Of Object)) Handles TreeGroupFolders.SelectedItemChanged

 Dim item = DirectCast(e.NewValue, DataRowView).Row
 Dim parent = item.GetParentRow("FK_FatherChild")

 Dim tv As TreeView = TreeGroupFolders
 Dim lbl As New Label

'' lbl.Top = ...
'' lbl.Width = ...

   Canvas.Children.Add(lbl)

End Sub