Navigating WPF’s TreeView

In Microsoft’s Windows Presentation Foundation, I think one of the more difficult things for programmers coming from WinForms and ASP.NET to conceptualize is how to work with the logical and visual control trees.   A common way this manifests itself is in how to navigate a TreeView control.   I struggled with this quite a lot, but finally got it with some help from the guys on the MDSN forum.  I’m posting the sample I made here.  Hope this is helpful to someone.