' Databound event handler for a TreeView   
    Protected Sub TreeView_DataBound(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim Tree As TreeView = sender
       
        ' Only prune the tree if a node is selected
        If Tree.SelectedNode IsNot Nothing Then
           
            Dim childNodes As New Generic.List(Of TreeNode)()
            Dim childNodeCollection As New TreeNodeCollection()
           
            FindNodesAtDepth(Tree.Nodes, Tree.SelectedNode.Depth, childNodes)
                       
            ' Copy from List(of TreeNode) to TreeNodeCollection
            For Each node As TreeNode In childNodes
                childNodeCollection.Add(node)
            Next
           
            ' Clear the TreeView nodes and add the nodes we found
            Tree.Nodes.Clear()
            CloneTreeNodeHierarchyRecursive(childNodeCollection, Tree.Nodes)
           
        End If
       
    End Sub

  This function, conceptually, is pretty straight forward.  Collect a list of the Nodes at the depth of the selected node, clear the nodes in the tree and add them back into the Tree.  However, in practice, the helpers needed make it a bit more complicated. 

    ' Recursively searches the tree for all nodes at a given depth
    ' and adds them to a list of TreeNodes
    Private Sub FindNodesAtDepth(ByVal collection As TreeNodeCollection, _
                                 ByVal targetDepth As Integer, _
                                 ByVal targetNodes As Generic.List(Of TreeNode))
       
        For Each node As TreeNode In collection
            If node.Depth = targetDepth Then
                targetNodes.Add(node)
            ElseIf node.Depth < targetDepth Then
                FindNodesAtDepth(node.ChildNodes, targetDepth, targetNodes)
            End If
        Next
    End Sub

There's not much to say about this function, it's pretty simple.  It's a depth-first recursive function that creates a list of all the nodes with Depth=targetDepth.

    ' Recursively creates a clone of a given TreeNode Hierarchy   
    Private Sub CloneTreeNodeHierarchyRecursive(ByVal inCollection As TreeNodeCollection, _
                                                ByVal outCollection As TreeNodeCollection)
       
        For Each node As TreeNode In inCollection
            Dim clonedNode As TreeNode = CType(node, ICloneable).Clone()           
            outCollection.Add(clonedNode)
           
            If node.ChildNodes.Count > 0 Then
                CloneTreeNodeHierarchyRecursive(node.ChildNodes, clonedNode.ChildNodes)
            End If                       
        Next
    End Sub