How to select all the leaf nodes of the parent node in WinForms TreeViewAdv?

private void treeViewAdv1_AfterSelect(object sender, System.EventArgs e)
{
   TreeNodeAdvCollection tn = new TreeNodeAdvCollection();
   foreach (TreeNodeAdv n in this.treeViewAdv1.SelectedNodes)
   {
      this.AddChildren(tn, n);
   }
   foreach (TreeNodeAdv n in tn)
   {
      this.treeViewAdv1.SelectedNodes.Add(n);
   }
}
private void AddChildren(TreeNodeAdvCollection tn, TreeNodeAdv n)
{
   foreach (TreeNodeAdv child in n.Nodes)
   {
      tn.Add(child);
      this.AddChildren(tn, child);
   }
}

Private Sub treeViewAdv1_AfterSelect(ByVal sender As Object, ByVal e As System.EventArgs)
    Dim tn As TreeNodeAdvCollection = New TreeNodeAdvCollection()
    For Each n As TreeNodeAdv In Me.treeViewAdv1.SelectedNodes
       Me.AddChildren(tn, n)
    Next n
    For Each n As TreeNodeAdv In tn
       Me.treeViewAdv1.SelectedNodes.Add(n)
    Next n
End Sub
Private Sub AddChildren(ByVal tn As TreeNodeAdvCollection, ByVal n As TreeNodeAdv)
    For Each child As TreeNodeAdv In n.Nodes
        tn.Add(child)
        Me.AddChildren(tn, child)
    Next child
End Sub