Set custom ScrollViewer property value with AttachedProperty

Question

I retemplated TreeView with my custom ScrollViewer. If I try to set AttachedProperty "ScrollViewer.PanningMode" of TreeView in code-behind, then my custom ScrollViewer doesn't seem to change (PanningMode of ScrollViewer stays the same).

Code-behind:

_treeView.SetValue(ScrollViewer.PanningModeProperty, PanningMode.None);

EDIT

Actually, I solved it like this:

ScrollViewer scrollViewer = _treeView.Template.FindName("ScrollViewer", _treeView) as ScrollViewer;
if (scrollViewer != null)
    scrollViewer.PanningMode = PanningMode.None;

But it is not very beautiful and doesn't answer my question.