why Wpf TreeView collapse action changes selection?

I have a wpf TreeView with mvvm object binded. when I have a child node selected and I collapse the parent node, the parrent node get 开发者_C百科selected. When I expand the parent again, the parent retains the selection. I want Windows Explorer behavior where when I collapse the parent, the parent shows highlight visual but never gets selection and when I expand it again, the original child shows its selection.

Thanks