Eto TreeGridView multi selection issue

HI All,
I am trying to handle a Eto tree view with multiple section possible. While I am able to easily select one item via the SelectedItem property I was not able to find anything usable for selecting multiple items within the tree grid view. The main problem is that our panel has collapsible nodes therefore it is hard to use the SelectedRows property to select multiple items.

Any suggestion?

Thanks
Alberto