The TreeView.ExpandCollapseCursorRow event utilizes this delegate:

Event data is passed via the Gtk.ExpandCollapseCursorRowArgs parameter.

To attach a Gtk.ExpandCollapseCursorRowHandler to an event, add the ExpandCollapseCursorRowHandler instance to the event. The methods referenced by the ExpandCollapseCursorRowHandler instance are invoked whenever the event is raised, until the ExpandCollapseCursorRowHandler is removed from the event.