This event is raised when the active document used by modeless user interface changes. On Mac Rhino this will get raised before the NewDocument , BeginOpenDocument and EndOpenDocument events. Mac Rhino will also raise this event with 0 for the document Id and a None document pointer when the last document is closed. Windows Rhino will raise this event after the NewDocument , BeginOpenDocument and EndOpenDocument events when a new or existing model is opened.