As the error message says, you need to set the
RoutedEvent
property of the
MouseButtonEventArgs
to the routed event you're raising.
Something like this should work:
e = new MouseButtonEventArgs(InputManager.Current.PrimaryMouseDevice, e.Timestamp, MouseButton.Left);
e.RoutedEvent = UIElement.MouseLeftButtonDownEvent;
currentDesigner.OnMouseLeftButtonDown(sender as AnnContainer, e);