RootActivityId property returns the id of the root recurring activity. This property is only applicable to occurrence or a variance of a recurring activity. It returns null for other activities.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2