The Id is limited to a specific session, and thus identifies a consistent unchanged definition. The Id can not be used to identify a definition across different sessions, which could have different actual definitions due to changing user code. See the Key property to identify a metric definition across different sessions.
Requirements
Target Platforms: Windows 7, 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