You need to call the Lock method when you are about to change or read the document and there are possibly other threads that want to gain aceess to it as well. Note that you must unlock the document after the reading / modifications are done.
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