Note In Microsoft Windows XP and later, when a file system filter driver calls IoRegisterFsRegistrationChange, its notification routine is also called immediately for all currently registered file systems (that is, file systems that have already called IoRegisterFileSystem but have not yet called IoUnregisterFileSystem).
Because the caller's notification routine can be called even before IoRegisterFsRegistrationChange returns, a filter driver should not call this routine until after it has created any data structures that it needs in order to process these notifications.
IoRegisterFsRegistrationChange increments the reference count on the filter driver's driver object.
Note In Update Rollup for Windows 2000 Service Pack 4 (SP4), Windows XP Service Pack 2 (SP2), and Microsoft Windows Server 2003 SP1 and later, if a file system filter driver calls IoRegisterFsRegistrationChange twice in succession (without calling IoUnregisterFsRegistrationChange in between), passing the same values for the DriverObject and DriverNotificationRoutine that it registered in the previous call to IoRegisterFsRegistrationChange, and no other filter drivers have registered since the first call, IoRegisterFsRegistrationChange returns STATUS_DEVICE_ALREADY_ATTACHED.
In Update Rollup for Windows 2000 SP4, file system filter drivers can call IoRegisterFsRegistrationChangeEx instead of IoRegisterFsRegistrationChange. The effect of IoRegisterFsRegistrationChangeEx is identical to that of IoRegisterFsRegistrationChange on Windows XP and later.