The
FileSystemWatcher
doesn't track mere file opening. It just tracks physical changes to a file (create, change, delete, and that kind of thing). The only thing I can think of that will come close to doing what you want is to maintain a watch of open file handles in the system. You MIGHT be able to get that info using WMI, or some system debugging mechanism. Google is your friend.
EDIT ============
I found this googling for "c# file handle count wmi":
Listing Used Files[
^]