It's transitory information which you shouldn't normally rely on. A file
that's opened/closed one moment could be closed/opened the next. The safest
route is to simply open the file with the appropriate "FileShare" access.
See this enumeration for details.

Why do you want this information?

Note that if you do not yourself open and lock the file, then checking  
whether the file is open at any given moment in time is pointless.  A  
split second after you make the check, the state could change.

If you need to open the file yourself, then just try to open it.  If you  
don't need to open the file yourself, then there is not usually really any  
use in knowing whether the file is already open or used by another process.

(One exception would be if you are trying to write a tool similar to the  
Sysinternals FileMon utility, but the kind of things that tool does  
requires much lower-level coding than .NET provides).

Pete