How To Detect When A File has been closed in a NT directory

[philips]

I have created an OCX that uses ReadDirectoryChangesW to
detect when a file has been copied into a directory on
a NT system.
(ReadDirectoryChangesW is used sychronously with a Thread
created by the OCX and I'm using FILE_NOTIFY_CHANGE_ATTRIBUTES filter with this)

This detects when the file has arrived on the file system
but not necessarily when the file is closed and accessable
to a Visual Basic open as input statement.

How can I detect when the whole file has been transferred
and has been closed ready for the open input statement???