FileSystemWatcher behavior

riceman0
riceman0 used Ask the Experts™
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I wonder if anyone out there can confirm and explain two of my observations about FileSystemWatcher:

a) if I have a process that is continuously writing to a file for some period of time, it doesn't report an event until the process is done, then it will report a change.
b) it seems to miss file deletes if I delete it from a DOS window, but it will catch the delete if I delete it from Windows Explorer

Thanks...
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Author

Commented:

Addendum: it is because of these two facts that I have changed my watcher mechanism to a timer-based function that continuously checks the file size.  Pretty inelegant, eh?

Commented:
Hi riceman0
What NotifyFilter do you use?
try  FileSystemWatcher1.NotifyFilter = NotifyFilters.LastWrite
Jack.net

Commented:
Oh, I forgot,
Yes, I made the same experience. Changes made in a Dos window (CMD) will not trigger the File System Watcher.
Jack.net

Author

Commented:

I've tried a variety of filters, similar results.  Do you know if it's supposed to catch changes by other processes/apps?  Or just by the user through the windows shell?  MSDN isn't entirely clear on that...

Author

Commented:

And... changes made in Notepad DO trigger the FileSystemWatcher... what's the difference?  

Commented:
I do a lot of inter-process communication using the file system watcher. It works between processes / applications. It also works if the file is altered by a service.
Jack.net

Author

Commented:

Any idea why not the DOS window then?  It's just another process, I would think.

Also, seen anything similar to my observation (a)?

Commented:
a) I found some missing event reporting using the Dos (CMD) window. But I did not follow up on this, since it was not an operational issue.

b) Question: do you close the write process between write statements? It will only report a write event if the write process closes. You can't access a file anyhow as long as the write process is not closed and the file is open for write by your porcess.

Author

Commented:

The other process keeps it open and writes to it every cycle until it is done with it.  Actually I've found I can access it (for reading) while it is opened for writing by the other process, if I open it with a shared flag as follows:

Dim fs As New FileStream(FilePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
Commented:
Yes, you can do this with FileShare.ReadWrite, but you have to be aware that you may read incomplete information.

If you keep the file open between writes, the FileSystemWatcher can not be used to check porgress of the write process. Then your file-size method is probably the best alternative.
Jack.net

Author

Commented:

Thanks.  You've got at least a share of the points... I'll keep this open a bit to see if anyone has an insight as to why a delete from the DOS window doesn't (and therefore which other programs might not) trigger the FileSystemWatcher...

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