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FileSystemWatcher behavior

Posted on 2006-10-19
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Last Modified: 2012-08-14

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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Question by:riceman0
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Author Comment

by:riceman0
ID: 17771692

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?
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Expert Comment

by:jrscherer
ID: 17775074
Hi riceman0
What NotifyFilter do you use?
try  FileSystemWatcher1.NotifyFilter = NotifyFilters.LastWrite
Jack.net
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Expert Comment

by:jrscherer
ID: 17775087
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
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Author Comment

by:riceman0
ID: 17775148

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...
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by:riceman0
ID: 17775244

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

by:jrscherer
ID: 17776169
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
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Author Comment

by:riceman0
ID: 17776215

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

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

by:jrscherer
ID: 17776358
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.
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Author Comment

by:riceman0
ID: 17776516

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)
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Accepted Solution

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jrscherer earned 500 total points
ID: 17777056
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
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Author Comment

by:riceman0
ID: 17777075

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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