File.Move atomic operation

Question

I am trying to generate a huge text file using C# and another process is constantly looking at the location and trying to pickup the file if available.

In order to make the file atomic below are the steps :

1 - Write to file : Filename_temp.txt
2 - Check if Filename.txt already exists then Delete
3 - Do a File.Move to the same destination     
    From filename : Filename_temp.txt 
    TO : Filename.txt

Since C# does not have a rename, I have to rely on File.Move, does this make sure the move operation will be atomic or is there another way to achieve this atomicity?

Can you update both applications logic? If yes - use Mutex to sync access — sll, Mar 07 '13 at 15:23
I recommend checking for existence and deleting first. You never know, when `Filename_temp.txt` might already exist. — Nolonar, Mar 07 '13 at 15:23
@Tigran, he wants the file to be complete before it is visible to the other process. — Anthony Pegram, Mar 07 '13 at 15:25
@Tigran Atomic in this context means "all or nothing" so basically the file is not available to the watcher until its completely written. — Murtaza Mandvi, Mar 07 '13 at 15:25
@sll the other application is out of my control and I have no rights to change that application. — Murtaza Mandvi, Mar 07 '13 at 15:26
This is essentially the same question http://stackoverflow.com/questions/2222119/atomic-file-copy-under-net — Jodrell, Mar 07 '13 at 15:26

score 15 · Accepted Answer · answered Mar 07 '13 at 15:30

According to the MSDN blog article How to do atomic writes in a file, renaming an NTFS file is an atomic operation:

The solution? Let's remember that metadata changes are atomic. Rename is such a case. So, we can just perform the write to a temporary file, and after we know that the writes are on the disk (completed and flushed) then we can interchange the old file with the new file.

Granted, this does not guarantee that File.Move just issues an NTFS rename operation, but I can't think of a valid reason why it should do anything more complicated.

Sure there's a reason: the file could be moved to a different device. — PythonNut, Jul 14 '15 at 03:32

Gareth Wilson · Answer 2 · 2013-03-07T15:38:48.950

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File.Move should be a 'rename' if the source and destination are on the same volume. So, regardless of file size, the Move should be 'instant'. I presume that is your concern?

From a FAQ from a MS employee on http://msdn.microsoft.com/en-gb/library/windows/desktop/aa365240%28v=vs.85%29.aspx we have;

'Frequently asked question: Is MoveFileEx atomic if the existing and new files are both on the same drive?

The simple answer is "usually, but in some cases it will silently fall-back to a non-atomic method, so don't count on it".'

I guess if it's 100% critical you could look at Transactional NTFS. I'm not sure if there are wrappers in .Net for this yet, so you may need to use P/Invoke.

edited Mar 07 '13 at 15:38

answered Mar 07 '13 at 15:23

Gareth Wilson

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Exactly, is that documented somewhere ? – Murtaza Mandvi Mar 07 '13 at 15:24
Can't find anything at the moment, it's more a feature of NTFS rather than .Net though. Will try to find references. – Gareth Wilson Mar 07 '13 at 15:31
+1. There seem to be contradicting sources *from Microsoft* on this point. – Heinzi Mar 07 '13 at 15:44

score 1 · Answer 3 · edited May 23 '17 at 11:53

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This is a feature of windows, rather than c# or the .Net Framework.

See here

Atomicity of File.Move

edited May 23 '17 at 11:53

Community

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answered Mar 07 '13 at 15:29

Jodrell

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score 0 · Answer 4 · answered Mar 07 '13 at 16:30

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You can write your file directly to the destination and use a zero-size signal file that is created after you huge file is ready. Your reader process can look for a signal file and read a huge file after the signal file is available. I think that can solve the "atomicity" problem.

answered Mar 07 '13 at 16:30

Slava

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Basically the semaphore files indicating DONE, however this requires the watching process to change which is unfortunately out of my bounds. – Murtaza Mandvi Mar 07 '13 at 16:36

File.Move atomic operation

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