Can I use FileStream to implement a file lock?

Question

Can I use the FileStream constructor to ensure only one process accesses a file at a time? Will the following code work?

public static IDisposable AcquireFileLock() {
    IDisposable lockObj;
    do {
        // spinlock - continually try to open the file until we succeed
        lockObj = TryOpenLockFile();

        // sleep for a little bit to let someone else have a go if we fail
        if (lockObj == null) Thread.Sleep(100); 
    }
    while (lockObj == null);

    return lockObj;
}

private static FileStream TryOpenLockFile() {
    try {
        return new FileStream(s_LockFileName, FileMode.Create, FileAccess.Read, FileShare.None);
    }
    catch (IOException) {
        return null;
    }
}

In particular, is the behaviour with FileMode.Create atomic WRT other processes? Is there something else I should be using?

EDIT: to be more specific, this is on the Microsoft CLR using local files on a single machine.

This looks good! Who cleans-up the lock file though? There is probably no functional need to do that since the `FileStream` ctor invocation always overwrites the existing lock file. — Steve Guidi, Nov 29 '10 at 18:17
For cleanup, I could always `try { File.Delete(...) }` and ignore any exceptions (as that means that another process has taken the lock in the meantime)... — thecoop, Nov 29 '10 at 18:24

Steve Townsend · Accepted Answer · 2010-11-29T18:26:12.033

2

This will do what you want. FileShare.None is the important part.

If all the writer apps are on the same computer, you could also use a named Mutex to achieve the same goal without a physical file being needed, or to mediate file access if you don't care who writes to the file last (just that writes are not concurrent). This could save you from having to reopen the file every time, and provide a blocking semantic instead of your Sleep loop (better perf).

Note that any process can squat on your file (Mutex) if the name is not treated as a secret, i.e. information that needs to be kept secure.

edited Nov 29 '10 at 18:26

answered Nov 29 '10 at 18:05

Steve Townsend

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A Mutex only works on the same computer. File locks work across a network. – Gabe Nov 29 '10 at 18:06
@Gabe - it's not clear what the use case is, but you are right – Steve Townsend Nov 29 '10 at 18:08

score 1 · Answer 2 · answered Nov 29 '10 at 18:16

1

This will work, given the following caveats:

You are on Windows (on Linux via Mono, this doesn't hold up since file locks are advisory, so your method always succeeds)
You are accessing a local file (SMB will also probably work, but NFS or WebDAV almost certainly won't)
Catching IOException is probably too broad, you need to specifically verify that it failed because of a sharing violation

answered Nov 29 '10 at 18:16

Ana Betts

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IOException is the one that is thrown when the file is already locked, I can't catch anything more specific :/ – thecoop Nov 29 '10 at 18:25
You can check the HResult to see if it has a Win32 error code, or failing that, you can (ugh) check the Message text – Ana Betts Nov 29 '10 at 18:28

Can I use FileStream to implement a file lock?

2 Answers2