Why does File.Move allow 2 threads to move the same file at the same time?

Question

We currently have one application that monitors a folder for new files. To make it fault tolerant and be able to process more files at once, we want to be able to run multiple instances of this application on different machines. We use File.Move to "lock" a file and make sure that only one thread can process a file at a time.

To test that only one application and/or thread can perform a File.Move on a file, I created a simple application (based on the original application's code), which created 10 threads per application and monitored a folder, when each thread detects a new file, it performs File.Move on it and changes the file's extension, to try and stop other thread's from doing the same.

I have seen an issue when running multiple copies of this application (and it running on its own), whereby 2 threads (either in the same application or different ones), both successfully perform File.Move with no exception thrown, but the thread that performed it last (I change the file's extension to include the DateTime.Now.ToFileTime()), successfully renamed the file. I have looked at what File.Move does and it checks to see if the file exists before it performs the operation, then it calls out to Win32Native.MoveFile to perform the move.

All the other threads/applications throw an exception, as I would expect.

The reasons why this is an issue are:

1. I thought only 1 thread can perform a File.Move on a file at a time.
2. I need to reliably have only one application/thread be able to process a file at a time.

Here is the code that performs the File.Move:

public bool TryLock(string originalFile, out string changedFileName)
{
    FileInfo fileInfo = new FileInfo(originalFile);
    changedFileName = Path.ChangeExtension(originalFile, ".original." + DateTime.Now.ToFileTime());
    try
    {
        File.Move(originalFile, changedFileName);
    }
    catch (IOException ex)
    {
        Console.WriteLine("{3} - Thread {1}-{2} File {0} is already in use", fileInfo.Name, Thread.CurrentThread.ManagedThreadId, id, DateTime.Now.ToLongTimeString());
        return false;
    }
    catch (Exception ex)
    {
        Console.WriteLine("{3} - Thread {1}-{2} File {0} error {4}", fileInfo.Name, Thread.CurrentThread.ManagedThreadId, id, DateTime.Now.ToLongTimeString(), ex);
        return false;
    }
    return true;
}

Note - id is just a sequential number I assigned to each thread for logging.

I am running Windows 7 Enterprise SP1 on a SSD with NTFS.

Answer 1

From the MSDN description I assume that File.Move does not open the file in exclusive mode.

If you try to move a file across disk volumes and that file is in use, the file is copied to the destination, but it is not deleted from the source.

Anyway, I think you are better off to create your own move mechanism and have it open the file in exclusive mode prior to copying it (and then deleting it):

File.Open(pathToYourFile, FileMode.Open, FileAccess.Read, FileShare.None);

Other threads won't be able to open it if the move operation is already in progress. You might have race condition issues between the moment the copy is finalized (thus you need to dispose of the file handle) and deleting it.

Answer 2

Using File.Move as a lock isn't going to work. As stated in @marceln's answer, it won't delete the source file it is already in use elsewhere and doesn't have a "locking" behavior, you can't relay on it.

What i would suggest is to use a BlockingCollection<T> to manage the processing of your files:

// Assuming this BlockingCollection is already filled with all string file paths
private BlockingCollection<string> _blockingFileCollection = new BlockingCollection<string>();

public bool TryProcessFile(string originalFile, out string changedFileName)
{
    FileInfo fileInfo = new FileInfo(originalFile);
    changedFileName = Path.ChangeExtension(originalFile, ".original." + DateTime.Now.ToFileTime());

    string itemToProcess;
    if (_blockingFileCollection.TryTake(out itemToProcess))
    {
        return false;
    }

    // The file should exclusively be moved by one thread,
    // all other should return false.

    File.Move(originalFile, changedFileName);
    return true;
}

Answer 3

Are you moving across volumes or within a volume? In the latter case no copying is necessary.

.

@usr In production, once a thread has "locked" a file, we will be moving it across network shares

I'm not sure whether that is a true move or a copy operation. In any case, you could:

1. open the file exclusively
2. copy the data
3. delete the source by handle (Deleting or Renaming a file using an open handle)

That allows you to lock other processes out of that file for the duration of the move. It is more of a workaround than a real solution. Hope it helps.

Note, that for the duration of the move the file is unavailable and other processes will receive an error accessing it. You might need a retry loop with a time delay between operations.

Here's an alternative:

1. Copy the file to the target folder with a different extension that is being ignored by readers
2. Atomically rename the file to remove the extension

Renaming on the same volume is always atomic. Readers might receive a sharing violation error for a very short period of time. Again, you need a retry loop or tolerate a very small window of unavailability.