C# FileStream Lock. How to wait for a file to get released and lock safely.

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3 Mar 2011CPOL

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FileStream has Lock and Unlock methods, but no way to see if file was unlocked by another process.

Let's say we have a file - the one to be used with multiple processes.

I start with an assumption that file gets Write locked to get changed. If that is not the case for you, then my approach will not work. So if it sounds reasonable for you, then see the solution.

public static void Lock(string path, Action<FileStream> action)
{
    var autoResetEvent = new AutoResetEvent(false);

    while(true)
    {
        try
        {
            using (var file = File.Open(path,
                                        FileMode.OpenOrCreate,
                                        FileAccess.ReadWrite,
                                        FileShare.Write))
            {
                action(file);
                break;
            }
        }
        catch (IOException)
        {
            var fileSystemWatcher =
                new FileSystemWatcher(Path.GetDirectoryName(path))
                        {
                            EnableRaisingEvents = true
                        };

            fileSystemWatcher.Changed +=
                (o, e) =>
                    {
                        if(Path.GetFullPath(e.FullPath) == Path.GetFullPath(path))
                        {
                            autoResetEvent.Set();
                        }
                    };

            autoResetEvent.WaitOne();
        }
    }
}

So as you can see, we do an infinite loop which exits only on non IOException or when the block finishes successfully.

To wait and not consume 100% CPU AutoResetEvent which gets Set by FileSystemWatcher.Changed event.

So the flow looks like that:

1) We try to open the file
2) If we have an IOException, we wait until the file gets changed
3) We try to open file again, if failed - wait again
4) If file opened successfully, we perform an action passed as a parameter

You should carefully handle IOException inside your action. See the example:

FileLocker.Lock(@"c:\file",
        (f) =>
            {
                try
                {
                    f.Write(buf, 0, buf.Length);
                }
                catch(IOException ioe)
                {
// handle IOException
                }
            });

If you have any ideas on how to improve the solution, feel free to comment.

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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using System; using System.IO; using System.Threading; namespace HQ.Util.General { public class FileTryActionOnFile { /// <summary> /// Try to do an action on a file until a specific amount of time /// </summary> /// <param name="path">Path of the file</param> /// <param name="action">Action to execute on file</param> /// <param name="milliSecondMax">Maimum amount of time to try to do the action</param> /// <returns>true if action occur and false otherwise</returns> public static bool TryTo(string path, Action<FileStream> action, int milliSecondMax = Timeout.Infinite) { bool result = false; DateTime dateTimestart = DateTime.Now; Tuple<AutoResetEvent, FileSystemWatcher> tuple = null; while (true) { try { using (var file = File.Open(path, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.Write)) { action(file); result = true; break; } } catch (IOException ex) { // Init only once and only if needed. Prevent against many instantiation in case of multhreaded // file access concurrency (if file is frequently accessed by someone else). Better memory usage. if (tuple == null) { var autoResetEvent = new AutoResetEvent(true); var fileSystemWatcher = new FileSystemWatcher(Path.GetDirectoryName(path)) { EnableRaisingEvents = true }; fileSystemWatcher.Changed += (o, e) => { if (Path.GetFullPath(e.FullPath) == Path.GetFullPath(path)) { autoResetEvent.Set(); } }; tuple = new Tuple<AutoResetEvent, FileSystemWatcher>(autoResetEvent, fileSystemWatcher); } int milliSecond = Timeout.Infinite; if (milliSecondMax != Timeout.Infinite) { milliSecond = (int)(DateTime.Now - dateTimestart).TotalMilliseconds; if (milliSecond >= milliSecondMax) { result = false; break; } } tuple.Item1.WaitOne(milliSecond); } } if (tuple != null && tuple.Item1 != null) // Dispose of resources now (don't wait the GC). { tuple.Item1.Dispose(); tuple.Item2.Dispose(); } return result; } } }

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C# FileStream Lock. How to wait for a file to get released and lock safely.

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