Avoid crashing when file is locked?

Question

Resolution from comments:
Application crash was caused by another issue

I am reading/writting to a file from 2 different applications, when the file is being read or written to, it will always be locked by app A or B and they both make use of FileShare.None.

My issue is that even wrapping the reader around try/catch it still crashes the application with IOException at the using line (does not happen with the writter).

I have also made the catch as catch (IOException ... which I believe makes no difference other then make it more readable.

What is the correct way to ignore when the file is locked and keep trying until the file is available ?

while (true)
{
    try
    {
        using (FileStream stream = new FileStream("test_file.dat", FileMode.Open, FileAccess.Read, FileShare.None))
        {
            using (TextReader reader = new StreamReader(stream))
            {
                // bla bla bla does not matter
            }
        }
    }
    catch
    {
        // bla bla bla does not matter again
    }
    Thread.Sleep(500);
}

Write

private bool WriteData(string data)
{
    try
    {
        using (FileStream stream = new FileStream("test_file.dat", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
        {
            stream.SetLength(0);
            using (TextWriter writer = new StreamWriter(stream))
            {
                writer.Write(data);
            }
        }
        return true;
    }
    catch
    {
        return false;
    }
}

Please note that I am not giving share rights (both writer and reader use FileShare.None) to anyone when the file is being used for whatever process be it reading or writting so basically I am handling the exception until the file is available which is not working.

Answer 1

Here is the code we use for this purpose.

/// <summary>
/// Executes the specified action. If the action results in a file sharing violation exception, the action will be
/// repeatedly retried after a short delay (which increases after every failed attempt).
/// </summary>
/// <param name="action">The action to be attempted and possibly retried.</param>
/// <param name="maximum">Maximum amount of time to keep retrying for. When expired, any sharing violation
/// exception will propagate to the caller of this method. Use null to retry indefinitely.</param>
/// <param name="onSharingVio">Action to execute when a sharing violation does occur (is called before the waiting).</param>
public static void WaitSharingVio(Action action, TimeSpan? maximum = null, Action onSharingVio = null)
{
    WaitSharingVio<bool>(() => { action(); return true; }, maximum, onSharingVio);
}

/// <summary>
/// Executes the specified function. If the function results in a file sharing violation exception, the function will be
/// repeatedly retried after a short delay (which increases after every failed attempt).
/// </summary>
/// <param name="func">The function to be attempted and possibly retried.</param>
/// <param name="maximum">Maximum amount of time to keep retrying for. When expired, any sharing violation
/// exception will propagate to the caller of this method. Use null to retry indefinitely.</param>
/// <param name="onSharingVio">Action to execute when a sharing violation does occur (is called before the waiting).</param>
public static T WaitSharingVio<T>(Func<T> func, TimeSpan? maximum = null, Action onSharingVio = null)
{
    var started = DateTime.UtcNow;
    int sleep = 279;
    while (true)
    {
        try
        {
            return func();
        }
        catch (IOException ex)
        {
            int hResult = 0;
            try { hResult = (int) ex.GetType().GetProperty("HResult", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(ex, null); }
            catch { }
            if (hResult != -2147024864) // 0x80070020 ERROR_SHARING_VIOLATION
                throw;
            if (onSharingVio != null)
                onSharingVio();
        }

        if (maximum != null)
        {
            int leftMs = (int) (maximum.Value - (DateTime.UtcNow - started)).TotalMilliseconds;
            if (sleep > leftMs)
            {
                Thread.Sleep(leftMs);
                return func(); // or throw the sharing vio exception
            }
        }

        Thread.Sleep(sleep);
        sleep = Math.Min((sleep * 3) >> 1, 10000);
    }
}

Example of use:

Utilities.WaitSharingVio(
    action: () =>
    {
        using (var f = File.Open(file, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
        {
            // ... blah, process the file
        }
    },
    onSharingVio: () =>
    {
        Console.WriteLine("Sharing violation. Trying again soon...");
    }
);

Answer 2

use reader & writer lock
correct syntax of catch to

catch(Exception e)

while (true)
{
    try
    {
        using (FileStream stream = new FileStream("test_file.dat", FileMode.Open, FileAccess.Read, FileShare.None))
        {
            using (TextReader reader = new StreamReader(stream))
            {
                // bla bla bla does not matter
            }
        }
    }
    catch(Exception e)
    {
        // bla bla bla does not matter again
    }
    Thread.Sleep(500);
}

Answer 3

You can use a mutex object to protect a shared resource from simultaneous access by multiple threads or processes.

Answer 4

I did this once using info from Is there a global named reader/writer lock?.

I suppose the outcome is somewhat like a ReaderWriterLockSlim that works in the case where multiple processes, not threads, are accessing a resource.

Answer 5

You can check for your file lock by writing the function as below:

protected bool IsFileLocked(FileInfo file)
{
    FileStream stream = null;

    try
    {
        stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None);
    }
    catch (IOException)
    {
        //the file is unavailable because it is:
        //still being written to
        //or being processed by another thread
        //or does not exist (has already been processed)
        return true;
    }
    finally
    {
        if (stream != null)
            stream.Close();
    }

    //file is not locked
    return false;
}

Answer 6

The answer from Timwi helped us alot (albeit in another context) but I found that the flag "BindingFlags.Public" also needs to be added if you want to fetch HResult from all IOExceptions:

public static int GetHresult(this IOException ex)
{
   return (int)ex.GetType().GetProperty("HResult", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance).GetValue(ex, null);
}