文件锁定时避免崩溃？

Question

来自 cmets 的解决方案：
应用程序崩溃是由另一个问题引起的

我正在从 2 个不同的应用程序读取/写入文件，当文件被读取或写入时，它将始终被应用程序 A 或 B 锁定，并且它们都使用FileShare.None。

我的问题是，即使将阅读器包裹在 try/catch 周围，它仍然会在 using 行使用 IOException 使应用程序崩溃（写入器不会发生）。

我也把它写成了catch (IOException ...，我相信除了让它更具可读性之外没有什么区别。

文件被锁定时忽略并继续尝试直到文件可用的正确方法是什么？

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);
}

写

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;
    }
}

请注意，当文件被用于读取或写入的任何进程时，我不会向任何人授予共享权限（作者和读者都使用FileShare.None）所以基本上我正在处理异常，直到文件可用这不起作用。

Answer 1

这是我们用于此目的的代码。

/// <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);
    }
}

使用示例：

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

我使用来自Is there a global named reader/writer lock? 的信息做过一次。

我想结果有点像ReaderWriterLockSlim，它适用于多个进程而不是线程访问资源的情况。

Answer 3

您可以使用互斥对象来保护共享资源不被多个线程或进程同时访问。

Answer 4

您可以通过编写如下函数来检查您的文件锁定：

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 5

Timwi 的回答对我们帮助很大（尽管在另一个上下文中），但我发现如果你想从所有 IOExceptions 中获取 HResult，还需要添加标志“BindingFlags.Public”：

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

Answer 6

使用读写锁
正确的 catch 语法

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);
}