问题

File.AppendAllText是否管理来自多个写入程序的冲突？

研究

我注意到MSDN文档并没有真正提供任何位置，所以我决定反映代码，看看它能做什么。以下是来自File.AppendAllText:的调用方法

private static void InternalAppendAllText(string path, string contents, Encoding encoding)
{
    using (StreamWriter streamWriter = new StreamWriter(path, true, encoding))
    {
        streamWriter.Write(contents);
    }
}

正如您所看到的，它只是利用了CCD_ 3。因此，如果我们深入研究一下，特别是它使用的构造函数，我们会发现它最终调用了这个构造函数：

internal StreamWriter(string path, bool append, Encoding encoding, int bufferSize, bool checkHost) : base(null)
{
    if (path == null)
    {
        throw new ArgumentNullException("path");
    }
    if (encoding == null)
    {
        throw new ArgumentNullException("encoding");
    }
    if (path.Length == 0)
    {
        throw new ArgumentException(Environment.GetResourceString("Argument_EmptyPath"));
    }
    if (bufferSize <= 0)
    {
        throw new ArgumentOutOfRangeException("bufferSize", Environment.GetResourceString("ArgumentOutOfRange_NeedPosNum"));
    }
    Stream streamArg = StreamWriter.CreateFile(path, append, checkHost);
    this.Init(streamArg, encoding, bufferSize, false);
}

具有以下值：

path:        the path to the file
append:      the text to append
encoding:    UTF8NoBOM
bufferSize:  1024
checkHost:   true

并且我们进一步发现CCD_ 4实现除了将CCD_ 5设置为CCD_。所以，如果我们继续挖掘，我们会发现CreateFile:

private static Stream CreateFile(string path, bool append, bool checkHost)
{
    FileMode mode = append ? FileMode.Append : FileMode.Create;
    return new FileStream(path, mode, FileAccess.Write, FileShare.Read, 4096, FileOptions.SequentialScan, Path.GetFileName(path), false, false, checkHost);
}

创建具有以下参数值的FileStream：

path:         the path to the file
mode:         FileMode.Append
access:       FileAccess.Write
share:        FileShare.Read
bufferSize:   4096
options:      FileOptions.SequentialScan
msgPath:      just the file name of the path provided
bFromProxy:   false
useLongPath:  false
checkHost:    true

因此，现在我们终于有所进展，因为我们即将利用Windows API，而这正是问题真正开始的地方，因为FileStream::ctor调用了一个名为Init的方法。这是一个相当长的方法，但我真的对一行感兴趣：

this._handle = Win32Native.SafeCreateFile(text3,
    dwDesiredAccess,
    share,
    secAttrs,
    mode,
    num,
    IntPtr.Zero);

它当然调用CreateFile，其中参数值为：

text3:            the full path to the file
dwDesiredAccess:  1073741824
share:            1 (FILE_SHARE_READ)
secAttrs:         null
mode:             4 (OPEN_ALWAYS)
num:              134217728 | 1048576 (FILE_FLAG_SEQUENTIAL_SCAN | FILE_FLAG_POSIX_SEMANTICS)

那么，如果我有两个线程试图同时访问同一路径的调用，Windows会怎么办？它会打开文件并缓冲写入，这样两个使用者都可以写入文件吗？还是我需要在调用AppendAllText时利用锁定对象和lock？

private static Stream CreateFile(string path, bool append, bool checkHost)
{
    FileMode mode = append ? FileMode.Append : FileMode.Create;
    return new FileStream(path, mode, FileAccess.Write, FileShare.Read, 4096, FileOptions.SequentialScan, Path.GetFileName(path), false, false, checkHost);
}

    EventWaitHandle waitHandle = new EventWaitHandle(true, EventResetMode.AutoReset, "SHARED_BY_ALL_PROCESSES");
    Task[] tasks = new Task[ThreadCount];
    for (int counter = 0; counter < ThreadCount; counter++)
    {
        var dividedList = ....
        tasks[counter] = await Task.Factory.StartNew(async () => await RunTask(counter, dividedList, waitHandle));
    }

    private static async Task RunTask(int threadNum, List<string> ids, EventWaitHandle waitHandle)
    {
        Console.WriteLine($"Start thread {threadNum}");
        foreach (var id in ids)
        {
            // start waiting
            waitHandle.WaitOne();
            File.AppendAllText(@".'Result.txt", text + Environment.NewLine);
            waitHandle.Set();
            // until release
            // ninja code
        }
        Console.WriteLine($"End thread {threadNum}");
    }