FileShare.None 是否让线程等待文件流关闭？

Question

当使用文件流时，将 FileShare 设置为 None，并假设两个用户同时访问同一功能想要 read/write 该文件。 FileShare.None 是让第二个用户请求等待还是第二个用户的请求会抛出异常？

//two users get to this this code at the same time

using (FileStream filestream = new FileStream(chosenFile, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
using (StreamReader sr = new StreamReader(filestream))
using (StreamWriter sw = new StreamWriter(filestream))
{
    //reading and writing to file
}

Msdn 说：None 拒绝共享当前文件。任何打开文件的请求（通过此进程或其他进程）都将失败，直到文件关闭。

但是请求会一直尝试直到文件流关闭吗？

Answer 1

当一个进程用 FileShare.None 打开一个文件 Read/Write 时，任何进程对该同一文件的任何后续访问都将导致 Acess Denied Exception。要回答您的问题，第二个用户将获得例外。

MSDN: FileShare.None - Declines sharing of the current file. Any request to open the file (by this process or another process) will fail until the file is closed.

有很多方法可以处理此类并发文件访问问题，以下代码演示了一种解决这种情况的简单方法。

//Retry 5 times when file access fails
int retryCounter = 5;

while (!isFileAccessSuccess && retryCounter > 0)
{
    try
    {
       //Put file access logic here

        //If the file has been accessed successfully set the flag to true
        isFileAccessSuccess = true;
    }
    catch (Exception exception)
    {
        //Log exception
    }
    finally
    {
        //Decrease the retry count
        --retryCounter;
    }

    if (!isFileAccessSuccess)
    {
        //Wait sometime until initiating next try
        Thread.Sleep(10000);
    }
}

Answer 2

不，IOException 将被抛出 HResult = -2147024864 和 Message = The process cannot access the file 'path' because it is being used by another process.

如果您想同步对文件的访问，您可以使用命名等待句柄。

public class FileAcessSynchronizer
{
    private readonly string _path;
    private readonly EventWaitHandle _waitHandle;

    public FileAcessSynch(string path)
    {
        _path = path;
        _waitHandle =  new EventWaitHandle(true, EventResetMode.AutoReset, "NameOfTheWaitHandle");
    }

    public void DoSomething()
    {
        try
        {
            _waitHandle.WaitOne();
            using (FileStream filestream = new FileStream(chosenFile, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
            using (StreamReader sr = new StreamReader(filestream))
            using (StreamWriter sw = new StreamWriter(filestream))
            {
                //reading and writing to file
            }
        }
        finally
        {
            _waitHandle.Set();
        }
    }
}

因为命名等待句柄创建了一个 critical section 应用程序的任何两个线程或进程（使用与等待句柄名称相同的名称）都不能同时执行其中的代码。因此，一个线程或进程进入该部分，以任何人（其他应用程序）都无法访问它的方式打开文件，执行命令，最后离开临界区以允许应用程序的其他线程或进程进入临界区。

FileShare.None 是否让线程等待文件流关闭？

Does FileShare.None make threads wait until the filestream is closed?

.net

c#

file

fileshare

thread-safety