C#中的FileStream和FileSystemWatcher,奇怪的问题"进程无法访问文件"

Question

我有这个复杂的代码库,它正在侦听某个文件夹上的FileCreated事件.当文件被创建(其中还包括将文件移动到该文件夹​​)时,我想读取该文件并对其执行某些操作.它适用于第一个文件,但在所有其他尝试之后抛出异常.在调试模式下(使用VisualStudio),将抛出错误,但如果我只是单击"继续"..它将工作(没有错误).

我发布了简化代码,它证明了这个问题.

例如,启动应用程序,单击"开始"按钮,然后"创建新的文本文件"

输出是:

Working

如果您以完全相同的方式创建2ed文件,则输出为:

Broken: The process cannot access the file 'C:\TestFolder\New Text Document (2).txt' because it is being used by another process.
Working, after breaking

在查看我的代码之后,您将看到上面的打印输出意味着首先抛出了"无法访问文件"异常,但是在catch语句中执行相同的调用突然起作用.

这是没有意义的我,因为文件显然没有使用别的(我只是创造了它)..和它的作品一秒钟后,无论如何....

以下是我的代码

XAML:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" >
    <StackPanel>
        <Button Click="Button_Click"  Content="Start"/>
    </StackPanel>
</Window>

代码背后:

using System;
using System.Diagnostics;
using System.IO;
using System.Threading;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;


namespace WpfApplication1
{ 
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            test();
        }


        String Folder = @"C:\TestFolder";

        private void test()
        {
            FileSystemWatcher watch = new FileSystemWatcher(Folder);
            watch.Created += new FileSystemEventHandler(FileCreated);
            watch.EnableRaisingEvents = true;

            Process.Start(Folder);
        }

        private void FileCreated(object sender, FileSystemEventArgs fsEvent)
        {

            if (File.Exists(fsEvent.FullPath))
            {

                // Thread.Sleep(1000);// Sleeping for 1 second seems to prevent the error from happening...?
                // If i am debugging, and pause on exceptions... then it also suddenly works (similar to the Sleep above)
                try
                {

                    FileStream fs = new FileStream(fsEvent.FullPath, FileMode.Open); 
                    Console.WriteLine("Working");
                    fs.Close();
                }
                catch (IOException ex)
                {
                    Console.WriteLine("Broken: " + ex.Message);
                    try
                    {                        
                        FileStream fs = new FileStream(fsEvent.FullPath, FileMode.Open);
                        Console.WriteLine("Working, after breaking");
                        fs.Close();

                    }
                    catch(IOException ex2)
                    {                        
                        FileStream fs = new FileStream(fsEvent.FullPath, FileMode.Open);
                        Console.WriteLine("really broken: " + ex2.Message);
                        fs.Close();
                    }
                }


            }
        }
    }
}

Answer 1

我已经看到了自.NET 1.0以来你所描述的行为,并且从未费心去发现它为什么会发生.看起来操作系统或.NET有时(？)在调用close和dispose后短时间内锁定文件.

我做了一个解决方法 - 或者如果你愿意,那就是黑客攻击 - 这对我们来说已经证明是非常强大的.我们每天在服务器场中处理数百万个文件,并且filewatchers检测到的所有文件在传递给进一步处理之前都会通过此方法.

它的作用是在文件上放置一个独占锁.如果失败,它将可选择等待最多10秒钟,以便在放弃之前关闭文件.

    public static bool IsFileClosed(string filepath, bool wait)
    {
        bool        fileClosed = false;
        int         retries = 20;
        const int   delay = 500; // Max time spent here = retries*delay milliseconds

        if (!File.Exists(filepath))
            return false;

        do
        {
            try 
            {
                // Attempts to open then close the file in RW mode, denying other users to place any locks.
                FileStream fs = File.Open(filepath, FileMode.Open, FileAccess.ReadWrite, FileShare.None);
                fs.Close();
                fileClosed = true; // success
            }
            catch (IOException) {}

            if (!wait) break;

            retries --;

            if (!fileClosed)
                Thread.Sleep( delay );
        }
        while (!fileClosed && retries > 0);

        return fileClosed;
    }