Windows服务中FileSystemWatcher触发文件读取时出现文件被占用IO异常

我开发了一个Windows服务，使用FileSystemWatcher监听新文件夹的创建事件。触发OnCreated事件时，服务会读取新文件夹中的insp_board.txt文件，将数据记录到log.txt中，然后等待下一次文件夹创建触发。

控制台应用中运行代码时一切正常，但作为Windows服务运行时，首次OnCreated函数执行正常，第二次触发OnCreated时会抛出IO异常："Could not access file because it is being used by another process"。

相关代码 #

服务启动类 #

public partial class SPIHydraImport : ServiceBase
{
    private SourceWatcher sourceWatcher;

    public SPIHydraImport()
    {
        InitializeComponent();
        sourceWatcher = new SourceWatcher();
        var eventLogger = new EventLogger("HydraSPIImport", "Application");
    }

    protected override void OnStart(string[] args)
    {
        try
        {
            sourceWatcher.StartWatching();
        }
        catch (Exception ex)
        {
            EventLog.WriteEntry("An error occurred: " + ex.Message, EventLogEntryType.Error);
            throw;
        }
    }

    protected override void OnStop()
    {
        sourceWatcher.StopWatching();
    }
}

Watcher类 #

public class SourceWatcher
{
    private FileSystemWatcher watcher;
    private Logger logger = new Logger();

    private string sourcePath = ConfigurationManager.AppSettings["SourcePath"];
    public SourceWatcher()
    {
        watcher = new FileSystemWatcher();
        watcher.Path = sourcePath;
        watcher.IncludeSubdirectories = true;
        watcher.Filter = "insp_board.txt";
        watcher.Created += new FileSystemEventHandler(OnCreated);
        watcher.EnableRaisingEvents = true;
    }

    public void StartWatching()
    {
        watcher.EnableRaisingEvents = true;
        this.logger.LogStandard($"Start: watcher");
    }

    private void OnCreated(object source, FileSystemEventArgs e)
    {
        StopWatching();
        BoardData boardData = new BoardData();
        string directoryPath = e.FullPath;

        try
        {
            this.logger.LogStandard($"{directoryPath}");
            boardData = boardData.ParseFile(directoryPath);
            this.logger.LogStandard($"BoardID: {boardData.boardId} , Operation: {boardData.operationId} , PCB ID: {boardData.pcbId} , Inspection time: {boardData.inspectionTime} , Tack time: {boardData.tackTime} , Is pass: {boardData.isPass}");
        }
        catch (Exception exc)
        {
            this.logger.LogError(exc.ToString());
            throw;
        }
        finally
        {
            StartWatching();
        }
    }

    public void StopWatching()
    {
        watcher.EnableRaisingEvents = false;
        this.logger.LogStandard("Stop: watcher");
    }
}

ParseFile函数 #

public BoardData ParseFile(string inspBoardPath)
{
    var boardData = new BoardData();
    var lines = File.ReadAllLines(inspBoardPath).Skip(2);

    foreach (var line in lines)
    {
        var parts = line.Split(',');

        boardData.boardId = parts[1];
        boardData.operationId = "0" + parts[3];
        boardData.pcbId = parts[4];
        boardData.inspectionTime = DateTime.ParseExact(parts[5], "yyyyMMddHHmmss", CultureInfo.InvariantCulture).ToString("yyyy-MM-dd HH:mm:ss");
        boardData.tackTime = double.Parse(parts[6], CultureInfo.InvariantCulture);
        if (parts[7].Equals("3"))
        {
            boardData.isPass = 1;
        } else
        {
            boardData.isPass = 0;
        }
    }

    return boardData;
}

我在控制台应用中曾遇到相同错误，通过在OnCreated开头禁用事件、结尾启用事件解决了问题，但Windows服务中同样实现该方案后问题依旧。尝试过释放Watcher，结果相同；添加文件存在检查，每次都能确认文件存在。

解决方案 #

1. 处理文件未完全写入的问题 #

FileSystemWatcher的Created事件会在文件开始创建时触发，此时文件可能仍被写入进程占用。服务环境下文件写入的锁机制或延迟与控制台不同，需等待文件可用后再读取：

添加文件可用性检查方法：

private bool IsFileReady(string filePath)
{
    try
    {
        // 尝试以独占方式打开文件，确认无其他进程占用
        using (var stream = File.Open(filePath, FileMode.Open, FileAccess.Read, FileShare.None))
        {
            return true;
        }
    }
    catch (IOException)
    {
        return false;
    }
}

修改OnCreated中的读取逻辑，增加重试等待：

private void OnCreated(object source, FileSystemEventArgs e)
{
    StopWatching();
    BoardData boardData = new BoardData();
    string filePath = e.FullPath;

    try
    {
        this.logger.LogStandard($"{filePath}");
        
        // 等待文件可用，最多重试10次，每次间隔500ms
        int retryCount = 0;
        while (!IsFileReady(filePath) && retryCount < 10)
        {
            System.Threading.Thread.Sleep(500);
            retryCount++;
        }

        if (retryCount >= 10)
        {
            this.logger.LogError($"文件 {filePath} 长时间被占用，放弃读取");
            return;
        }

        boardData = boardData.ParseFile(filePath);
        this.logger.LogStandard($"BoardID: {boardData.boardId} , Operation: {boardData.operationId} , PCB ID: {boardData.pcbId} , Inspection time: {boardData.inspectionTime} , Tack time: {boardData.tackTime} , Is pass: {boardData.isPass}");
    }
    catch (Exception exc)
    {
        this.logger.LogError(exc.ToString());
        throw;
    }
    finally
    {
        StartWatching();
    }
}

2. 优化FileSystemWatcher配置 #

• 移除构造函数中直接启用EnableRaisingEvents的代码，统一在StartWatching中控制，避免初始化时意外触发事件：

  public SourceWatcher()
  {
      watcher = new FileSystemWatcher();
      watcher.Path = sourcePath;
      watcher.IncludeSubdirectories = true;
      watcher.Filter = "insp_board.txt";
      watcher.Created += new FileSystemEventHandler(OnCreated);
      // 注释掉这一行
      // watcher.EnableRaisingEvents = true;
  }
  

• 添加NotifyFilter，只关注文件写入完成的相关事件，减少误触发：

  watcher.NotifyFilter = NotifyFilters.LastWrite | NotifyFilters.Size;
  

3. 检查Logger的文件占用 #

确认Logger类在写入log.txt时是否正确释放了文件句柄。如果Logger使用了持续打开的文件流，会导致文件被长期占用，需修改Logger实现，确保每次写入后关闭流，或使用File.AppendAllText这类自动释放资源的方法。

4. 确认服务运行权限 #

确保Windows服务的运行账户对监控目录和文件有读取和写入权限。可以尝试将服务运行账户改为本地系统账户（Local System）测试，排除权限导致的异常。

内容的提问来源于stack exchange，提问作者Wtor