VB.NET开发Windows服务时FileSystemWatcher不触发事件问题求助

问题现象 #

• 基于FileSystemWatcher开发的Windows文件夹监听服务可正常安装、启动，向监听目标路径C:\temp\新增、删除文件时，无任何监听事件触发
• 系统事件日志仅存在服务启动时写入的started watcher记录，无其他事件日志、无报错信息
• 已尝试将拆分的监听逻辑整合为单类、精简冗余代码，参考大量VB/C#的FileSystemWatcher实现案例仍未定位根因，初步怀疑与服务权限配置相关

原始问题代码 #

Imports System.IO

Public Class TestFileWatcher

    Dim WithEvents watcher As System.IO.FileSystemWatcher
    Dim myLog As System.Diagnostics.EventLog
    Dim folderName As String
    Dim fileTypes As String

    Protected Overrides Sub OnStart(ByVal args() As String)
        myLog = New System.Diagnostics.EventLog()
        If Not EventLog.SourceExists("My Test Log") Then
            EventLog.CreateEventSource("My Test Log", "Application")
        End If
        myLog.Source = "My Test Log"
        folderName = "C:\temp\"
        fileTypes = "*.*"
        Try
            watcher = New System.IO.FileSystemWatcher()
            DefineWatcher()
            watcher.EnableRaisingEvents = True
            myLog.WriteEntry("started watcher")
        Catch
            ErrorHandler(e)
            System.Environment.Exit(-1)
        End Try
    End Sub

    Protected Overrides Sub OnStop()
        StopWatcher()
    End Sub

    Public Sub StopWatcher()
        watcher.Dispose()
    End Sub

    Public Sub RecordOnEventLog(ByVal str As String)
        myLog.WriteEntry(str)
    End Sub

    Protected Sub ErrorHandler(ByVal e As ApplicationException)
        Dim err As String
        err = "ERROR" + vbCrLf
        err += "Message: " + e.Message + vbCrLf
        err += "Source: " + e.Source + vbCrLf
        err += "Method: " + e.TargetSite.Name + vbCrLf
        err += "Stack Trace: " + e.StackTrace + vbCrLf
        err += "Error String: " + e.ToString + vbCrLf
        err += "Application shut down at " & My.Computer.Clock.GmtTime.ToString
        myLog.WriteEntry(err)
    End Sub

    Public Sub DefineWatcher()
        watcher.BeginInit()
        watcher.Path = folderName
        watcher.Filter = fileTypes
        watcher.IncludeSubdirectories = False
        watcher.NotifyFilter = System.IO.NotifyFilters.FileName
        watcher.NotifyFilter = watcher.NotifyFilter Or System.IO.NotifyFilters.Attributes
        watcher.NotifyFilter = watcher.NotifyFilter Or System.IO.NotifyFilters.CreationTime
        watcher.NotifyFilter = watcher.NotifyFilter Or System.IO.NotifyFilters.Security
        AddHandler watcher.Created, AddressOf OnCreated
        AddHandler watcher.Deleted, AddressOf OnDeleted
        watcher.EndInit()
    End Sub

    Public Sub OnCreated(src As Object, e As FileSystemEventArgs)
        RecordOnEventLog("New file added to temp folder, path: " + e.FullPath)
    End Sub

    Public Sub OnDeleted(src As Object, e As FileSystemEventArgs)
        RecordOnEventLog("Item deleted")
    End Sub

End Class

根因与修复方案 #

代码存在多个会直接导致事件不触发、错误被掩盖的问题，按优先级修复即可：

• 修复异常捕获逻辑缺陷
  原代码OnStart中的Catch块未声明异常对象，却直接调用ErrorHandler(e)，只要初始化阶段抛出任何异常，Catch块自身就会触发未处理错误，完全无法写入错误日志，直接掩盖真实报错。需修改为显式捕获异常实例：

  Catch ex As Exception
      ErrorHandler(New ApplicationException("Watcher initialization failed", ex))
      System.Environment.Exit(-1)
  End Try
  

• 删除运行时无效的初始化方法调用
  BeginInit()、EndInit()是供Visual Studio设计器承载组件时使用的方法，运行时手动调用会干扰FileSystemWatcher的正常初始化状态，直接删除这两行即可。
• 补充FileSystemWatcher的Error事件绑定
  权限不足、内部缓冲区溢出、监听路径失效等运行时错误，只会通过Error事件抛出通知，原代码未绑定该事件，所有运行时错误都会被静默忽略。添加事件绑定与处理逻辑：

  ' 在DefineWatcher中添加事件绑定
  AddHandler watcher.Error, AddressOf OnWatcherError
  
  ' 新增错误处理方法
  Public Sub OnWatcherError(src As Object, e As ErrorEventArgs)
      Dim ex = e.GetException()
      RecordOnEventLog($"Watcher error: {ex.Message}, stacktrace: {ex.StackTrace}")
  End Sub
  

• 验证服务账户的目录权限
  打开服务管理器，找到对应服务打开「属性-登录」页，确认服务运行身份：
  • 若使用Local System账户，默认拥有本地磁盘绝大多数路径访问权限，手动检查C:\temp目录的访问控制列表，无特殊拒绝规则即可
  • 若使用Local Service或自定义用户账户运行，必须手动给该账户授予C:\temp目录的读取、列出文件夹内容、更改权限
• 调整监听配置避免事件丢失
  原代码仅配置了文件名、属性、创建时间、安全相关的通知过滤，可按需补充LastWrite、DirectoryName过滤项；默认8KB内部缓冲区在高频文件操作下极易溢出，建议设置为64KB（不要超过64KB，避免占用过多非分页内存）：

  watcher.InternalBufferSize = 65536 ' 64KB
  watcher.NotifyFilter = NotifyFilters.FileName Or NotifyFilters.Attributes Or 
                          NotifyFilters.CreationTime Or NotifyFilters.Security Or 
                          NotifyFilters.LastWrite Or NotifyFilters.DirectoryName
  

内容的提问来源于stack exchange，提问作者Amy