为什么FileSystemWatcher不触发事件，而在打开SMSTrace的情况下触发事件?

我创建了一个FileSystemWatcher对象来监视日志文件。它被初始化为侦听所有可能的事件(lastwrite, lastaccess等)，但当文件被写入时，它不会触发事件。

但是，如果我打开SMSTrace并使用它侦听该文件(并清楚地看到该文件不断更新)，则文件系统监视器会触发事件。SMSTrace对文件做了什么?如何解释这个问题，我该如何解决它?

这是代码:

    private FileSystemWatcher fileWatcher;
    private FileStream fileStream;
    private StreamReader streamReader;
    private String fullPath = null;
    private String dir = null;
    private String fileName = null;
    private void selectLogFile()
    {
        // TODO: try to restore previous settings before asking the user for the log file
        OpenFileDialog openFileDialog = new OpenFileDialog();
        openFileDialog.InitialDirectory = "C:''";
        openFileDialog.Filter = "txt files (*.txt)|*.txt|All files (*.*)|*.*";
        openFileDialog.FilterIndex = 2;
        openFileDialog.RestoreDirectory = true;
        if (openFileDialog.ShowDialog() == DialogResult.OK)
        {
            fullPath = openFileDialog.FileName;
            dir = Path.GetDirectoryName(fullPath);
            fileName = Path.GetFileName(fullPath);
        }
    }

    // TODO: what to do when file erases? (reboot) - clear the window?
    public LogListener()
    {
        try
        {
            Thread selectFileThread = new Thread(new ThreadStart(selectLogFile));
            selectFileThread.SetApartmentState(ApartmentState.STA);
            selectFileThread.Start();
            selectFileThread.Join();
            // The user did not select a file - nothing to do
            if (fullPath == null)
            {  
                return;
            }
            // Create file listener to listen on changes to log
            fileWatcher = new FileSystemWatcher(dir);
            // Create a file stream to read the data from the log file
            fileStream = new FileStream(fullPath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
            // Create a stream reader from the fileStream to read text easily
            streamReader = new StreamReader(fileStream);
            // Watch for changes in LastWrite 
            fileWatcher.NotifyFilter = NotifyFilters.LastWrite;
            // Only watch the log file.
            fileWatcher.Filter = fileName;
            // Add event handlers.
            fileWatcher.Changed += new FileSystemEventHandler(OnChanged);
            // Initial syncing of the file
            readFile();
            // Begin watching for events
            fileWatcher.EnableRaisingEvents = true;
            Log.Add("Started");
        }
        catch (Exception e)
        {
            Log.Add("Exception: " + e.Message);
        }
    }
    void OnChanged(object source, FileSystemEventArgs e)
    {
        readFile();
    }
    public void readFile()
    {
        String line;
        String bytesString = "";
        Log.Add(DateTime.Now+":readFile()...");
        // Some more code here...
    }