FileSystemWatcher changed event (for "LastWrite") is unreliable

Question

I am trying to get a notification when a file is updated on disk. I am interested in getting this notifications as soon as a flush occurs, yet, it seems that the FileSystemWatcher would only send an event when the stream opens or closes.

In the code below I am writing to a file and repeatedly flush the buffer to the disk. Yet, the FileSystemWatcher only notified me once in the beginning of the writes and once when they end.

Is there another way to get these notifications? Or should I use polling for that?

The code:

class Program
{
    static void Main(string[] args)
    {
        FileSystemWatcher watcher = new FileSystemWatcher(Environment.CurrentDirectory, "Test.txt");
        watcher.Changed += watcher_Changed;
        watcher.EnableRaisingEvents = true;

        using(TextWriter writer = new StreamWriter("Test.txt"))
        {
            WriteData(writer);
            WriteData(writer);
            WriteData(writer);
            WriteData(writer);
        }

        Thread.Sleep(10000);
    }

    private static void WriteData(TextWriter writer)
    {
        writer.WriteLine("Hello!");
        writer.Flush();
        Console.WriteLine(DateTime.Now.ToString("T") + "] Wrote data");
        Thread.Sleep(3000);
    }

    static void watcher_Changed(object sender, FileSystemEventArgs e)
    {
        Console.WriteLine(DateTime.Now.ToString("T") + "] Watcher changed!");
    }
}

UPDATE 1

I've corrected the ToString function of the DateTime to show seconds. Here is the output with the code above:

11:37:47 AM] Watcher changed!
11:37:47 AM] Wrote data
11:37:50 AM] Wrote data
11:37:53 AM] Wrote data
11:37:56 AM] Wrote data
11:37:59 AM] Watcher changed!

Thanks!

Answer 1

It has nothing to do with the FileSystemWatcher. The watcher reacts to updates on the LastWrite attribute for the filesystem.

E.g. NTFS does not update the LastWrite on every write. The value is cached and only written when the stream is closed or at some other unspecified time. This document says

Timestamps are updated at various times and for various reasons. The only guarantee about a file timestamp is that the file time is correctly reflected when the handle that makes the change is closed. [...] The NTFS file system delays updates to the last access time for a file by up to 1 hour after the last access

I assume a similar caching applies for write