FileSystemWatcherChanged event dosen't fire for a memory mapped file

Question

I have json file that I want to share between two processess. So I created a memory mapped file as follows.

private void CreateMemoryMappedFile()
        {
            var info = Directory.CreateDirectory(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData) + "/" + model.Data.Settings.OrcaUISpecificSettings.TimeOutFolder);
            string path = Path.Combine(info.FullName + @"\" + model.Data.Settings.OrcaUISpecificSettings.File);
            FullPath = path;
            try
            {
                mmf = MemoryMappedFile.CreateFromFile(path, FileMode.CreateNew, "MyMemoryFile", 1024 * 1024);
            }
            catch (Exception ex)
            {

            }
        }

        public MemoryMappedViewStream GetAccessor()
        {
            MemoryMappedViewStream FileMapView = null;
            if (FileMapView != null)
            {
                return FileMapView;
            }
            FileMapView = mmf.CreateViewStream();
            return FileMapView;
        }

And to read and write to the files I am doing the following

public void WriteToMemoryMappedFile(string Data)
        {
            try
            {
                mutex.WaitOne();
                byte[] bytes = Encoding.ASCII.GetBytes(Data);
                var accessor = GetAccessor();

                accessor.Write(bytes, 0, Data.Length);
                mutex.ReleaseMutex();
            }
            catch (Exception ex)
            {

            }
        }

public string ReadFromMemoryMappedFile()
        {
            mutex.WaitOne();
            var accessor = GetAccessor();
            using (BinaryReader binReader = new BinaryReader(accessor))
            {
                byte[] reader = binReader.ReadBytes((int)accessor.Length);
                string result = Encoding.Default.GetString(reader);
                mutex.ReleaseMutex();
                return result.Replace("NULL", "");
            }
        }

My problem is I have an Activity Monitor for my app. So after x amount of time I am updating the json file with InActiveStatus. Along the same lines I am listening to any file changes(Look at D_IDle event). Problem is If a normal file is changed I get the FileSytemWatcher changed event firing just fine. But when I use a Memory Mapped File to update the status, the FileSystemWatcher changed event never gets fired please help.

private void D_IsIdle(object sender, EventArgs e)
        {
            MonitorDirectory();
            //AppViewModel.SerializeData("InActive");
            AppViewModel.SerializeDataToMemoryMap("InActive");
            d.IsIdle -= D_IsIdle;
        }
public void MonitorDirectory()
        {
            FileSystemWatcher fileSystemWatcher = new FileSystemWatcher(AppViewModel.GetDriectory());
            fileSystemWatcher.NotifyFilter = NotifyFilters.LastWrite;
            fileSystemWatcher.Filter = "*.json";
            fileSystemWatcher.Changed += FileSystemWatcher_Changed;
            fileSystemWatcher.EnableRaisingEvents = true;
        }
private void FileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
//IT NEVER COMES HERE
}

Answer 1

Using FileSystemWatcher with memory mapped files must be some kind of anti-pattern:). If you are on a local system, use one of the process synchronization primitives (e.g. semaphores) to signal change.

My guess is that the FileSystemWatcher triggers when the file handle is closed to avoid other processed reading partially written files.