FileSystemWatcher Changed event is raised twice

Question

I have an application where I am looking for a text file and if there are any changes made to the file I am using the OnChanged eventhandler to handle the event. I am using the NotifyFilters.LastWriteTime but still the event is getting fired twice. Here is the code.

public void Initialize()
{
   FileSystemWatcher _fileWatcher = new FileSystemWatcher();
  _fileWatcher.Path = "C:\\Folder";
  _fileWatcher.NotifyFilter = NotifyFilters.LastWrite;
  _fileWatcher.Filter = "Version.txt";
  _fileWatcher.Changed += new FileSystemEventHandler(OnChanged);
  _fileWatcher.EnableRaisingEvents = true;
}

private void OnChanged(object source, FileSystemEventArgs e)
{
   .......
}

In my case the OnChanged is called twice, when I change the text file version.txt and save it.

Answer 1

Try this!

string temp="";

public void Initialize()
{
   FileSystemWatcher _fileWatcher = new FileSystemWatcher();
  _fileWatcher.Path = "C:\\Folder";
  _fileWatcher.NotifyFilter = NotifyFilters.LastWrite;
  _fileWatcher.Filter = "Version.txt";
  _fileWatcher.Changed += new FileSystemEventHandler(OnChanged);
  _fileWatcher.EnableRaisingEvents = true;
}

private void OnChanged(object source, FileSystemEventArgs e)
{
   .......
if(temp=="")
{
   //do thing you want.
   temp = e.name //name of text file.
}else if(temp !="" && temp != e.name)
{
   //do thing you want.
   temp = e.name //name of text file.
}else
{
  //second fire ignored.
}

}

Answer 2

Here's my approach :

// Consider having a List<String> named _changedFiles

private void OnChanged(object source, FileSystemEventArgs e)
{
    lock (_changedFiles)
    {
        if (_changedFiles.Contains(e.FullPath))
        {
            return;
        }
        _changedFiles.Add(e.FullPath);
    }

    // do your stuff

    System.Timers.Timer timer = new Timer(1000) { AutoReset = false };
    timer.Elapsed += (timerElapsedSender, timerElapsedArgs) =>
    {
        lock (_changedFiles)
        {
            _changedFiles.Remove(e.FullPath);
        }
    };
   timer.Start();
}

This is the solution I used to solve this issue on a project where I was sending the file as attachment in a mail. It will easily avoid the twice fired event even with a smaller timer interval but in my case 1000 was alright since I was happier with missing few changes than with flooding the mailbox with > 1 message per second. At least it works just fine in case several files are changed at the exact same time.

Another solution I've thought of would be to replace the list with a dictionary mapping files to their respective MD5, so you wouldn't have to choose an arbitrary interval since you wouldn't have to delete the entry but update its value, and cancel your stuff if it hasn't changed. It has the downside of having a Dictionary growing in memory as files are monitored and eating more and more memory, but I've read somewhere that the amount of files monitored depends on the FSW's internal buffer, so maybe not that critical. Dunno how MD5 computing time would affect your code's performances either, careful =\

Answer 3

This solution worked for me on production application:

Environment:

VB.Net Framework 4.5.2

Set manually object properties: NotifyFilter = Size

Then use this code:

Public Class main
    Dim CalledOnce = False
    Private Sub FileSystemWatcher1_Changed(sender As Object, e As IO.FileSystemEventArgs) Handles FileSystemWatcher1.Changed
            If (CalledOnce = False) Then
                CalledOnce = True
                If (e.ChangeType = 4) Then
                    ' Do task...
                CalledOnce = False
            End If
        End Sub
End Sub

Answer 4

One possible 'hack' would be to throttle the events using Reactive Extensions for example:

var watcher = new FileSystemWatcher("./");

Observable.FromEventPattern<FileSystemEventArgs>(watcher, "Changed")
            .Throttle(new TimeSpan(500000))
            .Subscribe(HandleChangeEvent);

watcher.EnableRaisingEvents = true;

In this case I'm throttling to 50ms, on my system that was enough, but higher values should be safer. (And like I said, it's still a 'hack').

Answer 5

Make it simple define one global variable var1 = true.

Private Sub FileWatchman_Changed(ByVal sender As System.Object, ByVal e As System.IO.FileSystemEventArgs) Handles FileWatchman.Changed
   If var1 = true 
       your logic goes here
       var1 = false
   Else
       var1 = true 
   End If
End Sub

Answer 6

I approached the double create issue like this, which ignores the first event:

Private WithEvents fsw As New System.IO.FileSystemWatcher
Private complete As New List(Of String)

Private Sub fsw_Created(ByVal sender As Object, _
    ByVal e As System.IO.FileSystemEventArgs) Handles fsw.Created

    If Not complete.Contains(e.FullPath) Then
        complete.Add(e.FullPath)

    Else
        complete.Remove(e.FullPath)
        Dim th As New Threading.Thread(AddressOf hprocess)
        th.Start(e)

    End If

End Sub