Why Does My FileSystemWatcher's Changed Event Fire Twice After a Single File Modification?

Understanding Double Event Triggers in FileSystemWatcher's Changed Event

Using FileSystemWatcher, you might observe the Changed event firing twice after a single file modification. This isn't a bug, but a known behavior. Let's examine a typical scenario:

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

private void OnChanged(object source, FileSystemEventArgs e)
{
   // ... event handling logic ...
}

This code monitors "Version.txt". Modifying and saving the file often triggers OnChanged twice. This happens because the file system performs multiple actions during the write process, each potentially triggering a separate event.

While refining NotifyFilter to restrict monitored actions can help, it's not a guaranteed solution. In some situations, you might need to implement custom duplicate event filtering (a workaround).

For more detailed information on this behavior and potential solutions, consult this helpful resource: FileSystemWatcher Tips.

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