How does System.IO.FileSystemWatcher works under the hood?

Question

I would like to understand how does System.IO.FileSystemWatcher works under the hood? Because I have a requirement where I need to watch all the files present under 100 or more Folders where each folder will have around 1K files.

I am not sure if I used FileSystemwatcher whether it will create many threads to monitor these files which would impact the performance of my application? So could you please let me know how exactly System.IO.FileSystemWatcher works under the hood or does it uses Threads internally to monitor these directories ?

Answer 1

Internally FileSystemWatcher uses the Windows API ReadDirectoryChangesW function (as can be seen from the FileSystemWatcher reference source). The underlying implementation of ReadDirectoryChangesW is not documented, but in answer to your specific question as to whether FileSystemWatcher creates seprate threads to monitor files, the answer is therefore "no".

It is however worth highlighting the following from the remarks on FileSystemWatcher in the documentation given that your directories contain many files:

The Windows operating system notifies your component of file changes in a buffer created by the FileSystemWatcher. If there are many changes in a short time, the buffer can overflow. This causes the component to lose track of changes in the directory, and it will only provide blanket notification. Increasing the size of the buffer with the InternalBufferSize property is expensive, as it comes from non-paged memory that cannot be swapped out to disk, so keep the buffer as small yet large enough to not miss any file change events. To avoid a buffer overflow, use the NotifyFilter and IncludeSubdirectories properties so you can filter out unwanted change notifications.

There are two takeaways from this:

1. With many FileSystemWatcher instances, there will be many buffers created to store the change events, and since these are allocated from non-paged memory (a limited resource).
2. If there are a large number of changes taking place in the directories you're monitoring, you may miss events.