App that monitors changes in a folder and reacts to them C#

I'm new to C# programming and I need some help. I have to write an app that, after an authentication phase, monitors the changes of a folder and when something happens (a file is added/deleted/updated) reacts by sending a notification to a server. What is the best way to do that? A windows service launched after the authentication? Please note that this monitoring activity should be performed while in parallel the user is navigating the ui.

Thank you all!

via Chebli Mohamed