I checked that the folder that's watched by FileSystemWatcher actually does not check or tells the changes in the folder while the program is not running. How can I check the changes in the folder when the program is off? I mean, how can the program tell 'the changes in the folder when the program is off'?
If you want to know that the changes have happened when they happen, you'll need to use a service (see here: https://learn.microsoft.com/en-us/dotnet/framework/windows-services/introduction-to-windows-service-applications).