Use FileSystemWatcher on a single file in C#

Your error is setting the Path property with a full filename

watcher.Path = Path.GetDirectoryName(filePath1); 
watcher.Filter = Path.GetFileName(filePath1);

should work.

Not related to your proper question, but, of course, to enable the FileSystemWatcher’s functionality, it is imperative to set the EnableRaisingEvents property to true.