how to use file watcher in c#

private void watch()
{
  FileSystemWatcher watcher = new FileSystemWatcher();
  watcher.Path = path;
  watcher.NotifyFilter = NotifyFilters.LastWrite;
  watcher.Filter = "*.*";
  watcher.Changed += new FileSystemEventHandler(OnChanged);
  watcher.EnableRaisingEvents = true;
}

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