The reason may be that watcher is declared as local variable to a method and it is garbage collected when the method finishes. You should declare it as a class member. Try the following:
FileSystemWatcher watcher;
private void watch()
{
watcher = new FileSystemWatcher();
watcher.Path = path;
watcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite
| NotifyFilters.FileName | NotifyFilters.DirectoryName;
watcher.Filter = "*.*";
watcher.Changed += new FileSystemEventHandler(OnChanged);
watcher.EnableRaisingEvents = true;
}
private void OnChanged(object source, FileSystemEventArgs e)
{
//Copies file to another directory.
}