public static void AddSongToPlaylist(int songId, int playlistId, bool allowDuplicates)
{
if (CPlaylists.Get(playlistId) == null)
{
throw new ArgumentException("invalid playlistId");
}
if (allowDuplicates || !PlaylistContainsSong(songId, playlistId))
{
CPlaylists.AddSong(playlistId, songId);
CPlaylists.Save(playlistId);
}
else
{
throw new ArgumentException("song exists in this playlist");
}
}
