public IEnumerable <DeckDTO> GetAllPaidDecks()
{
List <Deck> decks = courseDetails.GetAllPaidDecks().ToList();
return(decks.Count > 0
? converterToDTO.ConvertToDeckListDTO(decks)
: throw new ArgumentNullException());
}
public List <DeckDTO> GetDecksByUser(string userLogin)
{
List <Deck> decks = userProfile.GetDecksByUser(userLogin).ToList();
return(converterToDTO.ConvertToDeckListDTO(decks));
}