OptionalTypeReservationExaple.cs

 public Option<ReservationViewModel> AddItemToReservation(string id, string itemId)
        {
            var reservationOptinal = _restaurant.GetReservation(id);

            var itemOptional = _inventory.GetItem(itemId);
            
           return reservationOptinal.Intersect(itemOptional)
                .Select(AddItemFunc)
                .Select(reservation => (reservation, _ClientsManger.GetUser(reservation.UserId)))
                .Select(MapToViewModel);


            Reservation AddItemFunc(( Reservation , Item ) tuple)
            {
                var (reservation, item) = tuple;
                reservation.Items.Add(item);
                return reservation;
            }

            ReservationViewModel MapToViewModel((Reservation , Option<User>) tuple)
            {
                var (reservation, userOption) = tuple;

                return new ReservationViewModel()
                {
                    From = reservation.From,
                    User = userOption.Select(user => $"{user.FirstName} {user.SecondName}").ValueOr("Not a client"),
                    Id = reservation.Id,
                    Name = reservation.Name,
                    To = reservation.To
                };
            }

           


        }