public static class BookingHelper
    {
        public static string OverlappingBookingsExist(Booking booking, IBookingRepository bookingRepository)
        {
            if (booking.Status == "Cancelled")
                return string.Empty;

            var bookings = bookingRepository.GetActiveBookings(booking.Id);

            var overlappingBooking =
                bookings.FirstOrDefault(
                    b =>
                        booking.ArrivalDate < b.DepartureDate
                        && b.ArrivalDate < booking.DepartureDate
                );

            return overlappingBooking == null ? string.Empty
                : overlappingBooking.Reference;
        }
    }

    public class BookingRepository : IBookingRepository
    {
        public IQueryable<Booking> GetActiveBookings(int? excludedBookingId = null)
        {
            var unitOfWork = new UnitOfWork();
            var bookings =
                unitOfWork.Query<Booking>()
                    .Where(
                        b => b.Id != excludedBookingId && b.Status != "Cancelled");

            return bookings;
        }
    }

    public interface IBookingRepository
    {
        IQueryable<Booking> GetActiveBookings(int? excludedBookingId = null);
    }

    public class UnitOfWork
    {
        public IQueryable<T> Query<T>()
        {
            return new List<T>().AsQueryable();
        }
    }

    public class Booking
    {
        public string Status { get; set; }
        public int Id { get; set; }
        public DateTime ArrivalDate { get; set; }
        public DateTime DepartureDate { get; set; }
        public string Reference { get; set; }
    }