API/TOOHUCardAPI/Data/Repositories/CardRepository.cs

using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;
using TOOHUCardAPI.Data.Models;

namespace TOOHUCardAPI.Data.Repositories
{
    public class CardRepository
    {
        private readonly AppDbContext _context;

        public CardRepository(AppDbContext context)
        {
            _context = context;
        }

        public async Task<Card> GetCardByItemCode(string itemCode, bool allowNull = false)
        {
            Card card = await _context.Cards.FirstOrDefaultAsync(card => card.ItemCode == itemCode);
            if (card == null && !allowNull)
            {
                throw new InvalidCardException();
            }

            return card;
        }
    }
}