BasketEither.cs

public interface IBasketService
{
    Task<Either<int, QuantityValidationError>> AddItemToBasket(int basketId, int catalogItemId, decimal price, int quantity = 1);
}

public class QuantityValidationError
{
    public string Message { get; set; }

    public QuantityValidationError(string message)
    {
        Message = message;
    }
}

public class BasketService : IBasketService
{
    private readonly IAsyncRepository<Basket> _basketRepository;

    public BasketService(IAsyncRepository<Basket> basketRepository)
    {
        _basketRepository = basketRepository;
    }

    public async Task<Either<int, QuantityValidationError>> AddItemToBasket(int basketId, int catalogItemId, decimal price, int quantity = 1)
    {
        if (quantity < 1)
        {
            return new Either<int, QuantityValidationError>(new QuantityValidationError("Quantity must be greater than 0."));
        }
        
        var basket = await _basketRepository.GetByIdAsync(basketId);

        // This returns the quantity of the product we have in our shopping cart.
        var result = basket.AddItem(catalogItemId, price, quantity);

        await _basketRepository.UpdateAsync(basket);

        return new Either<int, QuantityValidationError>(result);
    }
}