Clean Domain-driven Design article - Update Product in the Cart

RequestHandler.cs

public class RequestHandler : TxRequestHandlerBase<UpdateItemInCartRequest, UpdateItemInCartResponse>
{
  private readonly ICatalogGateway _catalogGateway;
  private readonly IShippingGateway _shippingGateway;
  private readonly IPromoGateway _promoGateway;

  public RequestHandler(IUnitOfWorkAsync uow, IQueryRepositoryFactory qrf,
    ICatalogGateway catalogGateway, IShippingGateway shippingGateway,
    IPromoGateway promoGateway) : base(uow, qrf)
  {
    _catalogGateway = catalogGateway;
    _shippingGateway = shippingGateway;
    _promoGateway = promoGateway;
  }

  public override async Task<UpdateItemInCartResponse> Handle(UpdateItemInCartRequest request,
    CancellationToken cancellationToken)
  {
    var cartCommander = UnitOfWork.Repository<Domain.Cart>();
    var cartQuery = QueryFactory.QueryEfRepository<Domain.Cart>();

    var cart = await cartQuery
      .GetFullCartAsync(request.CartId);

    var cartItem = cart.FindCartItem(request.ProductId);

    // if not exists then it should be a new item
    if (cartItem == null)
    {
      cart.InsertItemToCart(request.ProductId, request.Quantity);
    }
    else
    {
      // otherwise is updating the current item in the cart
      cart.AccumulateCartItemQuantity(cartItem.Id, request.Quantity);
    }

    await cart.CalculateCartAsync(TaxType.NoTax, _catalogGateway, _promoGateway, _shippingGateway);

    await cartCommander.UpdateAsync(cart);
    await UnitOfWork.SaveChangesAsync(cancellationToken);

    return new UpdateItemInCartResponse {Result = cart.ToDto()};
  }
}