Serializable.cs

public class QuickOrderTrxHandler : IRequestHandler<QuickOrderCommand>
{
    private readonly ICatalogService _catalogService;
    private readonly IWarehouseService _warehouseService;
    private readonly IOrderRepository _orderRepository;

    public QuickOrderTrxHandler(ICatalogService catalogService, IWarehouseService warehouseService, IOrderRepository orderRepository)
    {
        _catalogService = catalogService;
        _warehouseService = warehouseService;
        _orderRepository = orderRepository;
    }

    public async Task Handle(QuickOrderCommand request)
    {
        using var trx = _orderRepository.BeginTransaction(IsolationLevel.Serializable);

        if (await _warehouseService.QuantityOnHand(request.Sku, trx) <= 0)
        {
            throw new InvalidOperationException("Product has no quantity on hand.");
        }

        var price = await _catalogService.Price(request.Sku, trx);

        var quickOrder = new QuickOrder(request.Sku, price);
        _orderRepository.Save(quickOrder);

        trx.Commit();
    }
}