public async Task Buy(Guid userId, Guid productId, int quantity)
{
var product = DomainRegistry.ProductService().GetProduct(productId);
if (product == null)
{
throw new NotFoundException("对不起,未能获取产品信息,请重试~");
}
var cart = DomainRegistry.MarketService().GetCartOfUser(userId);
cart.AddCartItem(productId, quantity, product.Price);
await DomainRegistry.Repository <ICartRepository>().SaveAsync <Guid, Domain.Market.Cart>(cart, true);
}