Clean Domain-driven Design article

Cart.cs

public sealed class Cart : AggregateRootBase
{
  private Cart() : base(GenerateId())
  {
  }

  private Cart(Guid id) : base(id)
  {
  }

  public static Cart Load()
  {
    return new Cart();
  }

  public static Cart Load(Guid id)
  {
    return new Cart(id);
  }

  [Required] public double CartItemTotal { get; private set; }

  [Required] public double CartItemPromoSavings { get; private set; }

  [Required] public double ShippingTotal { get; private set; }

  [Required] public double ShippingPromoSavings { get; private set; }

  [Required] public double CartTotal { get; private set; }

  [Required] public List<CartItem> CartItems { get; private set; } = new List<CartItem>();

  public bool IsCheckout { get; private set; }

  public CartItem FindCartItem(Guid productId)
  {
    var cartItem = CartItems.FirstOrDefault(x => x.Product.ProductId == productId);
    return cartItem;
  }

  public Cart InsertItemToCart(Guid productId, int quantity, double promoSavings = 0.0D)
  {
    CartItems.Add(CartItem.Load(productId, quantity, 0.0D, promoSavings));
    return this;
  }

  public Cart RemoveCartItem(Guid itemId)
  {
    CartItems = CartItems.Where(y => y.Id != itemId).ToList();
    return this;
  }

  public Cart AccumulateCartItemQuantity(Guid cartItemId, int quantity)
  {
    var cartItem = CartItems.FirstOrDefault(x => x.Id == cartItemId);

    if(cartItem ==null) throw new DomainException($"Couldn't find cart item #{cartItemId}");

    cartItem.AccumulateQuantity(quantity);
    return this;
  }

  public async Task<Cart> CalculateCartAsync(
    TaxType taxType, ICatalogGateway catalogGateway,
    IPromoGateway promoGateway, IShippingGateway shippingGateway)
  {
    if (CartItems != null && CartItems?.Count() > 0)
    {
      CartItemTotal = 0.0D;
      foreach (var cartItem in CartItems)
      {
        var product = await catalogGateway.GetProductByIdAsync(cartItem.Product.ProductId);
        if (product == null)
        {
          throw new Exception("Could not find product.");
        }

        cartItem
          .FillUpProductInfo(product.Name, product.Price, product.Desc)
          .ChangePrice(product.Price);

        CartItemPromoSavings = CartItemPromoSavings + cartItem.PromoSavings * cartItem.Quantity;
        CartItemTotal = CartItemTotal + cartItem.Product.Price * cartItem.Quantity;
      }

      shippingGateway.CalculateShipping(this);
    }

    promoGateway.ApplyShippingPromotions(this);

    switch (taxType)
    {
      case TaxType.NoTax:
        CartTotal = CartItemTotal + ShippingTotal;
        break;
      case TaxType.TenPercentage:
        var cartTotal = CartItemTotal + ShippingTotal;
        CartTotal = (cartTotal * 10 / 100) + cartTotal;
        break;
      default:
        throw new ArgumentOutOfRangeException(nameof(taxType), taxType, null);
    }

    return this;
  }

  public Cart Checkout()
  {
    IsCheckout = true;
    return this;
  }
}