Rich Domain Model

Order.cs

    public class Order
    {
        private Order() { }

        public string OrderId { get; private set; }
        public string CustomerId { get; private set; }
        public decimal Discount { get; private set; }
        public bool Paid { get; private set; }
        public DateTime? PaymentTime { get; private set; }
        public DateTime CreationTime { get; private set; }

        private List<OrderItem> _items; // The original source is private and can only be modified by this class.
        public IEnumerable<OrderItem> Items => _items.AsReadOnly(); // Exposes a readonly copy of order items

        public static Order New(string orderId, string customerId, DateTime? creationTime = null)
        {
            if (string.IsNullOrWhiteSpace(orderId)) throw new ArgumentNullException(nameof(orderId));
            if (string.IsNullOrWhiteSpace(customerId)) throw new ArgumentNullException(nameof(customerId));

            return new Order
            {
                OrderId = orderId,
                CustomerId = customerId,
                CreationTime = creationTime ?? DateTime.UtcNow,
                _items = new List<OrderItem>()
            };
        }

        public decimal TotalDiscountOnItems => Items.Sum(i => i.Discount);
        public decimal GrossTotal => Items.Sum(i => i.Quantity * i.Price);
        public decimal OrderTotal => GrossTotal - TotalDiscountOnItems - Discount;

        public void AddItem(OrderItem item)
        {
            if (Paid) throw new InvalidOperationException("The order is paid already.");
            if (_items.Count > 5) throw new InvalidOperationException("Cannot add more than 5 items in an order.");
            if (TotalDiscountOnItems + item.Discount > GrossTotal) throw new ArgumentException("Discount on item cannot exceed the order total.");

            _items.Add(item);
        }

        public void AddOrderDiscount(decimal discount)
        {
            if (Paid) throw new InvalidOperationException("The order is already paid.");
            if (OrderTotal - discount < 0) throw new ArgumentException("Discount cannot exceed the order total.", nameof(discount));

            Discount = discount;
        }

        public void PayOrder()
        {
            if (Paid) return;

            Paid = true;
            PaymentTime = DateTime.UtcNow;
        }
    }

OrderItem.cs

    public class OrderItem
    {
        private OrderItem() { }

        public static OrderItem New(string productId, int quantity, decimal price, decimal discount = 0)
        {
            if (string.IsNullOrWhiteSpace(productId)) throw new ArgumentNullException(nameof(productId));
            if (quantity <= 0) throw new ArgumentOutOfRangeException(nameof(quantity));
            if (price <= 0) throw new ArgumentOutOfRangeException(nameof(price));
            if (discount <= 0) throw new ArgumentOutOfRangeException(nameof(discount));
            if (discount > price * quantity) throw new ArgumentException("The discount cannot be more than the total of the order item.", nameof(discount));
            
            return new OrderItem
            {
                ProductId = productId,
                Quantity = quantity,
                Price = price,
                Discount = discount
            };
        }

        public string ProductId { get; private set; }
        public int Quantity { get; private set; }
        public decimal Price { get; private set; }
        public decimal Discount { get; private set; }
    }