Order.cs

public class Order : Entity, IAggregateRoot
{
    // DDD Patterns comment
    // This Order AggregateRoot's method "AddOrderitem()" should be the only way to add Items to the Order,
    // so any behavior (discounts, etc.) and validations are controlled by the AggregateRoot 
    // in order to maintain consistency between the whole Aggregate. 
    public void AddOrderItem(int productId, string productName, decimal unitPrice, decimal discount, string pictureUrl, int units = 1)
    {
        var existingOrderForProduct = _orderItems.SingleOrDefault(o => o.ProductId == productId);

        if (existingOrderForProduct != null)
        {
            //if previous line exist modify it with higher discount  and units..

            if (discount > existingOrderForProduct.GetCurrentDiscount())
            {
                existingOrderForProduct.SetNewDiscount(discount);
            }

            existingOrderForProduct.AddUnits(units);
        }
        else
        {
            //add validated new order item

            var orderItem = new OrderItem(productId, productName, unitPrice, discount, pictureUrl, units);
            _orderItems.Add(orderItem);
        }
    }
    
    public void SetStockConfirmedStatus()
    {
        if (_orderStatusId == OrderStatus.AwaitingValidation.Id)
        {
            AddDomainEvent(new OrderStatusChangedToStockConfirmedDomainEvent(Id));

            _orderStatusId = OrderStatus.StockConfirmed.Id;
            _description = "All the items were confirmed with available stock.";
        }
    }
}