public void Add(Product product, uint quantity)
        {
            var existingLine = lines.FirstOrDefault(x => x.ProductId == product.Id);

            if (existingLine != null)
                existingLine.Quantity += quantity;
            else
                lines.Add(new Line(product.Id, product.Name, quantity));
        }
 public ProductDetails(Product product)
 {
     ProductId = product.Id;
     ProductName = product.Name;
     UnitPrice = product.UnitPrice;
 }