求一个集合的集合下所有集合元素求值

场景是这样的：Order下有一个Suppler的集合，即一个订单下可能有多个供应商；Supplier下有一个Product的集合，即对一个供应商采购多个产品。

需求是这样的：算出所有订购产品的总价

模型这样：

    public class Order
    {
        public int OrderId { get; set; }
        public ICollection<Supplier> Suppliers { get; set; }

        public Order()
        {
            Suppliers = new List<Supplier>();
        }
    }

    public class Supplier
    {
        public int SupplierId { get; set; }

        public ICollection<Product> Products { get; set; }

        public Supplier()
        {
            Products = new List<Product>();
        }
    }

    public class Product
    {
        public int ProductId { get; set; }
        public decimal UnitPrice { get; set; }
        public int Quantity { get; set; }
    }

这样算出总价：

        static void Main(string[] args)
        {

            var orders = new List<Order>
            {
                new Order
                {
                    OrderId = 1,
                    Suppliers = new List<Supplier>
                    {
                        new Supplier {SupplierId=11, Products= new List<Product>
                        {
                            new Product {ProductId=111, Quantity=1, UnitPrice=10 },
                            new Product {ProductId = 112, Quantity =1, UnitPrice = 20 }
                        } },
                        new Supplier {SupplierId =12, Products = new List<Product>
                        {
                            new Product {ProductId =113, Quantity =2, UnitPrice=30 },
                            new Product {ProductId=114, Quantity=1, UnitPrice=50 }
                        } }
                    }
                }
            };

            var suppliers = orders.Select(t => t.Suppliers);

            var productTotalPrice = suppliers.Sum(t => t.Sum(p => p.Products.Sum(o => o.Quantity * o.UnitPrice)));

            Console.WriteLine(productTotalPrice);
            Console.ReadKey();
        }