LINQ to Objectsで、複数Join・複合キーでGroup化・並び替えをして、サマリを出す

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LinqSample
{
    class Program
    {
        static void Main(string[] args)
        {
            var items = new List<Item>
            {
                new Item(){ ID = 1, ItemName = "りんご" },
                new Item(){ ID = 2, ItemName = "みかん" },
                new Item(){ ID = 3, ItemName = "ぶどう" }
            };

            var customers = new List<Customer>
            {
                new Customer(){ ID = 1, CustomerName = "hoge" },
                new Customer(){ ID = 2, CustomerName = "fuga" }
            };

            var orders = new List<Order>
            {
                new Order() { No = 1, CustomerID = 1, ItemID = 1, Amount = 1 },
                new Order() { No = 2, CustomerID = 1, ItemID = 1, Amount = 2 },
                new Order() { No = 3, CustomerID = 1, ItemID = 2, Amount = 1 },
                new Order() { No = 4, CustomerID = 2, ItemID = 1, Amount = 1 },
                new Order() { No = 5, CustomerID = 1, ItemID = 1, Amount = 3 },
                new Order() { No = 6, CustomerID = 2, ItemID = 1, Amount = 2 }, // カンマ付もOK
            };


            //  o:Order, c:Customer, i:Item, oc:Order&Customer とする
            var results = orders.Join(customers, o => o.CustomerID, c => c.ID, (o, c) => new { o, c })
                                .Join(items, oc => oc.o.ItemID, i => i.ID, (oc, i) => new { oc, i })
                                .GroupBy(g => new
                                {
                                    //  匿名型では同じIDという名前を持てないので、新しく名前をつける
                                    //  エラー「匿名型では、同じ名前を持つ複数のプロパティを含むことはできません」
                                    CustomerID = g.oc.c.ID,
                                    ItemID = g.i.ID
                                })
                                .OrderByDescending(d => d.Key.CustomerID)
                                .ThenBy(t => t.Key.ItemID)
                                .Select(s => new
                                {
                                    //  CustomerNameとItemNameは、グループ化されていれば全部同じはずなので、最初のを使う
                                    CustomerName = s.First().oc.c.CustomerName,
                                    ItemName = s.First().i.ItemName,
                                    Total = s.Sum(sum => sum.oc.o.Amount)
                                });


            foreach (var result in results)
            {
                Console.WriteLine("================");
                Console.WriteLine(result.CustomerName);
                Console.WriteLine("----------------");
                Console.WriteLine("品名：" + result.ItemName + "、数量：" + result.Total.ToString());
                Console.WriteLine("----------------");
                Console.WriteLine("");
            }

            Console.WriteLine("Enterキーで終了");
            Console.ReadKey();
        }


        public class Order
        {
            public int No { get; set; }
            public int CustomerID { get; set; }
            public int ItemID { get; set; }
            public int Amount { get; set; }
        }

        public class Item
        {
            public int ID { get; set; }
            public string ItemName { get; set; }
        }

        public class Customer
        {
            public int ID { get; set; }
            public string CustomerName { get; set; }
        }
    }
}