using System;
using System.Collections.Generic;
using System.Linq;

namespace TestApp
{

    class Program
    {
        public class Order
        {
            public Order()
            {
                Items = new List<OrderItem>();
            }

            public string Code { get; set; }
            public ICollection<OrderItem> Items { get; set; }
        }

        public class OrderItem
        {
            public OrderItem()
            {
                SubItems = new List<SubItem>();
            }

            public decimal Quantity { get; set; }
            public decimal Price { get; set; }
            public ICollection<SubItem> SubItems { get; set; }
        }

        public class SubItem
        {
            public DateTime Date { get; set; }
            public decimal Quantity { get; set; }
            public string UserName { get; set; }
        }

        static void Main(string[] args)
        {
            Order order = new Order();
            order.Code = "123";
            order.Items.Add(new OrderItem()
            {
                Price = 30,
                Quantity = 3,
                SubItems = new List<SubItem>()
                                        {
                                            new SubItem() { Date = DateTime.Now, Quantity = 1, UserName = "User1" }, 
                                            new SubItem() { Date = DateTime.Now, Quantity = 2, UserName = "User2" }
                                        }
            });


            order.Items.Add(new OrderItem()
            {
                Price = 500,
                Quantity = 50,
                SubItems = new List<SubItem>()
                                        {
                                            new SubItem() { Date = DateTime.Now, Quantity = 20, UserName = "User1" }, 
                                            new SubItem() { Date = DateTime.Now, Quantity = 20, UserName = "User2" },
                                            new SubItem() { Date = DateTime.Now, Quantity = 10, UserName = "User3" }
                                        }
            });


            order.Items.Add(new OrderItem()
                                {
                                    Price = 1000,
                                    Quantity = 50,
                                    SubItems = new List<SubItem>()
            });


            var res = order.Items
                           .SelectMany(i => i.SubItems.DefaultIfEmpty(), (Item, Sub) => new { Item, Sub })
                           .Select(r => new { order.Code, r.Item.Price, ItemQuantity = r.Item.Quantity, SubItemQuantity = r.Sub == null ? null : (Decimal?) r.Sub.Quantity ,UserName = r.Sub == null ? (string)null : r.Sub.UserName });

            foreach(var x in res)
            {
                System.Console.WriteLine(x.ToString());
            }
        }
}
}