using System;

using System.Collections.Generic;

using System.Linq;

​

namespace RandomStuff

{

    // Группа товаров

    class Group

    {

        public int Id { get; set; }

        public string Name { get; set; }

    }

​

    // Товар

    class Product

    {

        public int Id { get; set; }

        public string Name { get; set; }

        public int GroupId { get; set; } // Идентификатор группы товаров (внешний ключ)

        public Group Group { get; set; } // Navigation property

    }

​

    class Program

    {

        static void Main(string[] args)

        {

            var productIds = new List<int> { 0, 2, 3 };

​

            IEnumerable<Group> groups = new[]

            {

                new Group { Id = 0, Name = "Food" },

                new Group { Id = 1, Name = "Drinks" }

            };

​

            var food = groups.First();

            var drinks = groups.Last();

​

            Product CreateFood(int id, string name)

            {

                return new Product { Id = id, Name = name, Group = food, GroupId = food.Id };

            }

​

            Product CreateDrink(int id, string name)

            {

                return new Product { Id = id, Name = name, Group = drinks, GroupId = drinks.Id };

            }

​

            IEnumerable<Product> products = new[]

            {

                CreateDrink(0, "Milk"),

                CreateDrink(1, "Coffee"),

                CreateFood(2, "Chocolate"),

                CreateFood(3, "Meat"),

                CreateFood(4, "Potato")

            };

​

            // 2.1

            var result1 = productIds.Join(products, id => id, product => product.Id,

                (id, product) => new

                {

                    ProductName = product.Name,

                    GroupName = product.Group.Name

                }).ToList();

​

            // 2.2

            var random = new Random(Environment.TickCount);

            foreach (var product in products)

            {

                product.Group = null;

                product.GroupId = random.Next(5);

            }

​

            const string EmptyGroupName = "No Group";

            var result2 = productIds

                .Join(products, id => id, product => product.Id, (id, product) => product)

                .GroupJoin(groups, product => product.GroupId, group => group.Id,

                    (product, matchedGroups) => new

                    {

                        ProductName = product.Name,

                        GroupName = matchedGroups.FirstOrDefault()?.Name ?? EmptyGroupName

                    }).ToList();

​

            Console.WriteLine("2.1");

            Console.WriteLine(string.Join(Environment.NewLine, result1));

            Console.WriteLine();

​

            Console.WriteLine("2.2");

            Console.WriteLine(string.Join(Environment.NewLine, result2));

        }

    }

}

​