using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{   

    public class Category

        {

            public Category(int id, string name, ICollection<Forum> forums)

            {

                Id = id;

                Name = name;

                Forums = forums;

            }

​

            public int Id { get; set; }

            public string Name { get; set; }

            public ICollection<Forum> Forums { get; set; }

        }

​

        public class Forum

        {

            public Forum(int id, string name, ICollection<Topic> topics)

            {

                Id = id;

                Name = name;

                Topics = topics;

            }

​

            public int Id { get; set; }

            public string Name { get; set; }

            public ICollection<Topic> Topics { get; set; }

        }

​

        public class Topic

        {

            public Topic(int id, string name, string owner, DateTime postDateTime)

            {

                Id = id;

                Name = name;

                Owner = owner;

                PostDateTime = postDateTime;

            }

​

            public int Id { get; set; }

            public string Name { get; set; }

            public string Owner { get; set; }

            public DateTime PostDateTime { get; set; }

        }

​

    public static void Main()

    {

            var categories = new List<Category>

            {

                new Category(1, "Cat 1", new List<Forum>

                {

                    new Forum(1, "Forum 1", new List<Topic>

                    {

                        new Topic(1, "Top 1", "User111", DateTime.Now.AddMinutes(-100)),

                        new Topic(2, "Top 2", "User222", DateTime.Now.AddMinutes(-20))

                    }),

                    new Forum(2, "Forum 2", new List<Topic>

                    {

                        new Topic(3, "Top 1", "User111", DateTime.Now.AddMinutes(-30)),

                        new Topic(4, "Top 2", "User222", DateTime.Now.AddMinutes(-40))

                    })

                }),

                new Category(2, "Cat 2", new List<Forum>

                {

                    new Forum(3, "Forum 3", new List<Topic>

                    {

                        new Topic(1, "Top 1", "User111", DateTime.Now.AddMinutes(-50)),

                        new Topic(2, "Top 2", "User222", DateTime.Now.AddMinutes(-60))

                    }),

                    new Forum(4, "Forum 4", new List<Topic>

                    {

                        new Topic(3, "Top 1", "User111", DateTime.Now.AddMinutes(-70)),

                        new Topic(4, "Top 2", "User222", DateTime.Now.AddMinutes(-800))

                    })

                })

            };

            var users = categories.Select(x=> new {x.Id, LastUser = x.Forums.SelectMany(t => t.Topics).OrderByDescending(o=>o.PostDateTime).FirstOrDefault().Owner});

            foreach (var item in users) Console.WriteLine("[" + item.Id + "] " + item.LastUser);

    }

}