direct-handler-xml-file.cs

 public static class ConsoleExtension
    {
        public static void ToConsole<T>(this IEnumerable<T> s, string header)
        {
            Console.WriteLine($"*********BEGIN {header}**************");
            foreach (T item in s)
            {
                Console.WriteLine(item);
            }
            Console.WriteLine($"*********END {header}**************");
            Console.ReadLine();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.InputEncoding = Encoding.Unicode;
            Console.OutputEncoding = Encoding.Unicode;

            XDocument doc = XDocument.Load("http://users.nik.uni-obuda.hu/prog3/_data/people.xml");
            var AllPeople = doc.Descendants("person").Select(person => person.Element("name")?.Value);
            AllPeople.ToConsole("ALL PEOPLE NAME");

            //Q1 - Determine the number of workers who work in the AII department!

            var q1 = doc.Descendants("person").Where(person => person.Element("dept")?.Value == "Alkalmazott Informatikai Intézet").Count();
            Console.WriteLine($"QUERY 1\nNumber of worker in 'All Department': {q1}");
            Console.ReadLine();

            //Q2 - Display those who live in the third floor in a “paginated” list, we should press ENTER after every 15 elements
            int current = 0, pagesize = 15;
            while (current < q1)
            {
                var q2 = doc.Descendants("person").Where(person => person.Element("room").Value.StartsWith("BA.3")).Select(person =>
                person.Element("name")?.Value).Skip(current).Take(15);
                q2.ToConsole("QUERY 2");
                current += pagesize;
            }

            //Q3 - Who have the shortest or the longest names?
            var q3 = from person in doc.Descendants("person")
                     let minNameLength = doc.Descendants("person").Min(x => x.Element("name")?.Value.Length)
                     let maxNameLength = doc.Descendants("person").Max(x => x.Element("name")?.Value.Length)
                     where person.Element("name")?.Value.Length == minNameLength ||
                     person.Element("name")?.Value.Length == maxNameLength
                     select new { Name = person.Element("name")?.Value, Length = person.Element("name")?.Value.Length };
            q3.ToConsole("QUERY 3");

            // Q4 - Determine the number of workers for every department!

            var q4 = from person in doc.Descendants("person")
                     group person by person.Element("dept")?.Value into g
                     select new { Dept = g.Key, NumberOfWorker = g.Count() };

            q4.ToConsole("QUERY 4");

            // Q5 - Determine the biggest department!

            var q5 = q4.OrderByDescending(person => person.NumberOfWorker).FirstOrDefault();
            Console.WriteLine($"QUERY 5 \n{q5}");
            Console.ReadLine();

            //Q6 - List the workers for the biggest department!
            var q6 = doc.Descendants("person").Where(person => person.Element("dept")?.Value == q5.Dept).Select(person => person.Element("name")?.Value);
            q6.ToConsole("QUERY 6");

            //Q7 - List the people who work in the third biggest department ordered by their room number in descending order!
            var thirdBiggestDept = q4.OrderByDescending(person => person.NumberOfWorker).Skip(2).FirstOrDefault();
            var q7 = doc.Descendants("person").Where(person => person.Element("dept")?.Value == thirdBiggestDept.Dept).Select(person => new { Name = person.Element("name")?.Value, Room = person.Element("room")?.Value }).OrderByDescending(person => person.Room);
            q7.ToConsole("QUERY 7");
        }
    }

handler-xml-file-via-class-list.cs

class Person
    {
        public string Name { get; set; }
        public string Email { get; set; }
        public string Dept { get; set; }
        public string Rank { get; set; }
        public string Phone { get; set; }
        public string Room { get; set; }

        public static List<Person> Persons(string url)
        {
            XDocument document = XDocument.Load(url);
            return document.Descendants("person").Select(node => new Person()
            {
                Name = node.Element("name")?.Value,
                Email = node.Element("email")?.Value,
                Dept = node.Element("dept")?.Value,
                Rank = node.Element("rank")?.Value,
                Phone = node.Element("phone")?.Value,
                Room = node.Element("room")?.Value
            }).ToList();
        }

    }

    public static class ConsoleExtension
    {
        public static void ToConsole<T>(this IEnumerable<T> s, string header)
        {
            Console.WriteLine($"*********BEGIN {header}**************");
            foreach (T item in s)
            {
                Console.WriteLine(item);
            }
            Console.WriteLine($"*********END {header}**************");
            Console.ReadLine();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.InputEncoding = Encoding.Unicode;
            Console.OutputEncoding = Encoding.Unicode;

            List<Person> People = Person.Persons("http://users.nik.uni-obuda.hu/prog3/_data/people.xml");
            People.Select(p => p.Name).ToConsole("ALL PEOPLE NAME");

            //Q1 - Determine the number of workers who work in the AII department!

            var q1 = People.Where(person => person.Dept == "Alkalmazott Informatikai Intézet").Count();
            Console.WriteLine($"QUERY 1\nNumber of worker in 'All Department': {q1}");
            Console.ReadLine();

            //Q2 - Display those who live in the third floor in a “paginated” list, we should press ENTER after every 15 elements

            int current = 0, pagesize = 15;
            while (current < q1)
            {
                var q2 = People.Where(person => person.Room.StartsWith("BA.3")).Select(person => person.Name).Skip(current).
                    Take(pagesize);
                q2.ToConsole("QUERY 2");
                current += pagesize;
            }

            //Q3 - Who have the shortest or the longest names?

            var q3 = from person in People
                     let maxNameLength = People.Max(x => x.Name.Length)
                     let minNameLength = People.Min(x => x.Name.Length)
                     where person.Name.Length == minNameLength ||
                     person.Name.Length == maxNameLength
                     select new { Name = person.Name, Length = person.Name.Length };
            q3.ToConsole("QUERY 3");

            //Q4 - Determine the number of workers for every department!

            var q4 = from person in People
                     group person by person.Dept into g
                     select new { Dept = g.Key, NumberOfWorker = g.Count() };
            q4.ToConsole("QUERY 4");

            //Q5 - Determine the biggest department!

            var q5 = q4.OrderByDescending(x => x.NumberOfWorker).FirstOrDefault();
            Console.WriteLine($"QUERY 5\nBiggest Department: {q5}");
            Console.ReadLine();

            //Q6 - List the workers for the biggest department!

            var q6 = People.Where(person => person.Dept == q5.Dept).Select(person => person.Name);
            q6.ToConsole("QUERY 6");

            //Q7 - List the people who work in the third biggest department ordered by their room number in descending order!

            var thirdBiggest = q4.OrderByDescending(x => x.NumberOfWorker).Skip(2).FirstOrDefault();
            var q7 = People.Where(person => person.Dept == thirdBiggest.Dept).Select(person => new { person.Name, person.Room }).OrderByDescending(person => person.Room);
            q7.ToConsole("QUERY 7");
        }
    }