FP1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Policy;

namespace GroupBy
{
    public static class Program
    {
        public static void Main()
        {
            var workers = new List<People>()
            {
                new People("Bob", 18),
                new People("Anna", 24),
                new People("Petya", 18),
                new People("Misha", 18),
            };

            var groupWorkersByAge = workers.MyGroupBy(worker => worker.Age);

            foreach (var pair in groupWorkersByAge)
            {
                Console.Write("{0}: ", pair.Key);
                foreach (var value in pair.Value)
                {
                    Console.Write("{0} ", value.Name);
                }
                Console.WriteLine();
            }
        }

        public static Dictionary<TKey, List<TValue>> MyGroupBy<TKey, TValue>(
            this IEnumerable<TValue> collection, Func<TValue, TKey> keySelector)
        {
            var groups = new Dictionary<TKey, List<TValue>>();
            foreach (TValue value in collection)
            {
                var key = keySelector(value);
                if (!groups.ContainsKey(key)) groups[key] = new List<TValue>();
                groups[key].Add(value);
            }

            return groups;
        }
    }

    public class People
    {
        public string Name { get; }
        public int Age { get; }

        public People(string name, int age)
        {
            Name = name;
            Age = age;
        }
    }
}