#region IEnumerable<T>.MapReduce
        public static Dictionary<TKey, TResult> MapReduce<TInput, TKey, TValue, TResult>(
            this IEnumerable<TInput> list,
            Func<TInput, IEnumerable<KeyValuePair<TKey, TValue>>> map,
            Func<TKey, IEnumerable<TValue>, TResult> reduce)
        {
            Dictionary<TKey, List<TValue>> mapResult = new Dictionary<TKey, List<TValue>>();
            foreach (var item in list)
            {
                foreach (var one in map(item))
                {
                    List<TValue> mapValues;
                    if (!mapResult.TryGetValue(one.Key, out mapValues))
                    {
                        mapValues = new List<TValue>();
                        mapResult.Add(one.Key, mapValues);
                    }
                    mapValues.Add(one.Value);
                }
            }
            var result = new Dictionary<TKey, TResult>();
            foreach (var m in mapResult)
            {
                result.Add(m.Key, reduce(m.Key, m.Value));
            }
            return result;
        }
        #endregion

    public class Person
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
    }

        static void Main(string[] args)
        {
            List<Person> list=new List<Person> ();
            list.Add(, Name= });
            list.Add(, Name =  });
            list.Add(, Name =  });
            list.Add(, Name =  });
            list.Add(, Name =  });

            var result = list.MapReduce<Person, int, string, string>(Map,
                (key, values) => string.Join(",", values));

            foreach (var d in result)
            {
                Console.WriteLine(d.Key + ":" + d.Value);
            }
        }

        public static IEnumerable<KeyValuePair<int, string>> Map(Person p)
        {
            )
                yield return new KeyValuePair<int, string>(p.Age, p.Name);
        }