using System;    

using System.Collections.Generic;    

using System.Linq;    

​

namespace LinqGroup    

{    

    public class Program    

    {

        public static void Main(string[] args)    

        {

            string[] listOfNames = new string[] { "John", "Axel", "Alex", "Peter", "Adam", "Jesus", "Michael", "Jeffrey", "Alex", "Peter", "John", "Adam" }; 

            int[] listOfInts = new int[] { 5, 10, 50, 23, 33, -5, 5, 12, 33 };

            listOfNames.MapReduce(s => s.Length > 10);

        }

    }

    public static class CustomMapReduce 

    {

        public static IEnumerable<TResult> CustomSelect<T, TResult>(IEnumerable<T> source, Func<T, TResult> predicate) 

        {

            foreach (var item in source) 

            {

                yield return predicate(item);

            }

        }

        public static IGrouping<T, TKey> CustomGroupBy(IEnumerable<T> source, Func<T, IEnumerable<T>> predicate) 

        {

        }

        public static T CustomReduce<T>(IEnumerable<T> source, Func<T,T,T> predicate) 

        {

            IEnumerator<T> enumerator = source.GetEnumerator();

            T result = enumerator.Current;

            while(enumerator.MoveNext()) 

            {

                result = predicate(result, enumerator.Current);

            }

            return result;

        }

    }

}

​