GroupBy

 
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        string[] names = { "Java", "C#", "Javascript", "SQL", "Oracle", "Python", "C++", "C", "HTML", "CSS" };

      IEnumerable< IGrouping<int,string>> grouping = names.GroupBy(s => s.Length);

       foreach (IGrouping<int, string> group in grouping)
        {
            Console.WriteLine("Key: " + group.Key);
            foreach (string filename in group) 
                Console.WriteLine("  - " + filename);
        }

    }
}