using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    class smth

    {

        public int header;

        public int id;

    }

​

    public static void Main()

    {

        List<smth> list = new List<smth>();

        list.Add(new smth() {header = 1, id = 1});

        list.Add(new smth() {header = 1, id = 2});

        list.Add(new smth() {header = 1, id = 3});

        list.Add(new smth() {header = 2, id = 4});

        list.Add(new smth() {header = 3, id = 5});

        IEnumerable<IGrouping<int, smth>> groups = list.GroupBy(x => x.header);

        foreach (var group in groups)

        {

            Console.WriteLine(group.Key);

            foreach (var element in group.AsEnumerable()    )

            {

                Console.WriteLine(String.Format("{0} - {1}", element.header, element.id));  

            }

        }

    }

}