using System;

using System.Collections.Generic;

using System.Linq;

public class TermData

{

    public int Sequence { get; set; }

    public string Terms { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        var list = new List<TermData>();

        list.Add(new TermData() { Sequence = 1438690, Terms = "weather" });

        list.Add(new TermData() { Sequence = 1438690, Terms = "the elements" });

        list.Add(new TermData() { Sequence = 9672410, Terms = "dogs" });

        list.Add(new TermData() { Sequence = 9672410, Terms = "cats" });

​

        var lookup = list.ToLookup( item => item.Sequence, item => item.Terms);

        foreach (var item in lookup)

        {

            Console.WriteLine("Sequence {0} has these terms: {1}", item.Key, string.Join(",", item));

        }

    }

}