using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<TrackingInfo> tracking = new List<TrackingInfo>

            {

                new TrackingInfo { Template = "Template A", ProfilesIDs = new List<int> { 1,3,5 }},

                new TrackingInfo { Template = "Template B", ProfilesIDs = new List<int> { 3,5,6 }},

                new TrackingInfo { Template = "Template C", ProfilesIDs = new List<int> { 2,4,6 }}

            };

        var groupedByprofiles = tracking.SelectMany(x => x.ProfilesIDs, (trackObj, profileIds) => new { trackObj, profileIds }).GroupBy(x => x.profileIds);

​

        foreach (var grp in groupedByprofiles)

        {

            // Each group or bucket has a Key property  

            Console.WriteLine("ProfileID: " + grp.Key);

​

            foreach (var temp in grp)

            {

                Console.WriteLine(temp.trackObj.Template);

            }

            Console.WriteLine();

        }

    }

}

​

 public class TrackingInfo

    {

        public string Template { get; set; }

        public List<int> ProfilesIDs { get; set; }

    }