using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<SqsMessage> messages = new List<SqsMessage>

        {

            new SqsMessage("Product1"),

            new SqsMessage("Product2", "Media1", "Media2"),

            new SqsMessage("Product3"),

            new SqsMessage("Product1", "Media3", "Media4"),

            new SqsMessage("Product3"),

            new SqsMessage("Product1", "Media5", "Media6"),

            new SqsMessage("Product1", "Media3")

        };

        Console.WriteLine("Message count: " + messages.Count);

        var productGroups = messages.GroupBy(m => m.CoreId);

        Console.WriteLine("Message group count: " + productGroups.Count());

        Console.WriteLine();

        foreach (var group in productGroups)

        {

            // Key will be the CoreId, since that was what was used for the grouping

            Console.WriteLine("Group for " + group.Key);

            foreach (var message in group)

            {

                if (message.MediaIds == null)

                {

                    Console.WriteLine("No MediaIds found");

                }

                else

                {

                    Console.WriteLine("MediaIds: " + string.Join(", ", message.MediaIds));

                }

            }

            var distinctMediaIds = group.Where(m => m.MediaIds != null).SelectMany(m => m.MediaIds).Distinct().ToList();

            if (distinctMediaIds.Count > 0)

            {

                Console.WriteLine("Distinct MediaIds: " + string.Join(", ", distinctMediaIds));

            }

            Console.WriteLine();            

        }

    }

}

​

public class SqsMessage

{

    public SqsMessage(string coreId)

    {

        CoreId = coreId;

    }

    public SqsMessage(string coreId, params string[] mediaIds)

        : this(coreId)

    {

        MediaIds = new List<string>(mediaIds);

    }

    public string CoreId { get; set; }

    public List<string> MediaIds { get; set; }

}