using System;

using System.Linq;

public class Program

{

    private class TimestampDetail

    {

        public DateTime Timestamp { get; set; } 

    }

    private class FeedDetails

    {

        public int Feed { get; set; }

        public TimestampDetail? TimestampDetail { get; set; }

    }       

    public static void Main()

    {

        DateTime? storedTimestamp = null;

        var testData = new []

        {

            new FeedDetails { Feed = 1, TimestampDetail = new TimestampDetail { Timestamp = DateTime.Now } },

            new FeedDetails { Feed = 2, TimestampDetail = null },

            new FeedDetails { Feed = 3, TimestampDetail = new TimestampDetail { Timestamp = DateTime.Now } },

            new FeedDetails { Feed = 4, TimestampDetail = new TimestampDetail { Timestamp = DateTime.Now.AddSeconds(10) } },

            new FeedDetails { Feed = 5, TimestampDetail = new TimestampDetail { Timestamp = DateTime.Now.AddSeconds(10) } }

        };

                var feedToProcess = testData

                    .Where(ofg =>

                        ofg!.TimestampDetail?.Timestamp != null &&

                        (storedTimestamp == null ||

                        storedTimestamp <= ofg!.TimestampDetail?.Timestamp))

                    .Select(ofg => ofg.Feed);

                var feedOutOfOrder = testData

                    .Where(ofg =>

                           ofg!.TimestampDetail?.Timestamp == null ||

                           storedTimestamp > ofg!.TimestampDetail?.Timestamp)

                    .Select(ofg => ofg.Feed);

​

        Console.WriteLine("To process:");

        foreach (var x in feedToProcess)

            Console.WriteLine(x);

        Console.WriteLine("Out of order:");

        foreach (var x in feedOutOfOrder)

            Console.WriteLine(x);

    }

}