using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<Output> records = GetData().ToList();

        var activityNames = new Dictionary<Guid, string>();

        List<Output> recordsWithStart = records.Where(record => record.Level == "Start").ToList();

        recordsWithStart.ForEach(record =>

                                 {

                                     record.ActivityName = record.Description;

                                     activityNames.Add(record.ActivityId, record.ActivityName);

                                 });

        records.ForEach(record =>

                        {

                            string activityName;

                            if (activityNames.TryGetValue(record.ActivityId, out activityName))

                            {

                                record.ActivityName = activityName;

                            }

                        });

        records.Dump(); // requires LinqPad or .NET Fiddle

    }

    private static IEnumerable<Output> GetData()

    {

        return new List<Output>

        {

            new Output

            {

                Description = "Test21",

                Level = "Information",

                ActivityId = new Guid("a54221e2-ad37-434a-8f0d-101f7abc2221")

            },

                new Output

            {

                Description = "Test24",

                Level = "Information",

                ActivityId = new Guid("d30741c2-da73-434a-8f0d-101f7ceb2228")

            },

                new Output

            {

                Description = "Alpha",

                Level = "Start",

                ActivityId = new Guid("00000000-0000-0000-0000-000000000000")

            },

                new Output

            {

                Description = "Test31",

                Level = "Information",

                ActivityId = new Guid("00000000-0000-0000-0000-000000000000")

            },

                new Output

            {

                Description = "Test51",

                Level = "Verbose",

                ActivityId = new Guid("3b081a36-43d5-4ecc-b381-628c33316205")

            },

                new Output

            {

                Description = "Bravo",

                Level = "Start",

                ActivityId = new Guid("aa5a5b9c-4b24-43af-9f49-32656385e17d")

            },

                new Output

            {

                Description = "Charlie",

                Level = "Start",

                ActivityId = new Guid("d30741c2-da73-434a-8f0d-101f7ceb2228")

            },

                new Output

            {

                Description = "Test71",

                Level = "Error",

                ActivityId = new Guid("aa5a5b9c-4b24-43af-9f49-32656385e17d")

            },

                new Output

            {

                Description = "Test41",

                Level = "Transfer",

                ActivityId = new Guid("00000000-0000-0000-0000-000000000000")

            }

        };

    }

    public sealed class Output

    {

        public string Description { get; set; }

        public string Level { get; set; }

        public string ActivityName { get; set; }

        public Guid ActivityId { get; set; }            

    }

}

​