using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<TrackList> inputList = new List<TrackList>

        {

            new TrackList { Id = 1,  Name = "aaa", Apr_By = 1, Reg_By = 1 },

            new TrackList { Id = 2,  Name = "bbb", Apr_By = 2, Reg_By = null },

            new TrackList { Id = 3,  Name = "ccc", Apr_By = null, Reg_By = 3 },

            new TrackList { Id = 4,  Name = "ddd", Apr_By = null, Reg_By = null }

        };

        List<TrackListMain> result = (inputList.Where(x => x.Apr_By == null && x.Reg_By == null)

                                         .Select(x => new TrackListMain

                                                 {

                                                     Id = x.Id,

                                                     Name = x.Name,

                                                     Status = null

                                                 })

                                    )

                                    .Union(inputList.Where(x => x.Apr_By != null)

                                     .Select(x => new TrackListMain

                                             {

                                                 Id = x.Id,

                                                 Name = x.Name,

                                                 Status = "AB"

                                             })

                                    )

                                    .Union(inputList.Where(x => x.Reg_By != null)

                                     .Select(x => new TrackListMain

                                             {

                                                 Id = x.Id,

                                                 Name = x.Name,

                                                 Status = "RB"

                                             })

                                    )

                                    .OrderBy(x => x.Id)

                                    .ToList();

        FiddleHelper.WriteTable(result);

    }

}

​

public class TrackList

{

    public int Id { get; set; }

    public string Name { get; set; }

    public int? Apr_By { get; set; }

    public int? Reg_By { get; set; }

}

​

public class TrackListMain

{

    public int Id { get; set; }

    public string Name { get; set; }

    public string Status { get; set; }

}

​