using System;

using System.Data;

using System.Xml;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public class Station

    {

        public int ID {get;set;}

        public string DEP {get;set;}

        public string ARR {get;set;}

        public string KEY {get;set;}

    }

    public static void Main()

    {

        List<Station> lstTemp1 = new List<Station>();

        lstTemp1.Add(new Station() { ARR = "CDG", DEP = "TPE" , KEY = "c" });

        lstTemp1.Add(new Station() { ARR = "ADG", DEP = "CAN" , KEY = "b" });

        lstTemp1.Add(new Station() { ARR = "CDG", DEP = "AXX" , KEY = "a" });

        lstTemp1.Add(new Station() { ARR = "BHR", DEP = "BKK" , KEY = "a" });

        lstTemp1.Add(new Station() { ARR = "LHR", DEP = "AXX" , KEY = "a" });

        lstTemp1.Add(new Station() { ARR = "BHR", DEP = "AXX" , KEY = "a" });

        foreach (var str in lstTemp1)

        {

            Console.WriteLine(str.ARR + "," +str.DEP + "," +str.KEY + "," );

        }

            Console.WriteLine("----------------------------------------");

        List<Station> lstTemp2 = lstTemp1.OrderBy(s => s.KEY).ThenBy(s => s.DEP).ThenBy(s => s.ARR).ToList();

        foreach (var str in lstTemp2)

        {

            Console.WriteLine(str.ARR + "," +str.DEP + "," +str.KEY + "," );

        }

            Console.WriteLine("----------------------------------------");

        List<Station> lstTemp3 = (from a in lstTemp1

                                    orderby a.KEY ascending, a.DEP ascending, a.ARR ascending

                                    select a).ToList();

​

        foreach (var str in lstTemp3)

        {

            Console.WriteLine(str.ARR + "," +str.DEP + "," +str.KEY + "," );

        }

            Console.WriteLine("----------------------------------------");  

        string orderKey = "a";

        Dictionary<string, int> list = new Dictionary<string, int>();

        Dictionary<string, int> lstOrderKey = orderKey.Split(',').Select((s, index) => new { s, index }).ToDictionary(o => o.s , o => o.index);

        var lstTemp4 = (from a in lstTemp1

                  join b in lstOrderKey on a.KEY equals b.Key

                  orderby b.Value , a.DEP

                  select a

                 ).Union(from a in lstTemp1

                  join b in lstOrderKey on a.KEY equals b.Key into g

                  from b in g.DefaultIfEmpty()

                  where b.Key == null

                  orderby a.DEP

                  select a).ToList();

        foreach (var str in lstTemp4)

        {

            Console.WriteLine(str.ARR + "," +str.DEP + "," +str.KEY + "," );

        }

    }

}