using System;

using System.Xml;

using System.Xml.Serialization;

using System.IO;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<Test> tests = new List<Test>() 

        { 

            new Test(){ No = 101 },

            new Test(){ No = 201 },

            new Test(){ No = 300 },

            new Test(){ No = 401 },

            new Test(){ No = 500 },

            new Test(){ No = 601 },

            new Test(){ No = 778 }, 

            new Test(){ No = 500 },

        };

​

        int[] newOrder = new int[7] { 201, 401, 300, 101, 601, 500, 777 };

        var results = from a in tests.Select((r, i) => new {item = r, Index = i})

                      from b in newOrder.Select((r, i) => new { item = r, Index = i }).Where(b => a.item.No == b.item).DefaultIfEmpty() 

                      orderby (b == null ? tests.Count() + a.Index : b.Index) // Not in order list

                      select a.item;

        Console.WriteLine("Records:" + results.Count());

        foreach (var item in results)

        {

            Console.WriteLine(string.Join(",", item.No));

        }

    }   

    public class Test

    {

        public int No { get; set; }

    }

}

​

​