using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<WatchList> original = new List<WatchList>

        {

            new WatchList{WatchListId=1, Position=1},

            new WatchList{WatchListId=2, Position=2},

            new WatchList{WatchListId=3, Position=3},

            new WatchList{WatchListId=4, Position=4},

            new WatchList{WatchListId=5, Position=5}

        };

​

        List<WatchList> input = new List<WatchList>

        {

            new WatchList{WatchListId=1, Position=5},

            new WatchList{WatchListId=3, Position=1},

            new WatchList{WatchListId=5, Position=4}

        };

​

        List<WatchList> ordered = original.Where(w => !input.Any(iw => iw.WatchListId == w.WatchListId)).OrderBy(w => w.Position).ToList();

        foreach (var inputWatchlist in input)

        {

            int indexToInsert = 0;

            while (indexToInsert < ordered.Count)

            {

                if (ordered[indexToInsert].Position <= inputWatchlist.Position)

                {

                    indexToInsert++;

                } 

                else

                {

                    break;

                }

            }

​

            ordered.Insert(indexToInsert, inputWatchlist);

        }

​

        foreach (var w in ordered)

        {

            Console.WriteLine("Id: " + w.WatchListId + " P: " + w.Position);

        }

    }

}

​

class WatchList

{

    public int WatchListId;

    public int Position;

}