using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

     public class WatchList : IEquatable<WatchList>

    {

        public WatchList(int id)

        {

            Id = id;

        }

​

        public int Id { get; set; }

​

        public string Name { get; set; }

​

        public int UserId { get; set; }

​

        public int Position { get; set; }

​

        public bool Equals(WatchList other)

        {

            if (other == null) return false;

            if (ReferenceEquals(this, other)) return true;

            return Id == other.Id;

        }

​

        public override bool Equals(object obj)

        {

            WatchList other = obj as WatchList;

            return Equals(other);

        }

​

        public override int GetHashCode()

        {

            return Id;

        }

​

        public static void UpdateWatchListsSort(string userId, List<WatchList> watchListsWithModifiedPosition)

        {

            List<WatchList> allUserWatchLists = GetWatchListsFromDb(userId);

            // mapping WatchListId --> WatchList (from DB)

            Dictionary<int, WatchList> dbWatchListIdLookup = allUserWatchLists.ToDictionary(w => w.Id);

​

            if (watchListsWithModifiedPosition.Count == allUserWatchLists.Count)

                allUserWatchLists = watchListsWithModifiedPosition;

            else

                foreach (WatchList modified in watchListsWithModifiedPosition.OrderBy(w => w.Position))

                {

                    WatchList dbWatchList = dbWatchListIdLookup[modified.Id];

                    int newIndex = modified.Position - 1;

                    int oldIndex = allUserWatchLists.IndexOf(dbWatchList); // might be at a different position meanwhile( != db-position )

                    allUserWatchLists.RemoveAt(oldIndex);

                    // if moved forwards index is index-1 because the watchlist was already removed at List.RemoveAt, 

                    // if moved backwards index isn't affected

                    bool movedForwards = newIndex > oldIndex;

                    if (movedForwards)

                        newIndex--;

                    allUserWatchLists.Insert(newIndex, dbWatchList);

                }

​

            var changeInfos = allUserWatchLists

                .Select((wl, index) => new {WatchList = wl, NewPosition = index + 1})

                .Where(x => x.WatchList.Position != x.NewPosition)

                .ToList();

            foreach (var change in changeInfos)

            {

                WatchList wl = change.WatchList;

                wl.Position = change.NewPosition;

                // check if the new position is equal to the position given as parameter

                bool success = wl.Position == watchListsWithModifiedPosition

                    .Where(w => w.Id == wl.Id)

                    .Select(w => w.Position)

                    .DefaultIfEmpty(wl.Position)

                    .First();

                Console.WriteLine("WatchLists has correct position? " + success);

            }

            // check if allUserWatchLists contains duplicate Positions which is invalid

            bool containsDuplicatePositions = allUserWatchLists

                .Select(w => w.Position)

                .Distinct()

                .Count() < allUserWatchLists.Count;

            Console.WriteLine("No duplicate positions? " + !containsDuplicatePositions);

​

            // update changeInfos.Select(x => x.WatchList) via table-valued-parameter in DB (not related) .....

        }

​

        private static List<WatchList> GetWatchListsFromDb(string userId)

        {

            var allDbWatchLists = new List<WatchList>

            {

                new WatchList(1) {Position = 1},

                new WatchList(2) {Position = 2},

                new WatchList(3) {Position = 3},

                new WatchList(4) {Position = 4},

                new WatchList(5) {Position = 5}

            };

            return allDbWatchLists;

        }

    }

​

​

   public static void Main(string[] args)

   {

       var changedWatchLists = new List<WatchList>

       {

           new WatchList(1) {Position = 5}, new WatchList(3) {Position = 1}, new WatchList(5) {Position = 4}

       };

       WatchList.UpdateWatchListsSort("123", changedWatchLists);

   }

}