using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class Item

    {

        public int Id { get; set; }

        public int Weight { get; set; }

        public int Phase { get; set; }

    }

    public class ItemComparison

    {

        public int Id { get; init; }

        public string Weight => GetComparisonString(_weightPrevious, _weightCurrent);

        public string Phase => GetComparisonString(_phasePrevious, _phaseCurrent);

        public ItemComparison(Item previousItem, Item currentItem)

        {

            // TODO Error handling: previousItem and currentItem may

            //  - both be null

            //  - both be not null and have different Id values

            Id = (previousItem ?? currentItem).Id;

            _weightPrevious = previousItem?.Weight;

            _weightCurrent = currentItem?.Weight;

            _phasePrevious = previousItem?.Phase;

            _phaseCurrent = currentItem?.Phase;

        }

        private int? _weightPrevious;

        private int? _weightCurrent;

        private int? _phasePrevious;

        private int? _phaseCurrent;

        private string GetComparisonString(int? previousValue, int? currentValue)

        {

            if (previousValue == currentValue)

            {

                return "not changed";

            }

            return $"{GetStringOrUnknown(previousValue)} --> {GetStringOrUnknown(currentValue)}"; 

        }

        private string GetStringOrUnknown(int? value)

        {

            return value.HasValue ? value.ToString() : "?";

        }

    }

    public static void Main()

    {

        List<Item> queryResult1 = new()

        {

            new() { Id = 1, Weight = 123, Phase = 1 },

            new() { Id = 2, Weight = 55, Phase = 1 },

            new() { Id = 3, Weight = 122, Phase = 2 },

            new() { Id = 4, Weight = 7, Phase = 2 },

        };

        List<Item> queryResult2 = new()

        {

            new() { Id = 1, Weight = 123, Phase = 1 },

            new() { Id = 4, Weight = 7, Phase = 1 },

            new() { Id = 5, Weight = 122, Phase = 2 },

        };

        var updatedItems = queryResult1

            .Join(queryResult2,

                previous => previous.Id,

                current => current.Id,

                (previous, current) => new ItemComparison(previous, current));

        var removedItems = queryResult1

            .ExceptBy(queryResult2.Select(qr2 => qr2.Id), qr1 => qr1.Id)

            .Select(removed => new ItemComparison(removed, null));

        var addedItems = queryResult2

            .ExceptBy(queryResult1.Select(qr1 => qr1.Id), qr2 => qr2.Id)

            .Select(added => new ItemComparison(null, added));

        List<ItemComparison> comparisonResult = updatedItems

            .Concat(removedItems)

            .Concat(addedItems)

            .OrderBy(item => item.Id)

            .ToList();

        // Horrible printing, but to give an idea of the result

        Console.WriteLine($"{nameof(ItemComparison.Id)}   {nameof(ItemComparison.Weight)}          {nameof(ItemComparison.Phase)}");

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

        foreach (var comparison in comparisonResult)

        {

            Console.WriteLine($"{comparison.Id}    {comparison.Weight}       {comparison.Phase}");

        }

    }

}