Program.cs

using DynamicData;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Reactive.Linq;

namespace ComparerStreamIssue;

public sealed class Program
{
    private static readonly SourceCache<TestUserItem, string>
        _sourceCache = new(x => x.Id);

    public static void Main()
    {
        int limit = 10;
        for (int i = 1; i <= limit; i++)
        {
            _sourceCache.AddOrUpdate(new TestUserItem(
                $"{i}",
                ((limit - i) * 10).ToString(),
                new string((char)('a' + (i % 13)), 7)));
        }

        _ = GetTestUserItemSourceChangeSetStream()
            .Bind(out ReadOnlyObservableCollection<TestUserItem> _testUserItems)
            .Subscribe(_ =>
            {
                Console.WriteLine("-= List =-");

                foreach (TestUserItem testUserItem in _testUserItems)
                {
                    Console.WriteLine($"{testUserItem.Number} - {testUserItem.Name}");
                }

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

        _ = Console.ReadKey();
    }

    private static IObservable<IChangeSet<TestUserItem, string, VirtualContext<TestUserItem>>> GetTestUserItemSourceChangeSetStream()
    {
        return _sourceCache
            .Connect()
            .SortAndVirtualize(
                GetComparerStream(),
                Observable.Return(new VirtualRequest(0, 20_000)),
                new SortAndVirtualizeOptions
                {
                    InitialCapacity = 100,
                    ResetThreshold = 100,
                    UseBinarySearch = true
                });
    }

    private static IObservable<IComparer<TestUserItem>> GetComparerStream()
    {
        return Observable
            .Timer(TimeSpan.FromSeconds(0), TimeSpan.FromSeconds(5))
            .Select(timer => timer % 2 == 0
                ? TestUserNameComparer.Instance
                : TestUserNumberComparer.Instance)
            .Do(comparer => Debug.WriteLine($">>> ComparerStream emitted: {comparer}"))
            .Replay(1)
            .RefCount();
    }

    public class TestUserItem(string Id, string Number, string Name)
    {
        public string Id { get; } = Id;

        public string Number { get; } = Number;

        public string Name { get; } = Name;
    }

    public class TestUserNameComparer : IComparer<TestUserItem>
    {
        public static readonly IComparer<TestUserItem> Instance = new TestUserNameComparer();

        public int Compare(TestUserItem? x, TestUserItem? y)
        {
            return x!.Name.CompareTo(y!.Name);
        }

        public override string ToString()
        {
            return "TestUserNameComparer";
        }
    }

    public class TestUserNumberComparer : IComparer<TestUserItem>
    {
        public static readonly IComparer<TestUserItem> Instance = new TestUserNumberComparer();

        public int Compare(TestUserItem? x, TestUserItem? y)
        {
            return x!.Number.CompareTo(y!.Number);
        }

        public override string ToString()
        {
            return "TestUserNumberComparer";
        }
    }
}