How does ThenBy work with Comparer?

[Morgfrimen]

Good afternoon, colleagues. I have a question.

Program class:

var list = new List<SiteMapModel>()
{
    new() { Level = 0, Name = "A0", Publish = DateTime.UtcNow },
    new() { Level = 1, Name = "A1", Publish = DateTime.UtcNow.AddDays(5) },
    new() { Level = 1, Name = "A11", Publish = DateTime.UtcNow.AddDays(4) },
    new() { Level = 1, Name = "B1", Publish = DateTime.UtcNow.AddDays(6) },
    new() { Level = 1, Name = "C1", Publish = DateTime.UtcNow.AddDays(4) },
    new() { Level = 0, Name = "B0", Publish = DateTime.UtcNow.AddDays(1) },
    new() { Level = 0, Name = "C0", Publish = DateTime.Now },
    new() { Level = 2, Name = "A2", Publish = DateTime.UtcNow.AddDays(5) }
};

var comparerList = list.OrderBy(x => x.Name).ToList().OrderBy(x => x, new SiteMapComparer());

foreach (var model in comparerList)
{
    Console.WriteLine($"{new string(' ', 2 * model.Level)}{model.Name}(level:{model.Level})");
}

Model:

public sealed class SiteMapModel
{
    public string Name { get; set; }
    public int Level { get; set; }
    public DateTime Publish { get; set; }
}

IComparer object:

public sealed class SiteMapComparer : IComparer<SiteMapModel>
{
    public int Compare(SiteMapModel? x, SiteMapModel? y)
    {
        if (ReferenceEquals(x, y))
            return 0;

        if (ReferenceEquals(null, y))
            return 1;

        if (ReferenceEquals(null, x))
            return -1;

        var nameComparison = string.Compare(x.Name, y.Name, StringComparison.Ordinal);
        var dateCompare = DateTime.Compare(x.Publish, y.Publish);

        if (x.Level != y.Level)
        {
            return nameComparison;
        }

        if (x.Level == y.Level)
        {
            return dateCompare;
        }

        return default;
    }
}

If writing is not beautiful:

var comparerList = list.OrderBy(x => x.Name).ToList().OrderBy(x => x, new SiteMapComparer());

Sorting works as I want!
But

var comparerList = list.OrderBy(x => x.Name).ThenBy(x => x, new SiteMapComparer());

I don't get into Comparer and sorting doesn't work as it should!

Why is this happening?

[heathbm]

First, in your initial chained OrderBy, you can drop the ToList().

var comparerList = list.OrderBy(x => x.Name).ToList().OrderBy(x => x, new SiteMapComparer());
to
var comparerList = list.OrderBy(x => x.Name).OrderBy(x => x, new SiteMapComparer());

To answer the ThenBy question:

Essentially, ThenBy does not perform a complete OrderBy again, otherwise, it would just duplicate OrderBy().
OrderBy will order the list by a key (x.Name in this case).
ThenBy allows you to apply subsequent ordering on the items belonging to the same key.
Since your provided list has no duplicate Name keys, the SiteMapComparer will never be called as there is only 1 item to sort, so it is skipped.

[Clockwork-Muse]

First, in your initial chained OrderBy, you can drop the ToList().

Officially, per the documentation:

Because IOrderedEnumerable inherits from IEnumerable, you can call OrderBy or OrderByDescending on the results of a call to OrderBy, OrderByDescending, ThenBy or ThenByDescending. Doing this introduces a new primary ordering that ignores the previously established ordering.

... this means that the two legally may have distinct behaviors, because by capturing the list the collection is guaranteed to be ordered by the first key, while if it is dropped the results of the first OrderBy call could be ignored completely.
(It seems to produce the expected results currently, although I'm not sure whether this is intended or merely an implementation detail)