NaturalStringComparer can't be passed where a non-generic IComparer is required (ArrayList.Sort, WPF CollectionView.CustomSort, WinForms ListView sorting) #74

What's wrong

NaturalStringComparer (Sorting/NaturalStringComparer.cs:16) implements only IComparer<string?>. Several common .NET sorting APIs accept only the non-generic System.Collections.IComparer, and sorting file names naturally is a typical reason to use them:

• WPF: ListCollectionView.CustomSort, the usual way to sort a bound ListBox or DataGrid with custom logic
• WinForms: ListView.ListViewItemSorter and DataGridView.Sort(IComparer)
• ArrayList.Sort, Array.Sort(Array, IComparer) and the non-generic SortedList

None of these accept the comparer today, so every caller has to write their own adapter class.

Repro (main @ f7c0884, net10.0)

using System.Collections;
using ktsu.Sorting;

var list = new ArrayList { "file10.txt", "file2.txt", "file1.txt" };
list.Sort(new NaturalStringComparer());

• Observed: the code doesn't compile: error CS1503: Argument 1: cannot convert from 'ktsu.Sorting.NaturalStringComparer' to 'System.Collections.IComparer?'. Also, new NaturalStringComparer() is IComparer evaluates to False.
• Expected: it compiles and sorts the list to file1.txt, file2.txt, file10.txt.

Suggested fix

Implement System.Collections.IComparer alongside the generic interface. Either:

• add an explicit int IComparer.Compare(object? x, object? y) that forwards to Compare(string?, string?), or
• derive from Comparer<string?>, which provides both interfaces.

Adding an interface keeps source and binary compatibility. Re-run package validation either way, because this overlaps with #72 and #73.

Acceptance criteria

• new NaturalStringComparer() can be passed anywhere a System.Collections.IComparer is expected.
• Through that interface it orders strings exactly as the generic Compare does, including null ordering and the ordinal tie-break from [patch] Break natural-order ties ordinally so distinct strings never compare equal #65.
• Arguments that aren't strings throw ArgumentException, as Comparer<T> does. Two nulls compare equal, and a null sorts before any string.
• New tests cover ArrayList.Sort (or Array.Sort(object[], IComparer)) and the wrong-type case.
• The generic Compare still allocates nothing: Compare_AllocatesNothing still passes.