Implement the non-generic IComparer on NaturalStringComparer [minor]

CLAUDE.md

@@ -4,7 +4,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 ## Project Overview
 
-ktsu.Sorting is a .NET library providing advanced sorting utilities for specialized sorting scenarios. The primary component is `NaturalStringComparer`, which implements `IComparer<string?>` to sort strings containing embedded numbers in a human-friendly order (e.g., "file2" before "file10").
+ktsu.Sorting is a .NET library providing advanced sorting utilities for specialized sorting scenarios. The primary component is `NaturalStringComparer`, which implements `IComparer<string?>` (and the non-generic `IComparer`) to sort strings containing embedded numbers in a human-friendly order (e.g., "file2" before "file10").
 
 ## Build Commands
 

README.md

@@ -81,6 +81,7 @@ var sortedFiles = files.OrderBy(file => file, new NaturalStringComparer());
 - Handles mixed alphanumeric strings
 - Properly compares strings with leading zeros
 - Fully compatible with .NET's `IComparer<string>` interface
+- Also implements the non-generic `System.Collections.IComparer`, for APIs that accept only that: `ArrayList.Sort`, WPF's `ListCollectionView.CustomSort`, WinForms' `ListView.ListViewItemSorter`. Arguments that aren't strings throw `ArgumentException`
 - Can be used with `Array.Sort()`, LINQ's `OrderBy()`, and other sorting mechanisms
 
 ## Examples

Sorting.Test/NaturalStringComparerTests.cs

@@ -4,6 +4,7 @@
 
 namespace ktsu.Sorting.Tests;
 
+using System.Collections;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
 [TestClass]
@@ -438,4 +439,56 @@
 
 		Assert.AreEqual(0L, allocated, $"{allocated} bytes allocated over {pairs.Length * 2000} comparisons");
 	}
+
+	[TestMethod]
+	public void NonGenericComparer_SortsAnArrayListNaturally()
+	{
+		ArrayList list = ["file10.txt", "file2.txt", null, "file1.txt"];
+
+		list.Sort(NonGeneric());
+
+		CollectionAssert.AreEqual(new object?[] { null, "file1.txt", "file2.txt", "file10.txt" }, list);
+	}
+
+	[TestMethod]
+	public void NonGenericComparer_OrdersLikeTheGenericCompare()
+	{
+		IComparer nonGeneric = NonGeneric();
+		string?[] values = [null, "", "file5", "file005", "file10", "file\u0665", "a5c", "5a", "File2"];
+		foreach (string? x in values)
+		{
+			foreach (string? y in values)
+			{
+				Assert.AreEqual(Math.Sign(_comparer.Compare(x, y)), Math.Sign(nonGeneric.Compare(x, y)), $"\"{x}\" vs \"{y}\"");
+			}
+		}
+	}
+
+	[TestMethod]
+	public void NonGenericComparer_SortsAnObjectArrayNaturally()
+	{
+		object[] items = ["img12", "img007", "img7", "img1"];
+
+		Array.Sort(items, NonGeneric());
+
+		CollectionAssert.AreEqual(new object[] { "img1", "img007", "img7", "img12" }, items);
+	}
+
+	[TestMethod]
+	public void NonGenericComparer_ThrowsForArgumentsThatAreNotStrings()
+	{
+		IComparer nonGeneric = NonGeneric();
+
+		Assert.ThrowsExactly<ArgumentException>(() => nonGeneric.Compare(5, "file"));
+		Assert.ThrowsExactly<ArgumentException>(() => nonGeneric.Compare("file", 5));
+	}
+
+	private IComparer NonGeneric()
+	{
+		// Checked at run time, so the tests build, and fail, against a comparer without the interface.
+		object comparer = _comparer;
+		IComparer? nonGeneric = comparer as IComparer;
+		Assert.IsNotNull(nonGeneric, "NaturalStringComparer doesn't implement System.Collections.IComparer");
+		return nonGeneric;
+	}
 }

Sorting/NaturalStringComparer.cs

@@ -2,6 +2,7 @@
 
 namespace ktsu.Sorting;
 
+using System.Collections;
 using System.Globalization;
 
 /// <summary>
@@ -12,8 +13,10 @@
 /// in and whether or not it lies outside the Basic Multilingual Plane, and is compared by the value
 /// it spells rather than by its code points. A number compared with text orders exactly as the
 /// equivalent ASCII digits would, so every digit script sorts in the same place relative to text.
+/// The comparer also implements the non-generic <see cref="IComparer"/>, so it can be handed to APIs
+/// that accept only that, such as <see cref="ArrayList.Sort(IComparer)"/>.
 /// </remarks>
-public partial class NaturalStringComparer : IComparer<string?>
+public partial class NaturalStringComparer : IComparer<string?>, IComparer
 {
 	/// <summary>
 	/// Compares two strings using natural sorting, where embedded numbers are compared as numeric values.
@@ -26,7 +29,7 @@
 	/// digit script are ordered next to each other, ordinally.
 	/// Greater than zero if x is greater than y.
 	/// </returns>
 	public int Compare(string? x, string? y)
 	{
 		if (x == null && y == null)
 		{
@@ -103,6 +106,31 @@
 		return string.CompareOrdinal(x, y);
 	}
 
+	/// <summary>
+	/// Compares two objects that are strings or <see langword="null"/>, exactly as
+	/// <see cref="Compare(string, string)"/> does.
+	/// </summary>
+	/// <param name="x">First object to compare.</param>
+	/// <param name="y">Second object to compare.</param>
+	/// <returns>The result of <see cref="Compare(string, string)"/> on the two strings.</returns>
+	/// <exception cref="ArgumentException">
+	/// An argument is neither a string nor <see langword="null"/>.
+	/// </exception>
+	int IComparer.Compare(object? x, object? y)
+	{
+		if (x is not (null or string))
+		{
+			throw new ArgumentException($"{nameof(NaturalStringComparer)} compares only strings, not {x.GetType()}.", nameof(x));
+		}
+
+		if (y is not (null or string))
+		{
+			throw new ArgumentException($"{nameof(NaturalStringComparer)} compares only strings, not {y.GetType()}.", nameof(y));
+		}
+
+		return Compare((string?)x, (string?)y);
+	}
+
 	/// <summary>
 	/// Reports whether the code point at <paramref name="index"/> is a Unicode decimal digit
 	/// (category <c>Nd</c>), reading a surrogate pair as the one code point it encodes.