Accept Nullable element types in the ordered containers without a comparer

Containers.Test/OrderedCollectionTests.cs

@@ -595,6 +595,22 @@ public void WorksWithStrings_MaintainsAlphabeticalOrder()
 		Assert.AreEqual("zebra", collection[3]);
 	}
 
+	[TestMethod]
+	public void Constructor_WithNullableType_WithoutComparer_SortsNullFirst()
+	{
+		OrderedCollection<int?> collection = [3, null, 1];
+		OrderedCollection<int?> withCapacity = new(10) { 2, null };
+		OrderedCollection<int?> fromCollection = new([2, null, 2]);
+
+		Assert.AreSequenceEqual([null, 1, 3], collection);
+		Assert.AreSequenceEqual([null, 2], withCapacity);
+		Assert.AreSequenceEqual([null, 2, 2], fromCollection);
+	}
+
+	[TestMethod]
+	public void Constructor_WithNullableOfNonComparableType_WithoutComparer_ThrowsArgumentException() =>
+		Assert.ThrowsExactly<ArgumentException>(() => new OrderedCollection<KeyValuePair<int, int>?>());
+
 	[TestMethod]
 	public void Constructor_WithNonComparableType_WithoutComparer_ThrowsArgumentException()
 	{

Containers.Test/OrderedMapTests.cs

@@ -420,6 +420,24 @@ public void Values_IsReadOnly()
 	/// <summary>
 	/// Tests error handling for constructor with non-comparable type.
 	/// </summary>
+	// OrderedMap constrains TKey to notnull, as Dictionary does, so a nullable key type draws a nullability
+	// warning at compile time. It must still construct and order keys at run time.
+#pragma warning disable CS8714 // Nullability of type argument doesn't match 'notnull' constraint
+	[TestMethod]
+	public void Constructor_NullableKeyType_SortsKeys()
+	{
+		OrderedMap<int?, string> map = new() { [3] = "three", [1] = "one" };
+		OrderedMap<int?, string> withCapacity = new(10) { [5] = "five", [2] = "two" };
+
+		Assert.AreSequenceEqual([1, 3], map.Keys);
+		Assert.AreSequenceEqual([2, 5], withCapacity.Keys);
+	}
+
+	[TestMethod]
+	public void Constructor_NullableOfNonComparableKeyType_ThrowsArgumentException() =>
+		Assert.ThrowsExactly<ArgumentException>(() => new OrderedMap<KeyValuePair<int, int>?, string>());
+#pragma warning restore CS8714
+
 	[TestMethod]
 	public void Constructor_NonComparableType_ThrowsArgumentException()
 	{

Containers.Test/OrderedSetTests.cs

@@ -62,6 +62,22 @@ public void Constructor_WithCollectionAndComparer_CreatesSetFromCollectionWithCo
 		Assert.AreSequenceEqual(expected, set);
 	}
 
+	[TestMethod]
+	public void Constructor_NullableTypeWithoutComparer_SortsNullFirst()
+	{
+		OrderedSet<int?> set = [3, null, 1, null];
+		OrderedSet<int?> withCapacity = new(10) { 2, null };
+		OrderedSet<int?> fromCollection = new([2, null, 2]);
+
+		Assert.AreSequenceEqual([null, 1, 3], set);
+		Assert.AreSequenceEqual([null, 2], withCapacity);
+		Assert.AreSequenceEqual([null, 2], fromCollection);
+	}
+
+	[TestMethod]
+	public void Constructor_NullableOfNonComparableTypeWithoutComparer_ThrowsArgumentException() =>
+		Assert.ThrowsExactly<ArgumentException>(() => _ = new OrderedSet<KeyValuePair<int, int>?>());
+
 	[TestMethod]
 	public void Constructor_NonComparableTypeWithoutComparer_ThrowsArgumentException()
 	{

Containers/Comparability.cs

@@ -0,0 +1,31 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Containers;
+
+/// <summary>
+/// Checks whether a type can be ordered by <see cref="Comparer{T}.Default"/> when no comparer is supplied.
+/// </summary>
+internal static class Comparability
+{
+	/// <summary>
+	/// Determines whether <typeparamref name="T"/> has a default ordering.
+	/// </summary>
+	/// <remarks>
+	/// A <see cref="Nullable{T}"/> implements neither comparison interface itself, but <see cref="Comparer{T}.Default"/>
+	/// orders it through its underlying type, with null first, so the underlying type is the one checked.
+	/// </remarks>
+	/// <typeparam name="T">The type to check.</typeparam>
+	/// <returns><see langword="true"/> if <typeparamref name="T"/> or its underlying type implements <see cref="IComparable{T}"/> or <see cref="IComparable"/>.</returns>
+	internal static bool HasDefaultOrdering<T>()
+	{
+		if (typeof(IComparable<T>).IsAssignableFrom(typeof(T)) || typeof(IComparable).IsAssignableFrom(typeof(T)))
+		{
+			return true;
+		}
+
+		Type? underlyingType = Nullable.GetUnderlyingType(typeof(T));
+		return underlyingType is not null
+			&& (typeof(IComparable<>).MakeGenericType(underlyingType).IsAssignableFrom(underlyingType)
+				|| typeof(IComparable).IsAssignableFrom(underlyingType));
+	}
+}

Containers/OrderedCollection.cs

@@ -79,10 +79,7 @@ public T this[int index]
 	/// <exception cref="ArgumentException">Thrown when T does not implement IComparable{T}.</exception>
 	public OrderedCollection()
 	{
-		if (
-			!typeof(IComparable<T>).IsAssignableFrom(typeof(T))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(T))
-		)
+		if (!Comparability.HasDefaultOrdering<T>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(T)} must implement IComparable<T> or IComparable when no comparer is provided."
@@ -116,10 +113,7 @@ public OrderedCollection(int capacity)
 	{
 		ArgumentOutOfRangeException.ThrowIfNegative(capacity);
 
-		if (
-			!typeof(IComparable<T>).IsAssignableFrom(typeof(T))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(T))
-		)
+		if (!Comparability.HasDefaultOrdering<T>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(T)} must implement IComparable<T> or IComparable when no comparer is provided."
@@ -156,10 +150,7 @@ public OrderedCollection(IEnumerable<T> collection)
 	{
 		Ensure.NotNull(collection);
 
-		if (
-			!typeof(IComparable<T>).IsAssignableFrom(typeof(T))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(T))
-		)
+		if (!Comparability.HasDefaultOrdering<T>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(T)} must implement IComparable<T> or IComparable when no comparer is provided."

Containers/OrderedMap.cs

@@ -53,8 +53,7 @@ private struct Entry(TKey key, TValue value)
 	/// </summary>
 	private readonly List<Entry> items =
 		comparer is null
-		&& !typeof(IComparable<TKey>).IsAssignableFrom(typeof(TKey))
-		&& !typeof(IComparable).IsAssignableFrom(typeof(TKey))
+		&& !Comparability.HasDefaultOrdering<TKey>()
 			? throw new ArgumentException(
 				$"Type {typeof(TKey)} must implement IComparable<TKey> or IComparable when no comparer is provided."
 			)
@@ -145,10 +144,7 @@ public OrderedMap(int capacity)
 	{
 		ArgumentOutOfRangeException.ThrowIfNegative(capacity);
 
-		if (
-			!typeof(IComparable<TKey>).IsAssignableFrom(typeof(TKey))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(TKey))
-		)
+		if (!Comparability.HasDefaultOrdering<TKey>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(TKey)} must implement IComparable<TKey> or IComparable when no comparer is provided."
@@ -187,10 +183,7 @@ public OrderedMap(IDictionary<TKey, TValue> dictionary)
 	{
 		Ensure.NotNull(dictionary);
 
-		if (
-			!typeof(IComparable<TKey>).IsAssignableFrom(typeof(TKey))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(TKey))
-		)
+		if (!Comparability.HasDefaultOrdering<TKey>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(TKey)} must implement IComparable<TKey> or IComparable when no comparer is provided."

Containers/OrderedSet.cs

@@ -69,10 +69,7 @@ public class OrderedSet<T> : ISet<T>
 	/// <exception cref="ArgumentException">Thrown when T does not implement IComparable{T}.</exception>
 	public OrderedSet()
 	{
-		if (
-			!typeof(IComparable<T>).IsAssignableFrom(typeof(T))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(T))
-		)
+		if (!Comparability.HasDefaultOrdering<T>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(T)} must implement IComparable<T> or IComparable when no comparer is provided."
@@ -106,10 +103,7 @@ public OrderedSet(int capacity)
 	{
 		ArgumentOutOfRangeException.ThrowIfNegative(capacity);
 
-		if (
-			!typeof(IComparable<T>).IsAssignableFrom(typeof(T))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(T))
-		)
+		if (!Comparability.HasDefaultOrdering<T>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(T)} must implement IComparable<T> or IComparable when no comparer is provided."
@@ -146,10 +140,7 @@ public OrderedSet(IEnumerable<T> collection)
 	{
 		Ensure.NotNull(collection);
 
-		if (
-			!typeof(IComparable<T>).IsAssignableFrom(typeof(T))
-			&& !typeof(IComparable).IsAssignableFrom(typeof(T))
-		)
+		if (!Comparability.HasDefaultOrdering<T>())
 		{
 			throw new ArgumentException(
 				$"Type {typeof(T)} must implement IComparable<T> or IComparable when no comparer is provided."