MinBy/MaxBy Null Handling

src/Polyfill/Polyfill_IEnumerable_Max.cs

@@ -26,11 +26,10 @@ static partial class Polyfill
     /// </remarks>
     [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.max?view=net-8.0#system-linq-enumerable-max-1(system-collections-generic-ienumerable((-0))-system-collections-generic-icomparer((-0)))")]
     public static TSource? Max<TSource>(
-        this IEnumerable<TSource> target,
+        this IEnumerable<TSource> source,
         IComparer<TSource>? comparer) =>
-        target
-            .OrderByDescending(_ => _, comparer)
-            .FirstOrDefault();
+        source
+            .MaxBy(_ => _, comparer);
 
 #endif
 }

src/Polyfill/Polyfill_IEnumerable_MaxBy.cs

@@ -15,7 +15,7 @@ static partial class Polyfill
     /// <summary>
     /// Returns the maximum value in a generic sequence according to a specified key selector function.
     /// </summary>
-    /// <typeparam name="TSource">The type of the elements of <paramref name="target" />.</typeparam>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
     /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
     /// <param name="source">A sequence of values to determine the maximum value of.</param>
     /// <param name="keySelector">A function to extract the key for each element.</param>
@@ -27,12 +27,12 @@ static partial class Polyfill
     /// </remarks>
     [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.maxby#system-linq-enumerable-maxby-2(system-collections-generic-ienumerable((-0))-system-func((-0-1)))")]
     public static TSource? MaxBy<TSource, TKey>(
-        this IEnumerable<TSource> target,
+        this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector) =>
-        MaxBy(target, keySelector, null);
+        MaxBy(source, keySelector, null);
 
     /// <summary>Returns the maximum value in a generic sequence according to a specified key selector function.</summary>
-    /// <typeparam name="TSource">The type of the elements of <paramref name="target" />.</typeparam>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
     /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
     /// <param name="source">A sequence of values to determine the maximum value of.</param>
     /// <param name="keySelector">A function to extract the key for each element.</param>
@@ -45,12 +45,81 @@ static partial class Polyfill
     /// </remarks>
     [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.maxby#system-linq-enumerable-maxby-2(system-collections-generic-ienumerable((-0))-system-func((-0-1))-system-collections-generic-icomparer((-1)))")]
     public static TSource? MaxBy<TSource, TKey>(
-        this IEnumerable<TSource> target,
+        this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector,
-        IComparer<TKey>? comparer) =>
-        target
-            .OrderByDescending(keySelector, comparer)
-            .FirstOrDefault();
+        IComparer<TKey>? comparer)
+    {
+        // Simplified from https://github.com/dotnet/runtime/blob/5d09a8f94c72ca4ef0a9c79eb9c58d06198e3ba9/src/libraries/System.Linq/src/System/Linq/Max.cs#L445-L526
+        if (source is null) throw new ArgumentNullException(nameof(source));
+        if (keySelector is null) throw new ArgumentNullException(nameof(keySelector));
+
+        comparer ??= Comparer<TKey>.Default;
+
+        using IEnumerator<TSource> e = source.GetEnumerator();
+
+        if (!e.MoveNext())
+        {
+            if (default(TSource) is null)
+            {
+                return default;
+            }
+            else
+            {
+                ThrowHelper.ThrowNoElementsException();
+            }
+        }
+
+        TSource value = e.Current;
+        TKey key = keySelector(value);
+
+        if (default(TKey) is null)
+        {
+            if (key is null)
+            {
+                TSource firstValue = value;
+
+                do
+                {
+                    if (!e.MoveNext())
+                    {
+                        // All keys are null, surface the first element.
+                        return firstValue;
+                    }
+
+                    value = e.Current;
+                    key = keySelector(value);
+                }
+                while (key is null);
+            }
+
+            while (e.MoveNext())
+            {
+                TSource nextValue = e.Current;
+                TKey nextKey = keySelector(nextValue);
+                if (nextKey is not null && comparer.Compare(nextKey, key) > 0)
+                {
+                    key = nextKey;
+                    value = nextValue;
+                }
+            }
+        }
+        else
+        {
+            while (e.MoveNext())
+            {
+                TSource nextValue = e.Current;
+                TKey nextKey = keySelector(nextValue);
+                if (comparer.Compare(nextKey, key) > 0)
+                {
+                    key = nextKey;
+                    value = nextValue;
+                }
+            }
+        }
+
+        return value;
+
+    }
 
 #endif
 }

src/Polyfill/Polyfill_IEnumerable_Min.cs

@@ -27,11 +27,10 @@ static partial class Polyfill
     /// </remarks>
     [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.min?view=net-8.0#system-linq-enumerable-min-1(system-collections-generic-ienumerable((-0))-system-collections-generic-icomparer((-0)))")]
     public static TSource? Min<TSource>(
-        this IEnumerable<TSource> target,
+        this IEnumerable<TSource> source,
         IComparer<TSource>? comparer) =>
-        target
-            .OrderBy(_ => _, comparer)
-            .FirstOrDefault();
+        source
+            .MinBy(_ => _, comparer);
 
 #endif
 

src/Polyfill/Polyfill_IEnumerable_MinBy.cs

@@ -16,7 +16,7 @@ static partial class Polyfill
     /// <summary>
     /// Returns the minimum value in a generic sequence according to a specified key selector function.
     /// </summary>
-    /// <typeparam name="TSource">The type of the elements of <paramref name="target" />.</typeparam>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
     /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
     /// <param name="source">A sequence of values to determine the minby value of.</param>
     /// <param name="keySelector">A function to extract the key for each element.</param>
@@ -28,30 +28,98 @@ static partial class Polyfill
     /// </remarks>
     [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.minby#system-linq-enumerable-minby-2(system-collections-generic-ienumerable((-0))-system-func((-0-1)))")]
     public static TSource? MinBy<TSource, TKey>(
-        this IEnumerable<TSource> target,
+        this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector) =>
-        MinBy(target, keySelector, null);
+        MinBy(source, keySelector, null);
 
     /// <summary>Returns the minimum value in a generic sequence according to a specified key selector function.</summary>
-    /// <typeparam name="TSource">The type of the elements of <paramref name="target" />.</typeparam>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
     /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
     /// <param name="source">A sequence of values to determine the minimum value of.</param>
     /// <param name="keySelector">A function to extract the key for each element.</param>
     /// <param name="comparer">The <see cref="IComparer{TKey}" /> to compare keys.</param>
     /// <returns>The value with the minimum key in the sequence.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
-    /// <exception cref="ArgumentException">No key extracted from <paramref name="target" /> implements the <see cref="IComparable" /> or <see cref="IComparable{TKey}" /> interface.</exception>
+    /// <exception cref="ArgumentException">No key extracted from <paramref name="source" /> implements the <see cref="IComparable" /> or <see cref="IComparable{TKey}" /> interface.</exception>
     /// <remarks>
     /// <para>If <typeparamref name="TKey" /> is a reference type and the source sequence is empty or contains only values that are <see langword="null" />, this method returns <see langword="null" />.</para>
     /// </remarks>
     [Link("https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.minby#system-linq-enumerable-minby-2(system-collections-generic-ienumerable((-0))-system-func((-0-1))-system-collections-generic-icomparer((-1)))")]
     public static TSource? MinBy<TSource, TKey>(
-        this IEnumerable<TSource> target,
+        this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector,
-        IComparer<TKey>? comparer) =>
-        target
-            .OrderBy(keySelector, comparer)
-            .FirstOrDefault();
+        IComparer<TKey>? comparer)
+    {
+        // Simplified from https://github.com/dotnet/runtime/blob/5d09a8f94c72ca4ef0a9c79eb9c58d06198e3ba9/src/libraries/System.Linq/src/System/Linq/Min.cs#L413-L503
+        if (source is null) throw new ArgumentNullException(nameof(source));
+        if (keySelector is null) throw new ArgumentNullException(nameof(keySelector));
+
+        comparer ??= Comparer<TKey>.Default;
+
+        using IEnumerator<TSource> e = source.GetEnumerator();
+
+        if (!e.MoveNext())
+        {
+            if (default(TSource) is null)
+            {
+                return default;
+            }
+            else
+            {
+                ThrowHelper.ThrowNoElementsException();
+            }
+        }
+
+        TSource value = e.Current;
+        TKey key = keySelector(value);
+
+        if (default(TKey) is null)
+        {
+            if (key is null)
+            {
+                TSource firstValue = value;
+
+                do
+                {
+                    if (!e.MoveNext())
+                    {
+                        // All keys are null, surface the first element.
+                        return firstValue;
+                    }
+
+                    value = e.Current;
+                    key = keySelector(value);
+                }
+                while (key is null);
+            }
+
+            while (e.MoveNext())
+            {
+                TSource nextValue = e.Current;
+                TKey nextKey = keySelector(nextValue);
+                if (nextKey is not null && comparer.Compare(nextKey, key) < 0)
+                {
+                    key = nextKey;
+                    value = nextValue;
+                }
+            }
+        }
+        else
+        {
+            while (e.MoveNext())
+            {
+                TSource nextValue = e.Current;
+                TKey nextKey = keySelector(nextValue);
+                if (comparer.Compare(nextKey, key) < 0)
+                {
+                    key = nextKey;
+                    value = nextValue;
+                }
+            }
+        }
+
+        return value;
+    }
 
 #endif
 

src/Polyfill/Polyfill_IEnumerable_ThrowHelper.cs

@@ -0,0 +1,13 @@
+// <auto-generated />
+#pragma warning disable
+
+namespace Polyfills;
+
+static partial class Polyfill
+{
+    private static class ThrowHelper
+    {
+        public static void ThrowNoElementsException() =>
+            throw new System.InvalidOperationException("Sequence contains no elements");
+    }
+}

src/Tests/PolyfillTests_IEnumerable.cs

@@ -1,52 +1,6 @@
 // ReSharper disable ReturnValueOfPureMethodIsNotUsed
 partial class PolyfillTests
 {
-    [Test]
-    public void MaxBy()
-    {
-        IEnumerable<int> enumerable = [1, 2];
-
-        Assert.AreEqual(2, enumerable.MaxBy(_ => _));
-    }
-
-    [Test]
-    public void MaxComparer()
-    {
-        IEnumerable<int> enumerable = [1, 2];
-
-        Assert.AreEqual(1, enumerable.Max(new ReverseComparer()));
-    }
-
-    [Test]
-    public void MaxByComparer()
-    {
-        IEnumerable<int> enumerable = [1, 2];
-
-        Assert.AreEqual(1, enumerable.MaxBy(_ => _, new ReverseComparer()));
-    }
-
-    [Test]
-    public void MinComparer()
-    {
-        IEnumerable<int> enumerable = [1, 2];
-
-        Assert.AreEqual(2, enumerable.Min(new ReverseComparer()));
-    }
-
-    [Test]
-    public void MinByComparer()
-    {
-        IEnumerable<int> enumerable = [1, 2];
-
-        Assert.AreEqual(2, enumerable.MinBy(_ => _, new ReverseComparer()));
-    }
-
-    class ReverseComparer : IComparer<int>
-    {
-        public int Compare(int x, int y) =>
-            y.CompareTo(x);
-    }
-
     [Test]
     public void TakeRange()
     {
@@ -86,14 +40,6 @@ public void Except()
         Assert.AreEqual(1, enumerable.Except(2).Single());
     }
 
-    [Test]
-    public void MinBy()
-    {
-        IEnumerable<int> enumerable = new List<int> {1, 2};
-
-        Assert.AreEqual(1, enumerable.MinBy(_ => _));
-    }
-
     [Test]
     public void TryGetNonEnumeratedCount()
     {