ThenByDescending extension method and unit test implementation.

ShittyLINQ/ThenByDescending.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace ShittyLINQ
+{
+    public static partial class Extensions
+    {
+        public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(this IOrderedEnumerable<TSource> source, Func<TSource, TKey> keySelector)
+        {
+            if (source == null) throw new ArgumentNullException("source");
+            return source.CreateOrderedEnumerable<TKey>(keySelector, null, true);
+        }
+
+        public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(this IOrderedEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
+        {
+            if (source == null) throw new ArgumentNullException("source");
+            return source.CreateOrderedEnumerable<TKey>(keySelector, comparer, true);
+        }
+    }
+}

ShittyLinqTests/ThenByDescendingTests.cs

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+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using ShittyLINQ;
+using System;
+
+namespace ShittyTests
+{
+    [TestClass]
+    public class ThenByDescendingTests
+    {
+        [TestMethod]
+        public void ThenByDescending_WithoutComparer()
+        {
+            // Arrange
+            var items = System.Linq.Enumerable.OrderBy(new (string prop1, int prop2)[]
+            {
+                ("test", 1),
+                ("test", 2),
+                ("test", 3),
+            }, i => i.prop1);
+
+            // Act
+            var result = items.ThenByDescending(i => i.prop2);
+
+            // Assert
+            Assert.IsNotNull(result);
+            using (var enumerator = result.GetEnumerator())
+            {
+                enumerator.MoveNext();
+                Assert.AreEqual(3, enumerator.Current.prop2);
+                enumerator.MoveNext();
+                Assert.AreEqual(2, enumerator.Current.prop2);
+                enumerator.MoveNext();
+                Assert.AreEqual(1, enumerator.Current.prop2);
+            }
+        }
+
+        [TestMethod]
+        public void ThenByDescending_WithComparer()
+        {
+            // Arrange
+            var items = System.Linq.Enumerable.OrderBy(new (string prop1, string prop2)[]
+            {
+                ("test", "a"),
+                ("test", "B"),
+                ("test", "c"),
+            }, i => i.prop1);
+
+            // Act
+            var result = items.ThenByDescending(i => i.prop2, StringComparer.OrdinalIgnoreCase);
+
+            // Assert
+            Assert.IsNotNull(result);
+            using (var enumerator = result.GetEnumerator())
+            {
+                enumerator.MoveNext();
+                Assert.AreEqual("c", enumerator.Current.prop2);
+                enumerator.MoveNext();
+                Assert.AreEqual("B", enumerator.Current.prop2);
+                enumerator.MoveNext();
+                Assert.AreEqual("a", enumerator.Current.prop2);
+            }
+        }
+    }
+}