From 041181cc5a4432bb0e54549a972031fc596c2ec6 Mon Sep 17 00:00:00 2001
From: Oliver Booth <me@olivr.me>
Date: Mon, 27 Feb 2023 21:11:37 +0000
Subject: [PATCH] Add IEnumerable<T?>.WhereNotNull()

---
 CHANGELOG.md                                  |  1 +
 X10D.Tests/src/Collections/EnumerableTests.cs | 21 +++++++++++++++
 X10D/src/Collections/EnumerableExtensions.cs  | 26 ++++++++++++++++++-
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6291042..5fcd310 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@
 - X10D: Added `IEnumerable<T>.LastWhereNot(Func<T, bool>)`
 - X10D: Added `IEnumerable<T>.LastWhereNotOrDefault(Func<T, bool>)`
 - X10D: Added `IEnumerable<T>.WhereNot(Func<T, bool>)`
+- X10D: Added `IEnumerable<T>.WhereNotNull()`
 - X10D: Added `IList<T>.RemoveRange(Range)`
 - X10D: Added `IList<T>.Swap(IList<T>)` (#62)
 - X10D: Added `IReadOnlyList<T>.IndexOf(T[, int[, int]])`
diff --git a/X10D.Tests/src/Collections/EnumerableTests.cs b/X10D.Tests/src/Collections/EnumerableTests.cs
index 1631040..c223c5b 100644
--- a/X10D.Tests/src/Collections/EnumerableTests.cs
+++ b/X10D.Tests/src/Collections/EnumerableTests.cs
@@ -287,6 +287,27 @@ public class EnumerableTests
         Assert.ThrowsException<ArgumentNullException>(() => ((IEnumerable<int>?)null)!.WhereNot(x => x % 2 == 0));
     }
 
+    [TestMethod]
+    public void WhereNotNull_ShouldContainNoNullElements()
+    {
+        object?[] array = Enumerable.Repeat(new object(), 10).ToArray();
+        array[1] = null;
+        array[2] = null;
+        array[8] = null;
+        array[9] = null;
+
+        const int expectedCount = 6;
+        var actualCount = 0;
+
+        foreach (object o in array.WhereNotNull())
+        {
+            Assert.IsNotNull(o);
+            actualCount++;
+        }
+
+        Assert.AreEqual(expectedCount, actualCount);
+    }
+
     private class DummyClass
     {
         public int Value { get; set; }
diff --git a/X10D/src/Collections/EnumerableExtensions.cs b/X10D/src/Collections/EnumerableExtensions.cs
index 93e1b67..4a2580a 100644
--- a/X10D/src/Collections/EnumerableExtensions.cs
+++ b/X10D/src/Collections/EnumerableExtensions.cs
@@ -1,4 +1,4 @@
-using System.Diagnostics.Contracts;
+using System.Diagnostics.Contracts;
 
 namespace X10D.Collections;
 
@@ -352,4 +352,28 @@ public static class EnumerableExtensions
 
         return source.Where(item => !predicate(item));
     }
+
+    /// <summary>
+    ///     Filters a sequence of values by omitting elements that are <see langword="null" /> (<see langword="Nothing" /> in
+    ///     Visual Basic).
+    /// </summary>
+    /// <param name="source">An <see cref="IEnumerable{T}" /> to filter.</param>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     An <see cref="IEnumerable{T}" /> that contains elements from the input sequence that are not <see langword="null" />
+    ///     (<see langword="Nothing" /> in Visual Basic).
+    /// </returns>
+    public static IEnumerable<TSource> WhereNotNull<TSource>(this IEnumerable<TSource?> source)
+    {
+#if NET6_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(source);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+#endif
+
+        return source.Where(item => item is not null).Select(item => item!);
+    }
 }