Add Index(...) LINQ method (#12)

Author: Tyrrrz

PolyShim.Tests/Net90/EnumerableExtensionsTests.cs

@@ -7,6 +7,19 @@ namespace PolyShim.Tests.Net90;
 
 public class EnumerableExtensionsTests
 {
+    [Fact]
+    public void Index_Test()
+    {
+        // Arrange
+        var source = new[] { 42, 13, 69, 17 };
+
+        // Act
+        var result = source.Index();
+
+        // Assert
+        result.Should().Equal((0, 42), (1, 13), (2, 69), (3, 17));
+    }
+
     [Fact]
     public void CountBy_Test()
     {

PolyShim/Net90/EnumerableExtensions.cs

@@ -11,6 +11,10 @@ namespace System.Linq;
 
 internal static partial class PolyfillExtensions
 {
+    // https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.index
+    public static IEnumerable<(int index, T value)> Index<T>(this IEnumerable<T> source) =>
+        source.Select((value, index) => (index, value));
+
     // https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.countby
     public static IEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(
         this IEnumerable<TSource> source,