multi-target net8 for LTS

Author: myssto

OpenSkillSharp/OpenSkillSharp.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>net9.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
         <Title>OpenSkillSharp</Title>
@@ -14,6 +13,7 @@
         <RepositoryType>git</RepositoryType>
         <PackageTags>ranking, rating, elo, trueskill, openskill, bayesian</PackageTags>
         <Copyright>Copyright (c) 2025 Patrick Lacni</Copyright>
+        <TargetFrameworks>net9.0;net8.0</TargetFrameworks>
     </PropertyGroup>
 
     <ItemGroup>

OpenSkillSharp/Util/Polyfills/EnumerablePolyfills.cs

@@ -0,0 +1,25 @@
+#if !NET9_0_OR_GREATER
+// ReSharper disable once CheckNamespace
+namespace System.Linq;
+
+internal static class EnumerableIndexPolyfill
+{
+    /// <summary>Returns an enumerable that incorporates the element's index into a tuple.</summary>
+    /// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
+    /// <param name="source">The source enumerable providing the elements.</param>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
+    public static IEnumerable<(int Index, TSource Item)> Index<TSource>(this IEnumerable<TSource> source)
+    {
+        ArgumentNullException.ThrowIfNull(source);
+
+        int index = -1;
+
+        foreach (TSource element in source)
+        {
+            index++;
+            yield return (index, element);
+        }
+    }
+}
+
+#endif