More helpers

src/Core/EnumerableExtensions.cs

@@ -49,6 +49,11 @@ public static T SingleOr<T>(this IEnumerable<T> @this, Func<T> defaultValueGener
         return value;
     }
 
+    public static T First<T>(this IReadOnlyList<T> @this) =>
+        @this[0];
+    public static T Last<T>(this IReadOnlyList<T> @this) =>
+        @this[^1];
+
     public static bool TryFirst<T>(this IEnumerable<T> @this, out T? found)
     {
         foreach (var element in @this)

src/Core/EnumerableExtensions.t.cs

@@ -1,3 +1,5 @@
+using System.Collections;
+
 class EnumerableExtensionsTests
 {
     [Test]
@@ -40,6 +42,33 @@ public void SingleOr_WithMoreThanOne_Throws()
             .Message.ShouldContain("more than one");
     }
 
+    class DirectIndexTest<T>(int count, int index, T value) : IReadOnlyList<T>
+    {
+        public IEnumerator<T> GetEnumerator() => throw new InvalidOperationException();
+        IEnumerator IEnumerable.GetEnumerator() => throw new InvalidOperationException();
+
+        public int Count => count;
+        public T this[int i] => i == index ? value : throw new InvalidOperationException();
+    }
+
+    [Test]
+    public void First_WithReadOnlyList_ShouldOnlyCallIndexer()
+    {
+        var list = new DirectIndexTest<int>(100, 0, 123);
+
+        list.First().ShouldBe(123);
+        Should.Throw<InvalidOperationException>(() => list.AsEnumerable().First());
+    }
+
+    [Test]
+    public void Last_WithReadOnlyList_ShouldOnlyCallIndexer()
+    {
+        var list = new DirectIndexTest<int>(100, 99, 234);
+
+        list.Last().ShouldBe(234);
+        Should.Throw<InvalidOperationException>(() => list.AsEnumerable().Last());
+    }
+
     [Test]
     public void TryFirst_WithNoMatch_ReturnsFalseDefault()
     {