# C# 15: Extension indexers

Extension indexers on one page: add square brackets to a class you can't change, and count from the end with ^1. From the blog post \[What's new in C# 15: Extension indexers\](/blog/whats-new-in-csharp-15-extension-indexers).

![The cheat sheet C# 15: Extension indexers, with Codey: the three points of the post on one page](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/11/e36f6f8933b0441997632156ba190479.webp)

- Category: C#
- Level: Apprentice
- Tags: csharp, csharp-15, whats-new
- Updated: November 11, 2026

Source: https://www.ask-codey.com/cheat-sheets/csharp-15-extension-indexers-cheat-sheet