# C# 15: Closed hierarchies

Closed hierarchies on one page: mark a family closed, switch with no default arm, and when to choose a union instead. From the blog post \[What's new in C# 15: Closed hierarchies\](/blog/whats-new-in-csharp-15-closed-hierarchies).

![The cheat sheet C# 15: Closed hierarchies, with Codey: the three points of the post on one page](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/11/d0dd44d3a007490fb08cae54889bf637.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-closed-hierarchies-cheat-sheet