---
title: "Union types in C# 15"
url: https://daily.dev/posts/union-types-in-c-15-00uk6sszd
source_url: https://www.damirscorner.com/blog/posts/20260821-UnionTypesInCs15.html
type: article
source: "Damir's Corner"
published: 2026-08-21T06:24:01.106Z
updated: 2026-08-21T06:24:26.264Z
tags: [".net", "c#"]
reading_time: 5
upvotes: 5
comments: 0
language: en
---

> ## Documentation Index
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# Union types in C# 15

**[Damir's Corner](https://daily.dev/sources/damirscorner)** · 5 min read · 5 upvotes · 0 comments

## Summary

C# 15 introduces native union types, letting developers combine unrelated types (e.g. public union Shape(Square, Circle)) without requiring a common base class or interface. Unlike prior workarounds - marker interfaces, abstract base classes, object typing, or the third-party OneOf library - union types allow exhaustive switch expressions without a discard pattern, and the compiler warns when a new type is added but not handled. The feature is still in preview, available via .NET 11 preview 7 by setting LangVersion to preview. A sample project comparing all five approaches is provided on GitHub.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://www.damirscorner.com/blog/posts/20260821-UnionTypesInCs15.html>

## Questions this post answers

### What are union types in C# 15 and how do they differ from using the OneOf library?

Union types are a new C# 15 language feature letting you declare a set of unrelated types, like public union Shape(Square, Circle), without a common base class. Unlike the OneOf library, switch expressions over a union type give compiler exhaustiveness warnings (CS8509) when a new type is added and not handled, without needing a discard pattern.

_daily.dev surfaces language updates like this for developers deciding how to model type unions in C#._

### How do I enable C# 15 union types in my project right now?

Union types are still a preview feature available starting with .NET 11 preview 7. To use them, set the LangVersion element to 'preview' inside a PropertyGroup in the project file; without this setting the union syntax will not compile.

_Developers testing preview language features can track setup details like this on daily.dev._

### Why does my C# switch expression give a CS8509 warning even after handling all known types?

CS8509 appears when a switch expression over a type hierarchy or union isn't provably exhaustive to the compiler. Using a discard pattern silences the warning but also hides future additions of new sibling types; C# 15 union types and closed class hierarchies fix this by making the compiler warn again when a new type is added and left unhandled.

_daily.dev helps developers troubleshooting compiler exhaustiveness warnings stay current on C# fixes._

---

Tags: [#.net](https://daily.dev/tags/.net), [#c#](https://daily.dev/tags/c#)

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