.NET 11 Reaches Release Candidate 1 as C# 15 Stabilizes Union Types and Closed Hierarchies
Microsoft's first .NET 11 release candidate ships with a go-live support license and locks in C# 15's union types, closed hierarchies, and four other preview-era language features as stable defaults.
Overview
Microsoft shipped .NET 11 Release Candidate 1 on September 8. “This is our first release candidate, which comes with a go-live support license so you can confidently use this release for your production applications,” according to the .NET Blog. The build makes C# 15 the default language version for .NET 11 projects and, per dotnet/core’s official release notes, stabilizes several language features that had spent the preview cycle behind an opt-in flag.
What We Know
According to the dotnet/core release notes, “C# 15 is now selected by default for projects targeting .NET 11,” and “The language version stabilizes the features introduced during the .NET 11 preview cycle: collection expression arguments, union types, non-virtual static interface members, closed class hierarchies, labeled break and continue, and extension indexers.” Concretely, the notes say “union types no longer require <LangVersion>preview</LangVersion>,” the compiler and Roslyn APIs “use the final CSharp15 language-version names,” and a new ITypeSymbol.UnionCaseTypes API now exposes a union’s cases to analyzers and other compiler tooling.
Union types let a developer declare that a value is exactly one of a fixed set of “case” types. As Microsoft’s C# 15 documentation describes it, a declaration such as public union Pet(Cat, Dog, Bird); creates a union whose cases the compiler tracks, and “the compiler ensures switch expressions are exhaustive across all case types.” The Machine Herald previously covered union types’ first in-SDK appearance in .NET 11 Preview 3 and their initial runtime scaffolding in Preview 5; RC1 is the point at which the feature exits preview status entirely.
Closed hierarchies, the other headline addition, restrict a class’s direct subclasses to its own assembly. Per Microsoft’s documentation, “A closed class can only be derived from within its declaring assembly, which fixes the set of direct descendants at compile time,” so a switch expression handling every direct descendant “is exhaustive and doesn’t need a default arm.”
Not every C# 15 effort stabilized in this build. The separate “Unsafe Evolution” memory-safety work remains a preview feature — the release notes flag it with “This is a preview feature for .NET 11” and say it “remains independent of C# 15 and still requires C# language preview and the feature flag for the new memory safety rules.” RC1 does add refinements to it: await is now allowed inside an unsafe context, the safe contextual keyword “can be applied anywhere that unsafe could mark a declaration as requires-unsafe,” supporting generated LibraryImport methods, and unsafe on delegates, static constructors, destructors, and type declarations “is now an error because it no longer establishes a meaningful unsafe context,” according to the release notes. The notes add that “Unsafe Evolution diagnostics are now reported consistently in the IDE.”
RC1 is supported in Visual Studio 2026 Insiders and Visual Studio Code with the C# Dev Kit, and downloads are available through the .NET Blog’s announcement post.
What We Don’t Know
Microsoft’s RC1 announcement does not state a specific general-availability date for .NET 11. The most recent Microsoft documentation to address timing — the .NET 11 “What’s new” overview, which notes it “was last updated for Preview 7” — says “The final release is expected in November 2026,” according to Microsoft Learn, but that estimate predates RC1 and has not been repeated in the RC1 materials themselves.
It is also unclear when, or in which release, the Unsafe Evolution memory-safety features will stabilize. The RC1 notes describe only incremental refinements to the preview feature, without a stated timeline for it to leave preview status the way union types and closed hierarchies just have.