Add closed hierarchies for C# 15, preview 5#54128
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Pull request overview
This PR adds C# 15 Preview 5 documentation for the new closed modifier (closed class hierarchies), including a new language reference article, updated pattern matching guidance on exhaustiveness/subsumption, a “What’s new” entry, and publication wiring for the feature specification.
Changes:
- Add
closedmodifier reference content and link it into the keyword index and TOC. - Extend pattern matching docs with a new “Closed hierarchy patterns” section and new snippets.
- Publish the
closed-hierarchiesproposal content via spec TOC anddocfx.jsonconfiguration.
Reviewed changes
Copilot reviewed 11 out of 11 changed files in this pull request and generated 6 comments.
Show a summary per file
| File | Description |
|---|---|
| docs/csharp/whats-new/csharp-15.md | Adds “Closed hierarchies” to the C# 15 preview feature list with examples and links. |
| docs/csharp/specification/toc.yml | Adds “Closed hierarchies” entry to the specification TOC pointing at the proposal. |
| docs/csharp/language-reference/toc.yml | Adds closed keyword entry to the language reference TOC. |
| docs/csharp/language-reference/operators/snippets/patterns/patterns.csproj | Updates snippet project TFM to net11.0 (but needs LangVersion for preview syntax). |
| docs/csharp/language-reference/operators/snippets/patterns/ClosedHierarchyPatterns.cs | Introduces new closed-hierarchy pattern matching snippets and narrative comments. |
| docs/csharp/language-reference/operators/patterns.md | Adds “Closed hierarchy patterns” section explaining exhaustiveness, accessibility, and subsumption. |
| docs/csharp/language-reference/keywords/snippets/shared/Closed.cs | Adds shared closed keyword snippets for the reference article. |
| docs/csharp/language-reference/keywords/snippets/keywords.csproj | Updates snippet project TFM to net11.0-windows (but needs LangVersion for preview syntax). |
| docs/csharp/language-reference/keywords/index.md | Adds closed to the contextual keywords list. |
| docs/csharp/language-reference/keywords/closed.md | Adds the new closed modifier reference article and links to related guidance/spec. |
| docfx.json | Wires in the new proposal file for build/metadata/title/description. |
Comments suppressed due to low confidence (1)
docs/csharp/language-reference/keywords/snippets/keywords.csproj:12
- The keywords snippet project now targets net11.0-windows and includes C# 15 preview syntax (the
closedmodifier), but the project doesn't setLangVersiontopreview. If the build defaults to a stable language version, the newclosedsnippets won't compile.
<PropertyGroup>
<ImplicitUsings>enable</ImplicitUsings>
<OutputType>WinExe</OutputType>
<TargetFramework>net11.0-windows</TargetFramework>
<Nullable>enable</Nullable>
<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
<StartupObject>Keywords.Program</StartupObject>
<GenerateDocumentationFile>False</GenerateDocumentationFile>
<UseWPF>true</UseWPF>
</PropertyGroup>
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Author
Publish the feature spec for closed hierarchies.
Add a new reference article for `closed`. Update snippets and the associated TOC.
The `closed` hierarchy impacts pattern exhaustiveness and subsumption. Add a section to discuss that in the patterns reference article.
Add the `closed` contextual keyword to the What's new in C# 15 article.
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Add note on exhaustiveness checks for type parameters.
Developers need to define the required attribute in Preview 5.
The first samples all used the same code. Show different scenarios that could be good for closed class hierarchies.
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wadepickett
reviewed
Jun 5, 2026
wadepickett
reviewed
Jun 5, 2026
wadepickett
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Jun 5, 2026
wadepickett
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Jun 5, 2026
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Approved. Looks great! I found a couple very minor items. My suggestions are inline.
Co-authored-by: Wade Pickett <wpickett@microsoft.com>
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Fixes #54013
closedcontextual keyword.closedhierarchies.Internal previews
isandswitchexpressions, and operatorsand,or, andnotin patterns