Add feature / version mismatch errors in C# 9#53662
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Add errors and warnings related to C# 9 features being unsupported before C# 9.
Add related codes that were missed earlier.
Integrate the new codes into the correct themes.
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Pull request overview
This PR consolidates several C# compiler diagnostics related to language feature availability, language version configuration, and runtime feature support into the language-reference “Feature or version missing” article, and updates F1 keyword routing and navigation so those diagnostics land on the new consolidated guidance instead of the generic “Sorry…” page or older per-error pages.
Changes:
- Expands
feature-version-errors.mdwith coverage for additional diagnostics (including the C# 9 feature/version mismatch errors) and adds structured resolution guidance. - Removes CS1617 and CS1638 standalone pages and adds redirects to the consolidated article.
- Updates F1 keyword mappings and TOC entries so the relevant diagnostics route to the consolidated article.
Reviewed changes
Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.
Show a summary per file
| File | Description |
|---|---|
| docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md | Removes several diagnostics from the generic fallback page now that they have dedicated guidance elsewhere. |
| docs/csharp/misc/cs1638.md | Deletes the standalone CS1638 page (replaced by redirect to consolidated article). |
| docs/csharp/misc/cs1617.md | Deletes the standalone CS1617 page (replaced by redirect to consolidated article). |
| docs/csharp/language-reference/toc.yml | Updates navigation and search displayName mappings to point diagnostics to the consolidated “Feature or version missing” article; removes CS1617/CS1638 entries. |
| docs/csharp/language-reference/compiler-messages/feature-version-errors.md | Adds diagnostics (including CS8830/CS8831/CS8888–CS8891) and significantly expands resolution guidance and categorization. |
| .openpublishing.redirection.csharp.json | Adds redirects for removed CS1617 and CS1638 pages to the consolidated article. |
These need to be updated before C# 15 ships.
gewarren
approved these changes
May 19, 2026
| Correct the `<LangVersion>` value in your project file to a valid language version string (**CS1617**, **CS8192**, **CS8303**). The valid values include `default`, `latest`, `preview`, `latestMajor`, or a specific version number such as `7.3`, `8.0`, `9.0`, `10`, `11`, `12`, `13`, or `14`. Don't include leading zeroes in the version number. See [C# language versioning](../language-versioning.md) for the full list of supported values. | ||
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| > [!TIP] | ||
| > To see a list of supported language versions, you reference the table in this article, compile with `-langversion:?`, or temporarily set `<LangVersion>?</LangVersion>` in your project file before building. |
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Suggested change
| > To see a list of supported language versions, you reference the table in this article, compile with `-langversion:?`, or temporarily set `<LangVersion>?</LangVersion>` in your project file before building. | |
| > To see a list of supported language versions, reference the table in this article, compile with `-langversion:?`, or temporarily set `<LangVersion>?</LangVersion>` in your project file before building. |
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| Upgrade the target framework so the compiler automatically selects the required language version. Each target framework maps to a default C# version. For example, .NET 8 defaults to C# 12, .NET 9 defaults to C# 13, and .NET 10 defaults to C# 14. See the table in [Language version configuration errors](#language-version-configuration-errors) for the full mapping. | ||
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| Set the `<LangVersion>` element in your project file to the required version or higher. For example, to enable C# 12 features, add `<LangVersion>12</LangVersion>` to a `<PropertyGroup>` in your project file. |
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Suggested change
| Upgrade the target framework so the compiler automatically selects the required language version. Each target framework maps to a default C# version. For example, .NET 8 defaults to C# 12, .NET 9 defaults to C# 13, and .NET 10 defaults to C# 14. See the table in [Language version configuration errors](#language-version-configuration-errors) for the full mapping. | |
| Set the `<LangVersion>` element in your project file to the required version or higher. For example, to enable C# 12 features, add `<LangVersion>12</LangVersion>` to a `<PropertyGroup>` in your project file. | |
| - Upgrade the target framework so the compiler automatically selects the required language version. Each target framework maps to a default C# version. For example, .NET 8 defaults to C# 12, .NET 9 defaults to C# 13, and .NET 10 defaults to C# 14. See the table in [Language version configuration errors](#language-version-configuration-errors) for the full mapping. | |
| - Set the `<LangVersion>` element in your project file to the required version or higher. For example, to enable C# 12 features, add `<LangVersion>12</LangVersion>` to a `<PropertyGroup>` in your project file. |
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Fixes #53546
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