feat: add C# syntax highlighting

[dev693-ai]

Linked issue

Closes #1527

Type of change

• New feature (with prior issue + design alignment)

What does this PR do

Adds C# (.cs) syntax highlighting, implementing the request in #1527 (proposed with a patch by @percyboy). It wires the @replit/codemirror-lang-csharp Lezer grammar into @getpaseo/highlight following the existing per-language pattern (Java/Rust/Swift), and aliases the csharp / c# markdown fence names to cs.

This makes C# highlight everywhere code is rendered through the shared highlighter — the file viewer, diff views, and ```csharp / ```cs / ```c# markdown code fences in agent messages.

One deliberate change from the patch in the issue: I register csharpLanguage.parser, not the package's raw parser export. The Replit package applies its highlight styleTags only inside csharpLanguage (via parser.configure), so the raw parser would produce a valid tree but render with no highlighting. This differs from @lezer/java etc., whose parser export ships highlight props baked in.

Changed files:

• packages/highlight/package.json — add @replit/codemirror-lang-csharp
• packages/highlight/src/parsers.ts — register cs: csharpLanguage.parser
• packages/app/src/components/highlighted-code-block.tsx — csharp / c# fence aliases
• packages/highlight/src/__tests__/* — parser + highlighter test cases

How did you verify it

• Added unit tests and ran the two changed suites:
  npx vitest run packages/highlight/src/__tests__/{parsers,highlighter}.test.ts → 23 passed, including the new C# cases (asserts class → keyword, string → string).

• npm run build:server then npm run typecheck → clean across all workspaces.

• npm run lint → 0 warnings / 0 errors. npm run format → clean.

• Opened a real .cs file (Program.cs) in the running Paseo app and confirmed full highlighting (keywords, types, method names, switch pattern matching, interpolated strings, LINQ, comments), on web and desktop (Electron):

  Web (browser):

Desktop (Electron):

Verified on web + desktop; iOS/Android render through the identical cross-platform component (no platform-gated code in the highlight path), so they were not separately captured.

Checklist

• One focused change. Unrelated cleanups split out.
• npm run typecheck passes
• npm run lint passes
• npm run format ran (Biome)
• UI change includes screenshots (web + desktop)
• Tests added or updated where it made sense

[greptile-apps]

Greptile Summary

This PR adds C# (.cs) syntax highlighting by wiring the @replit/codemirror-lang-csharp Lezer grammar into the @getpaseo/highlight package and aliasing the csharp / c# markdown fence names in the app component.

• packages/highlight/src/parsers.ts: Registers cs: csharpLanguage.parser, correctly using the language wrapper (not the raw parser export) so that the package's styleTags highlight props are applied.
• packages/app/src/components/highlighted-code-block.tsx: Adds csharp and c# fence aliases pointing to cs, consistent with every other alias in the table.
• Tests: New cases in both parsers.test.ts and highlighter.test.ts follow the existing per-language pattern and assert keyword and string token styles on a representative snippet.

Confidence Score: 5/5

Safe to merge — a minimal, focused language addition with no changes to existing logic.

The change touches only the extension registration map, the fence alias table, and the dependency list. It follows the established pattern exactly (import parser, add entry, add alias, add tests). The technical explanation for using csharpLanguage.parser over the raw export is correct and verified by the passing unit tests. No existing behavior is altered.

No files require special attention.

Important Files Changed

Filename	Overview
packages/highlight/src/parsers.ts	Adds cs: csharpLanguage.parser entry following the identical pattern used by Java, Rust, and Swift; import and registration are clean.
packages/app/src/components/highlighted-code-block.tsx	Adds csharp and c# aliases to the LANGUAGE_ALIASES map; consistent with the existing alias table and the fenceLanguageToExtension logic.
packages/highlight/package.json	Adds @replit/codemirror-lang-csharp: ^6.2.0 to dependencies; all peers are satisfied by existing CodeMirror 6 packages in the project.
packages/highlight/src/tests/highlighter.test.ts	New C# test case mirrors the Dart/Swift/Python pattern — asserts class → keyword and string literal → string style.
packages/highlight/src/tests/parsers.test.ts	Adds test.cs to isLanguageSupported and cs to getSupportedExtensions checks; follows the existing test structure.
package-lock.json	Adds only the @replit/codemirror-lang-csharp node and the one reference in packages/highlight; previously-flagged lockfile churn was addressed in the fixup commit.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Markdown fence / file path\ne.g. ```csharp or file.cs"] --> B["fenceLanguageToExtension\nor getParserForFile"]
    B --> C{"LANGUAGE_ALIASES lookup\n(highlighted-code-block.tsx)"}
    C -- "csharp / c# → cs" --> D["extension = 'cs'"]
    C -- "already an ext key\n(e.g. 'cs')" --> D
    D --> E["parsersByExtension\n(parsers.ts)"]
    E --> F["csharpLanguage.parser\n(@replit/codemirror-lang-csharp)"]
    F --> G["highlightCode → token stream\nwith keyword / string / type styles"]

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A["Markdown fence / file path\ne.g. ```csharp or file.cs"] --> B["fenceLanguageToExtension\nor getParserForFile"]
    B --> C{"LANGUAGE_ALIASES lookup\n(highlighted-code-block.tsx)"}
    C -- "csharp / c# → cs" --> D["extension = 'cs'"]
    C -- "already an ext key\n(e.g. 'cs')" --> D
    D --> E["parsersByExtension\n(parsers.ts)"]
    E --> F["csharpLanguage.parser\n(@replit/codemirror-lang-csharp)"]
    F --> G["highlightCode → token stream\nwith keyword / string / type styles"]

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