Make the C# TypeExpression handle every type kind by timotheeguerin · Pull Request #11596 · microsoft/typespec

Timothee Guerin (timotheeguerin) · 2026-08-07T21:50:40Z

The C# TypeExpression threw a hard Error for anything it did not recognise — tuples, string templates, enum members, template parameters and most of the intrinsics. An emitter hitting one of those got a crash with no source location instead of a diagnostic, so @typespec/http-server-csharp had to wrap every call site in try/catch.

Those kinds are now handled, and a genuinely unsupported type reports a diagnostic against the offending type and falls back to object, so compilation continues and the user gets a pointer to the spec that caused it. The diagnostics name the type, so it is actionable without bisecting the spec:

warning emitter-framework/csharp-unsupported-scalar: Scalar 'MyLib.ipAddress' has no
  C# equivalent, using 'object' instead. Extend a built-in scalar to control how it is emitted.

warning emitter-framework/csharp-unsupported-type: Type '@myLib.flag' of kind 'Decorator'
  is not supported in C#, using 'object' instead.

The two are kept separate because they call for different actions: the scalar one is fixable in the spec by extending a built-in scalar, while the other reports a gap in the emitter.

Two smaller bugs fixed along the way:

Unsupported scalars reported a TypeScript diagnostic (typescript-unsupported-scalar) from the C# components. C# now has its own src/csharp/lib.ts, matching the existing typescript/lib.ts and python/lib.ts, which makes that class of mistake a type error.
The null and never intrinsics emitted null and never, neither of which is a C# type. They now emit object and void.

Also adds an isCSharpValueType util, since deciding whether a TypeSpec type maps to a C# struct is something every C# emitter needs and every C# emitter was reimplementing.

Independent — targets main and can merge on its own.

One of 7 PRs moving @typespec/http-server-csharp onto the emitter framework instead of its private forks: #11596, #11597, #11598, #11599, #11600, #11601, #11602.

pkg-pr-new · 2026-08-07T21:52:07Z

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npm i https://pkg.pr.new/@typespec/emitter-framework@11596

commit: 3e407ef

github-actions · 2026-08-07T21:54:59Z

All changed packages have been documented.

✅ @typespec/emitter-framework

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`@typespec/emitter-framework` - fix ✏️

Make the C# TypeExpression handle every type kind instead of throwing,> ,> Tuple, StringTemplate, EnumMember, ModelProperty, UnionVariant, template parameters and the full Intrinsic set are now supported, and an unsupported type reports a diagnostic and falls back to object rather than throwing. The diagnostics now name the offending type instead of saying only "Unsupported scalar type":,> ,> ,> warning emitter-framework/csharp-unsupported-scalar: Scalar 'Currency' has no C# equivalent, using 'object' instead. Extend a built-in scalar to control how it is emitted.,> ,> ,> Also fixes the C# components reporting a TypeScript diagnostic for unsupported scalars, and corrects the C# expressions for the null and never intrinsics.

azure-sdk-automation · 2026-08-07T22:07:38Z

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Pull request overview

This PR improves the C# TypeExpression component in @typespec/emitter-framework so it no longer throws for previously-unhandled TypeSpec type kinds, instead reporting a diagnostic and falling back to object to keep emitting code.

Changes:

Added C#-specific diagnostics for unsupported scalars/types and used them in TypeExpression instead of throwing.
Expanded TypeExpression to handle additional TypeSpec kinds (e.g., tuples, string templates, enum members, template params, intrinsics) and corrected intrinsic mappings (null, never).
Introduced and exported a shared isCSharpValueType utility for determining whether a TypeSpec type maps to a C# struct.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/emitter-framework/src/lib.ts	Registers new C# emitter-framework diagnostic codes used for graceful fallback behavior.
packages/emitter-framework/src/csharp/components/utils/value-type.ts	Adds `isCSharpValueType` helper to centralize value-type vs reference-type decisions.
packages/emitter-framework/src/csharp/components/utils/index.ts	Re-exports the new `isCSharpValueType` utility.
packages/emitter-framework/src/csharp/components/type-expression.tsx	Refactors `TypeExpression` to be non-throwing, adds broader kind handling, and reports diagnostics with fallback types.
packages/emitter-framework/src/csharp/components/type-expression.test.tsx	Adds tests covering newly-supported kinds and “fallback instead of throwing” behavior.
.chronus/changes/ef-csharp-type-expression-total-2026-8-5.md	Adds a changelog entry describing the fix and behavior changes.

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Adds handling for Tuple, StringTemplate, EnumMember, ModelProperty, UnionVariant, template parameters and the full Intrinsic set. An unsupported type now reports a diagnostic and falls back to `object` instead of throwing. Also fixes the C# components reporting a TypeScript diagnostic for unsupported scalars, and corrects the C# expressions emitted for the `null` and `never` intrinsics. Adds an `isCSharpValueType` util.

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

…crosoft#11597) An override descriptor already accepted a `declaration` entry, but nothing ever dispatched to it — only `reference` was wired up. So an emitter could change how a type is *referenced* and had no say in how it is *declared*. The only way to customize a declaration was to copy the framework component into your own package and diverge from it, which is exactly what `@typespec/http-server-csharp` did for classes, properties and enums. `Experimental_OverridableComponent` now dispatches `declaration` too, and the C# `ClassDeclaration`, `Property` and `EnumDeclaration` render through it. An override can replace the declaration outright, or re-render the default with different props: ```tsx const overrides = Experimental_ComponentOverridesConfig().forTypeKind("ModelProperty", { declaration: (props) => isSecret(props.type) ? ( <props.Declaration {...props.declarationProps} public={false} internal /> ) : ( props.default ), }); ``` `props.Declaration` is the unwrapped body, so re-rendering it does not recurse back through the override. The declaration props type is generic and defaults to `Record<string, any>`, because the existing `Experimental_CustomTypeToProps` map is TypeScript-specific (`VarDeclarationProps`, `ObjectPropertyProps`) and means nothing for C#. Callers that want type safety pass the concrete props type: `.forTypeKind<"ModelProperty", PropertyProps>(...)`. --- Independent — targets `main` and can merge on its own. One of 7 PRs moving `@typespec/http-server-csharp` onto the emitter framework instead of its private forks: microsoft#11596, microsoft#11597, microsoft#11598, microsoft#11599, microsoft#11600, microsoft#11601, microsoft#11602.

The emitter carried a 217-line hand-maintained table of C# keywords and a `createLibrary` call re-declaring the `System.Text.Json.Serialization` attributes. `@alloy-js/csharp` ships both, and its versions are better: the local keyword list was missing `delegate`, and hand-declared symbols do not participate in Alloy's automatic `using` management. Both are deleted in favour of the Alloy equivalents (`isValidCSharpIdentifier`, `csharpKeywords`, the `System/Text/Json` builtins), along with the local `getDocComments` copy, which is already exported from `@typespec/emitter-framework/csharp`. One thing Alloy deliberately does *not* do is worth calling out: its name policy `@`-escapes real keywords, but the emitter needs namespace segments colliding with common BCL type names to be **renamed** — a namespace called `Type` shadows `System.Type` and breaks every `typeof()` in the generated converters. That rule survives, now isolated in `getCSharpNamespaceName` and built on Alloy's keyword sets rather than a parallel list. Generated output is unchanged. --- Independent — targets `main` and can merge on its own. One of 7 PRs moving `@typespec/http-server-csharp` onto the emitter framework instead of its private forks: microsoft#11596, microsoft#11597, microsoft#11598, microsoft#11599, microsoft#11600, microsoft#11601, microsoft#11602. --------- Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Timothee Guerin (timotheeguerin) mentioned this pull request Aug 7, 2026

Dispatch declaration overrides in Experimental_ComponentOverrides #11597

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microsoft-github-policy-service Bot added the emitter-framework Issues for the emitter framework label Aug 7, 2026

microsoft-github-policy-service Bot added the stale Mark a PR that hasn't been recently updated and will be closed. label Aug 22, 2026

Timothee Guerin (timotheeguerin) marked this pull request as ready for review August 27, 2026 16:36

Copilot AI lite review requested due to automatic review settings August 27, 2026 16:36

Timothee Guerin (timotheeguerin) requested review from catalinaperalta, iscai-msft, Laurent Mazuel (lmazuel) and Mark Cowlishaw (markcowl) as code owners August 27, 2026 16:36

Copilot started reviewing on behalf of Timothee Guerin (timotheeguerin) August 27, 2026 16:37 View session

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Comment thread packages/emitter-framework/src/csharp/components/type-expression.tsx

Comment thread packages/emitter-framework/src/csharp/components/type-expression.test.tsx

Comment thread packages/emitter-framework/src/csharp/components/type-expression.tsx

Copilot AI review requested due to automatic review settings August 27, 2026 16:51

Timothee Guerin (timotheeguerin) force-pushed the ef/csharp-type-expression-total branch from 1e8dbc8 to 3e407ef Compare August 27, 2026 16:51

Copilot started reviewing on behalf of Timothee Guerin (timotheeguerin) August 27, 2026 16:52 View session

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iscai-msft approved these changes Aug 28, 2026

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Timothee Guerin (timotheeguerin) enabled auto-merge August 28, 2026 16:26

Timothee Guerin (timotheeguerin) added this pull request to the merge queue Aug 28, 2026

Merged via the queue into microsoft:main with commit bed13e3 Aug 28, 2026
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Timothee Guerin (timotheeguerin) deleted the ef/csharp-type-expression-total branch August 28, 2026 17:11

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Make the C# TypeExpression handle every type kind - #11596

Make the C# TypeExpression handle every type kind#11596
Timothee Guerin (timotheeguerin) merged 1 commit into
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