fix(http-server-csharp): avoid duplicate nullable suffixes

[sophia-ramsey]

This pull request fixes an issue where optional nullable value parameters in generated C# interfaces and mocks would emit duplicate nullable suffixes (e.g., int??). The changes ensure that only a single nullable suffix is emitted for such parameters. The update includes logic changes, new helper functions, and additional tests to verify correct behavior.

Bug Fix: Prevent Duplicate Nullable Suffixes

• Updated parameter handling in both interface and mock generation to ensure that optional nullable value parameters (e.g., int32 | null) are rendered with only one nullable suffix (int?), preventing cases like int??. [1] [2] [3] [4] [5]

Helper Function

• Added getNullableValueTypeUnionInnerType to detect and extract the correct inner type for nullable value types in unions, used to avoid double nullable suffixes. [1] [2] [3]

Testing

• Added and updated tests to verify that only one nullable suffix is emitted for optional nullable value parameters in both interfaces and mocks. This includes new test cases in both the component and end-to-end test suites. [1] [2]

These changes ensure that the generated C# code is correct and idiomatic when handling optional nullable value parameters.

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commit: 29a344c

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-server-csharp

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@typespec/http-server-csharp - fix ✏️

Prevent optional nullable value parameters from emitting duplicate nullable suffixes in generated C# interfaces and mocks.

[azure-sdk-automation]

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[Copilot]

🟢 Approval recommended

The change is narrowly scoped, includes coverage in both component and end-to-end tests, and only leaves a minor test robustness nit.

Pull request overview

This PR fixes a C# server emitter bug where optional parameters whose TypeSpec type is already a nullable value-type union (e.g. int32 | null) could be emitted with a duplicated nullable suffix (int??) in generated business-logic interfaces and mock implementations.

Changes:

• Added a helper (getNullableValueTypeUnionInnerType) to detect nullable value-type unions and extract the non-null inner type for special handling.
• Updated interface and mock parameter type rendering to avoid double-appending ? for optional nullable value-type unions.
• Added targeted tests (component-level and end-to-end) to ensure int?? does not appear in generated signatures.

File summaries

File	Description
packages/http-server-csharp/test/nullable-parameters.test.ts	New end-to-end test validating optional nullable value parameters render with a single ? in both interfaces and mocks.
packages/http-server-csharp/src/components/type-expression/type-expression.tsx	Introduces helper for nullable value-type unions and refactors nullable-union rendering to use it.
packages/http-server-csharp/src/components/scaffolding/mock-implementations.tsx	Avoids T?? in mock method signatures by stripping nullable unions to their inner value type when also optional.
packages/http-server-csharp/src/components/interfaces/interfaces.tsx	Applies the same inner-type logic for optional parameters when building interface method parameters.
packages/http-server-csharp/src/components/interfaces/interfaces.test.tsx	Adds a component test asserting the interface signature uses only a single nullable suffix.
.chronus/changes/sramsey-csharp-duplicate-nullable-suffixes-2026-09-08.md	Adds a fix changelog entry for @typespec/http-server-csharp.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 1
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

The change is covered by component and end-to-end regression tests.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0 new
• Review effort level: Lite