fix(http-client-csharp): fix constructor and serialization generation for models rebased with hierarchyBuilding

[JoshLove-msft]

Fixes two @Legacy.hierarchyBuilding bugs in the C# emitter that made a rebased discriminated model impossible to generate/compile.

Bug 1 — uncompilable constructor (CS0103)

When a discriminated model is rebased onto another discriminated model, the emitter generated:

private protected VoiceConversationItem() : base(@type)
{
}

@type was forwarded to the base initializer but never declared as a parameter, producing:

error CS0103: The name 'type' does not exist in the current context

Cause: ModelProvider.BuildConstructorParameters only kept the inherited discriminator in the signature when includeDiscriminatorParameter && IsMultiLevelDiscriminator, but it always forwarded it in the base(...) initializer. ComputeIsMultiLevelDiscriminator returns false for abstract types, and a discriminated base type is always abstract — so the rebased base type dropped the parameter while still passing the argument.

Fix: in BuildConstructors, compute includeDiscriminatorParameter from whether the base model actually declares a discriminator property, and honor that flag directly in the signature filter.

Generated output is now:

private protected VoiceConversationItem(string @type) : base(@type)
{
}

Bug 2 — NullReferenceException during generation

Generating the reported spec also crashed the emitter with a NullReferenceException in SwitchStatement.Write.

Cause: MrwSerializationTypeDefinition.GetDiscriminatorSwitchCases allocated new SwitchCaseStatement[DerivedModels.Count - 1] and iterated only that many entries while continue-ing past the unknown variant. That is only correct when the unknown variant is last. hierarchyBuilding appends the rebased model after the unknown variant, leaving a null slot in the array.

Fix: iterate all derived models into a list and skip entries with no discriminator value.

Tests

• Microsoft.TypeSpec.Generator / DiscriminatorTests: 3 new tests covering the rebased-base constructor signature, the multi-level variant, and the emitted C# text.
• Microsoft.TypeSpec.Generator.ClientModel / MrwSerializationTypeDefinitions.DiscriminatorTests: 1 new test asserting no null switch cases when the unknown variant is not last, and that writing the provider does not throw.

All new tests were confirmed to fail before the corresponding fix.

Validation

• Microsoft.TypeSpec.Generator.Tests: 1913 passed
• Microsoft.TypeSpec.Generator.ClientModel.Tests: 1575 passed
• eng/scripts/Generate.ps1: no diff to checked-in generated libraries
• npm run cop: passed

End-to-end against the reported spec shape (a VoiceConversationItem rebased onto OpenAI.RealtimeConversationItem via hierarchyBuilding):

• Before: generation emitted private protected VoiceConversationItem() : base(@type) and the build failed with CS0103: The name 'type' does not exist in the current context at VoiceConversationItem.cs(21,58).
• After: generation emits private protected VoiceConversationItem(string @type) : base(@type) and the generated library builds with 0 errors and 0 warnings on net8.0.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11732

commit: 7d7f069

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[Copilot]

Pull request overview

Fixes a C# emitter constructor generation bug in http-client-csharp where a rebased discriminated base type could emit a parameterless ctor that still forwards an undeclared discriminator argument to base(...), producing uncompilable code (CS0103).

Changes:

• Adjust constructor generation to include the discriminator parameter for rebased discriminated base types whose base model also declares a discriminator.
• Update discriminator filtering logic so the “include discriminator parameter” switch is honored directly (not gated by multi-level discriminator detection), and remove a redundant base-discriminator guard.
• Add focused unit tests to lock the expected constructor signature and base-initializer forwarding behavior (including a multi-level base scenario).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs	Fixes constructor parameter filtering/forwarding so discriminator arguments are only forwarded when declared, covering rebased discriminated base cases.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/DiscriminatorTests.cs	Adds regression tests validating the discriminator parameter is declared and forwarded correctly, and that emitted C# text matches the previously broken scenario.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.