Preserve accessible serialization constructors during back compat - #11798
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JoshLove-msft merged 4 commits intoSep 1, 2026
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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 00636c74-c4f7-434a-8a70-fb0c3d221328
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 00636c74-c4f7-434a-8a70-fb0c3d221328
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Pull request overview
This PR refines the C# generator’s API-compat constructor restoration logic so that back-compat processing does not introduce a duplicate parameterless constructor when one already exists on a generated serialization partial (including cases where a visitor has made that constructor public). This directly targets the constructor churn scenario seen when ApiCompatVersion is enabled.
Changes:
- Update model back-compat constructor restoration to treat an accessible parameterless constructor on any serialization provider partial as already satisfying the contract.
- Add focused ClientModel tests covering:
- preserving an accessible parameterless serialization constructor (including visitor-customized routing),
- retaining the existing behavior of replacing an inaccessible parameterless serialization constructor by restoring one on the model partial.
- Update backward-compat documentation to clarify the “already exists” condition and the mocking-constructor removal behavior.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs | Avoid restoring a parameterless constructor onto the model partial when an accessible parameterless constructor already exists on a serialization partial. |
| packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BackCompat_AccessibleParameterlessSerializationConstructorIsPreserved/MockInputModel.cs | Adds last-contract custom code fixture defining a published public parameterless constructor. |
| packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/ScmModelProviderTests.cs | Adds/extends tests validating preservation vs. replacement behavior for parameterless serialization constructors under back-compat. |
| packages/http-client-csharp/generator/docs/backward-compatibility.md | Clarifies docs for parameterless-constructor restoration eligibility and mocking-constructor removal semantics. |
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Jorge Rangel (jorgerangel-msft)
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Sep 1, 2026
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Summary
This addresses the constructor churn observed in openai/openai-dotnet#1341 when ApiCompatVersion is enabled.
Validation
npm run buildeng/scripts/Generate.ps1npm testnpm run cop