Add Support for Union Extends #12478

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1. Jorge Rangel (jorgerangel-msft)

   added

   emitter:client:csharpIssue for the C# client emitter: @typespec/http-client-csharpIssue for the C# client emitter: @typespec/http-client-csharp

   on Nov 3, 2026
2. Jorge Rangel (jorgerangel-msft)

   self-assigned this

   on Nov 3, 2026
3. github-actions

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   featureNew feature or requestNew feature or request

   on Nov 3, 2026
4. github-actions commented on Nov 3, 2026

   

   github-actionsBot

   on Nov 3, 2026 – with GitHub Actions

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   🎯 Issue Triage

   Add C# client generation support for inherited unions (union extends) according to the linked cross-language design, validated by the existing union Spector scenarios.

   Category: Feature Request
   Area: emitter:client:csharp

   🔍 Analysis

   The affected component is @typespec/http-client-csharp. The compiler-side extends base type for unions was implemented separately in #2737; this request is specifically for the C# client emitter to represent those semantics in generated code. Related issues such as #7923 (adopting discriminated-union Spector scenarios) and #4202 (general heterogeneous-union generation) overlap in topic but do not describe the same request.

   The implementation should follow the linked cross-language design and use the cases under packages/http-specs/specs/type/union as conformance coverage.

   📝 Suggested next steps

   • Map inherited union metadata into the C# emitter's generated type model.
   • Enable the applicable type/union Spector scenarios for the C# client emitter.
   • Add focused generator tests for inheritance, serialization, and deserialization behavior.

   Generated by Agentic Triage
5. Jorge Rangel (jorgerangel-msft)

   assigned

   JoshLove01

   and unassigned

   JoshLove01

   on Nov 3, 2026
6. JoshLove01

   assigned

   JoshLove01

   and unassigned

   Jorge Rangel (jorgerangel-msft)

   on Nov 3, 2026
7. sub4biz

   added a commit that references this issue on Nov 14, 2026

   [http-client-csharp] Add discriminated union extends support (microso…

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