Skip Generating Convenience Methods for multipart/mixed Requests

[jorgerangel-msft]

We technically don't currently support this, and with the recent addition of multipart/form-data support, we attempt to generate a convenience method that is not correct which is worse. We should update the emitter to log a diagnostic and disable generating convenience methods for this content type until we have a concrete design for this.

[github-actions]

🎯 Issue Triage

The C# client emitter incorrectly generates convenience methods for multipart/mixed requests following the recent addition of multipart/form-data support — producing invalid client code for an unsupported content type.

Category: Bug
Area: emitter:client:csharp

⚠️ No reproduction provided. A minimal repro would help diagnose this issue.

🔍 Analysis

• Affected component: @typespec/http-client-csharp convenience method generation
• Root cause: The emitter does not differentiate between multipart/form-data (now supported) and multipart/mixed (not yet supported) when deciding whether to emit convenience methods. As a result, it generates an incorrect convenience method for multipart/mixed operations.
• Proposed fix: Emit a compiler diagnostic for multipart/mixed usage and suppress convenience method generation for that content type until a concrete design is in place.

📝 Suggested next steps

• Identify all code paths in the C# emitter that generate convenience methods for multipart operations
• Add a check to detect multipart/mixed content type and emit a diagnostic (warning or error)
• Skip convenience method generation when the content type is multipart/mixed
• Add a test case covering multipart/mixed to verify the diagnostic is emitted and no incorrect method is generated

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