[http-client-csharp] Single-member enum with @alternateType incorrectly collapses to constant instead of preserving enum type

[live1206]

Description

When a single-member enum (e.g., enum Colors { green }) is used as a property type and @alternateType("green") is applied, the C# emitter generates a constant "green" instead of keeping the Colors enum type.

TypeSpec to reproduce

enum Colors {
  green,
}

model EmployeeProperties {
  @Azure.ClientGenerator.Core.alternateType("green")
  favoriteColor: Colors;
}

Playground link

Expected behavior

The C# emitter should generate a struct named Colors (as an enum type). The type of favoriteColor should be Colors.

Actual behavior

The type of favoriteColor is generated as a constant "green" (a string literal), losing the Colors enum type entirely.

Root cause analysis

Investigation in Azure/azure-sdk-for-net#57771 found:

1. TCGC resolves the type incorrectly: When @alternateType("green") is applied to a property of type Colors (a single-member enum), TCGC resolves the alternate type as a string literal "green" (an SdkConstantType with kind: "constant", value: "green", valueType.kind: "string"), losing the reference to the Colors enum entirely. Without @alternateType, the single-member enum correctly emits as kind: "enum" with name: "Colors".

2. The C# emitter then handles the constant case: In type-converter.js fromSdkType(), the case "constant": branch:

   • For optional properties with constants → converts to extensible enum (correct behavior)
   • For required properties with constants → keeps as literal/constant (the buggy path)

The fix should likely be in the fromSdkType function in type-converter.ts (case "constant": branch) to check if the original property type was an enum and preserve that enum type instead of collapsing to a constant.

Alternatively, this could also be addressed in TCGC (@azure-tools/typespec-client-generator-core) so it does not resolve @alternateType("green") on an enum-typed property as a plain string constant.

Related issues

• Incorrect behavior for constant Azure/azure-sdk-for-net#57770
• [WIP] Fix incorrect behavior for constant in EmployeeProperties Azure/azure-sdk-for-net#57771 (investigation PR)