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[http-client-csharp] Preserve enum type when @alternateType collapses single-member enum to constant #10281

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[http-client-csharp] Preserve enum type when @alternateType collapses single-member enum to constant #10281

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26b702d
Initial plan
Copilot Apr 7, 2026
4a40a87
fix: preserve enum type when @alternateType collapses single-member e…
Copilot Apr 7, 2026
67380b3
refactor: improve condition clarity and add isFixed assertion per rev…
Copilot Apr 7, 2026
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12 changes: 9 additions & 3 deletions packages/http-client-csharp/emitter/src/lib/type-converter.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -118,12 +118,18 @@ export function fromSdkType<T extends SdkType>(
typeof sdkProperty.serializedName === "string" &&
sdkProperty.serializedName.toLocaleLowerCase() === "content-type";

// Check if the original property type was an enum (e.g., when @alternateType collapses
// a single-member enum to a constant) - preserve the enum type in this case
const isOriginalTypeEnum =
sdkProperty !== undefined && sdkProperty.__raw?.type?.kind === "Enum";

if (
sdkProperty &&
!isContentTypeHeader &&
(sdkProperty.optional || sdkProperty?.type.kind === "nullable") &&
sdkProperty?.type.kind !== "boolean" &&
sdkType.valueType.kind !== "boolean"
(((sdkProperty.optional || sdkProperty?.type.kind === "nullable") &&
sdkProperty?.type.kind !== "boolean" &&
sdkType.valueType.kind !== "boolean") ||
isOriginalTypeEnum)
) {
// turn the constant into an extensible enum
retVar = diagnostics.pipe(createEnumType(sdkContext, sdkType, namespace!));
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30 changes: 30 additions & 0 deletions packages/http-client-csharp/emitter/test/Unit/constant-type.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -149,4 +149,34 @@ describe("Constant enum conversion", () => {
strictEqual(enumType.access, undefined);
strictEqual(enumType.usage, UsageFlags.Input | UsageFlags.Json);
});

it("single-member enum with @alternateType should preserve enum type for required property", async () => {
const program = await typeSpecCompile(
`
enum Colors {
green,
}

model TestModel {
@alternateType("green")
prop: Colors;
}

op test(@body input: TestModel): void;
`,
runner,
{ IsTCGCNeeded: true },
);
const context = createEmitterContext(program);
const sdkContext = await createCSharpSdkContext(context);
const [root] = createModel(sdkContext);
const testModel = root.models.find((m) => m.name === "TestModel");
ok(testModel);
const propertyType = testModel.properties[0].type;
strictEqual(propertyType.kind, "enum");
const enumType = propertyType as InputEnumType;
strictEqual(enumType.isFixed, false);
strictEqual(enumType.values.length, 1);
strictEqual(enumType.values[0].value, "green");
});
});
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