Skip convenience method generation for multipart/mixed requests (#10967)

The C# emitter does not support `multipart/mixed` requests, yet since
the addition of `multipart/form-data` support it would emit an incorrect
convenience method for them. This disables convenience method generation
for multipart content types other than `multipart/form-data` and
surfaces a diagnostic instead.

The detection lives in the emitter so the generator simply honors the
resulting `generateConvenienceMethod` flag.

### Changes

- **`operation-converter.ts`**: In `fromSdkServiceMethodOperation`, when
an operation uses a `multipart/*` content type other than
`multipart/form-data`, `generateConvenienceMethod` is set to `false`
(protocol methods still emitted) and a warning diagnostic is reported. A
small `isUnsupportedMultipart` helper inspects the request media types.
- **`lib.ts`**: Added the `unsupported-multipart-convenience-method`
warning diagnostic.
- **Tests**: Added emitter unit tests in `operation-converter.test.ts` —
`multipart/mixed` turns off convenience method generation and emits the
warning, while `multipart/form-data` keeps convenience methods with no
diagnostic.

```ts
const requestMediaTypes = getRequestMediaTypes(method.operation);
if (generateConvenience && isUnsupportedMultipart(requestMediaTypes)) {
  diagnostics.add(
    createDiagnostic({
      code: "unsupported-multipart-convenience-method",
      format: { methodCrossLanguageDefinitionId: method.crossLanguageDefinitionId },
      target: method.__raw ?? NoTarget,
    }),
  );
  generateConvenience = false;
}
```

No existing test specs use `multipart/mixed`, so generated library
output is unchanged.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jorgerangel-msft <102122018+jorgerangel-msft@users.noreply.github.com>

Author: Copilot

packages/http-client-csharp/emitter/src/lib/lib.ts

@@ -95,6 +95,12 @@ const diags: { [code: string]: DiagnosticDefinition<DiagnosticMessages> } = {
       default: paramMessage`Convenience method is not supported for PATCH method, it will be turned off. Please set the '@convenientAPI' to false for operation ${"methodCrossLanguageDefinitionId"}.`,
     },
   },
+  "unsupported-multipart-convenience-method": {
+    severity: "warning",
+    messages: {
+      default: paramMessage`Convenience method is not supported for multipart content types other than 'multipart/form-data', it will be turned off. Please set the '@convenientAPI' to false for operation ${"methodCrossLanguageDefinitionId"}.`,
+    },
+  },
   "unsupported-service-method": {
     severity: "warning",
     messages: {

packages/http-client-csharp/emitter/src/lib/operation-converter.ts

@@ -196,6 +196,20 @@ export function fromSdkServiceMethodOperation(
     generateConvenience = false;
   }
 
+  const requestMediaTypes = getRequestMediaTypes(method.operation);
+  if (generateConvenience && isUnsupportedMultipart(requestMediaTypes)) {
+    diagnostics.add(
+      createDiagnostic({
+        code: "unsupported-multipart-convenience-method",
+        format: {
+          methodCrossLanguageDefinitionId: method.crossLanguageDefinitionId,
+        },
+        target: method.__raw ?? NoTarget,
+      }),
+    );
+    generateConvenience = false;
+  }
+
   operation = {
     name: method.name,
     isExactName: method.isExactName,
@@ -217,7 +231,7 @@ export function fromSdkServiceMethodOperation(
     uri: uri,
     path: method.operation.path,
     externalDocsUrl: getExternalDocs(sdkContext, method.operation.__raw.operation)?.url,
-    requestMediaTypes: getRequestMediaTypes(method.operation),
+    requestMediaTypes: requestMediaTypes,
     bufferResponse: true,
     generateProtocolMethod: shouldGenerateProtocol(sdkContext, method.operation.__raw.operation),
     generateConvenienceMethod: generateConvenience,
@@ -751,6 +765,15 @@ function toStatusCodesArray(range: number | HttpStatusCodeRange): number[] {
   return statusCodes;
 }
 
+function isUnsupportedMultipart(requestMediaTypes: string[] | undefined): boolean {
+  return (
+    requestMediaTypes?.some((mediaType) => {
+      const normalized = mediaType.toLowerCase();
+      return normalized.startsWith("multipart/") && normalized !== "multipart/form-data";
+    }) ?? false
+  );
+}
+
 function getRequestMediaTypes(op: SdkHttpOperation): string[] | undefined {
   const contentTypes = op.parameters.filter(
     (p) => p.kind === "header" && p.serializedName.toLocaleLowerCase() === "content-type",

packages/http-client-csharp/emitter/test/Unit/operation-converter.test.ts

@@ -622,3 +622,79 @@ describe("Test isExactName propagation on operations and parameters", () => {
     strictEqual(method.operation.isExactName, true);
   });
 });
+
+describe("Multipart convenience method generation", () => {
+  let runner: TestHost;
+
+  beforeEach(async () => {
+    runner = await createEmitterTestHost();
+  });
+
+  it("disables convenience method for multipart/mixed and reports a diagnostic", async () => {
+    const program = await typeSpecCompile(
+      `
+        model MultipartRequest {
+          id: HttpPart<string>;
+          profileImage: HttpPart<File>;
+        }
+
+        @post
+        @route("/upload")
+        op upload(
+          @header contentType: "multipart/mixed",
+          @multipartBody body: MultipartRequest,
+        ): NoContentResponse;
+      `,
+      runner,
+      { IsTCGCNeeded: true },
+    );
+    const context = createEmitterContext(program);
+    const sdkContext = await createCSharpSdkContext(context);
+    const [root, diagnostics] = createModel(sdkContext);
+
+    const operation = root.clients[0].methods[0].operation;
+    ok(operation);
+    ok(operation.requestMediaTypes?.includes("multipart/mixed"));
+    // Protocol methods are still generated, but convenience methods are turned off.
+    strictEqual(operation.generateProtocolMethod, true);
+    strictEqual(operation.generateConvenienceMethod, false);
+
+    const diagnostic = diagnostics.find(
+      (d) => d.code === "@typespec/http-client-csharp/unsupported-multipart-convenience-method",
+    );
+    ok(diagnostic);
+    strictEqual(diagnostic.severity, "warning");
+  });
+
+  it("keeps convenience method for multipart/form-data without a diagnostic", async () => {
+    const program = await typeSpecCompile(
+      `
+        model MultipartRequest {
+          profileImage: HttpPart<File>;
+        }
+
+        @post
+        @route("/upload")
+        op upload(
+          @header contentType: "multipart/form-data",
+          @multipartBody body: MultipartRequest,
+        ): NoContentResponse;
+      `,
+      runner,
+      { IsTCGCNeeded: true },
+    );
+    const context = createEmitterContext(program);
+    const sdkContext = await createCSharpSdkContext(context);
+    const [root, diagnostics] = createModel(sdkContext);
+
+    const operation = root.clients[0].methods[0].operation;
+    ok(operation);
+    ok(operation.requestMediaTypes?.includes("multipart/form-data"));
+    strictEqual(operation.generateConvenienceMethod, true);
+
+    const diagnostic = diagnostics.find(
+      (d) => d.code === "@typespec/http-client-csharp/unsupported-multipart-convenience-method",
+    );
+    strictEqual(diagnostic, undefined);
+  });
+});