Fix optional Content-Type headers being transformed to enums (emitter-only)

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

@@ -107,8 +107,18 @@ export function fromSdkType<T extends SdkType>(
       retVar = fromSdkArrayType(sdkContext, sdkType);
       break;
     case "constant":
+      // Don't transform optional Content-Type headers into enums - keep them as constants
+      const isContentTypeHeader =
+        sdkProperty &&
+        "kind" in sdkProperty &&
+        sdkProperty.kind === "header" &&
+        "serializedName" in sdkProperty &&
+        typeof sdkProperty.serializedName === "string" &&
+        sdkProperty.serializedName.toLocaleLowerCase() === "content-type";
+
       if (
         sdkProperty &&
+        !isContentTypeHeader &&
         (sdkProperty.optional || sdkProperty?.type.kind === "nullable") &&
         sdkProperty?.type.kind !== "boolean" &&
         sdkType.valueType.kind !== "boolean"

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

@@ -366,5 +366,95 @@ describe("Operation Converter", () => {
         strictEqual(response.bodyType, undefined);
       });
     });
+
+    describe("Optional Content-Type header", () => {
+      it("Optional body should have Content-Type remain as Constant (not transformed to enum)", async () => {
+        const program = await typeSpecCompile(
+          `
+          model BodyModel {
+            name: string;
+          }
+
+          @post
+          op withOptionalBody(@body body?: BodyModel): void;
+          `,
+          runner,
+        );
+        const context = createEmitterContext(program);
+        const sdkContext = await createCSharpSdkContext(context);
+        const root = createModel(sdkContext);
+
+        strictEqual(root.clients.length, 1);
+        strictEqual(root.clients[0].methods.length, 1);
+
+        const method = root.clients[0].methods[0];
+        ok(method);
+
+        // validate operation
+        const operation = method.operation;
+        ok(operation);
+
+        // Find Content-Type parameter
+        const contentTypeParam = operation.parameters.find((p) => p.name === "contentType");
+        ok(contentTypeParam, "Content-Type parameter should exist");
+        strictEqual(contentTypeParam.kind, "header");
+        strictEqual(contentTypeParam.serializedName, "Content-Type");
+        strictEqual(contentTypeParam.optional, true, "Content-Type should be optional");
+        strictEqual(
+          contentTypeParam.scope,
+          "Constant",
+          "Content-Type should remain Constant scope",
+        );
+        strictEqual(
+          contentTypeParam.type.kind,
+          "constant",
+          "Content-Type should remain a constant type, not transformed to enum",
+        );
+      });
+
+      it("Required body should have Content-Type with Constant scope", async () => {
+        const program = await typeSpecCompile(
+          `
+          model BodyModel {
+            name: string;
+          }
+
+          @post
+          op withRequiredBody(@body body: BodyModel): void;
+          `,
+          runner,
+        );
+        const context = createEmitterContext(program);
+        const sdkContext = await createCSharpSdkContext(context);
+        const root = createModel(sdkContext);
+
+        strictEqual(root.clients.length, 1);
+        strictEqual(root.clients[0].methods.length, 1);
+
+        const method = root.clients[0].methods[0];
+        ok(method);
+
+        // validate operation
+        const operation = method.operation;
+        ok(operation);
+
+        // Find Content-Type parameter
+        const contentTypeParam = operation.parameters.find((p) => p.name === "contentType");
+        ok(contentTypeParam, "Content-Type parameter should exist");
+        strictEqual(contentTypeParam.kind, "header");
+        strictEqual(contentTypeParam.serializedName, "Content-Type");
+        strictEqual(contentTypeParam.optional, false, "Content-Type should be required");
+        strictEqual(
+          contentTypeParam.scope,
+          "Constant",
+          "Content-Type should have Constant scope for required body",
+        );
+        strictEqual(
+          contentTypeParam.type.kind,
+          "constant",
+          "Content-Type should be a constant type",
+        );
+      });
+    });
   });
 });