[http-server-csharp] Emit C# class for models extending another model with no additional properties

.chronus/changes/fix-empty-derived-model-emission-2026-3-27.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Emit C# class for models that extend another model with no additional properties

packages/http-server-csharp/src/lib/utils.ts

@@ -1059,7 +1059,7 @@ export function getControllerReturnStatement(
 
 export function isEmptyResponseModel(program: Program, model: Type): boolean {
   if (model.kind !== "Model") return false;
-  if (model.properties.size === 0) return true;
+  if (model.properties.size === 0 && !model.baseModel) return true;
 
   return (
     model.properties.size === 1 &&

packages/http-server-csharp/test/generation.test.ts

@@ -3395,3 +3395,30 @@ describe("collection type: defined as emitter option", () => {
     );
   });
 });
+
+it("emits class for model extending another model with no additional properties", async () => {
+  await compileAndValidateMultiple(
+    tester,
+    `
+      model Foo {
+        id: int32;
+        name: string;
+      }
+
+      model Baz extends Foo {}
+
+      @route("/foo/{id}") @get op getFoo(id: int32): Foo;
+      `,
+    [
+      [
+        "Foo.cs",
+        [
+          "public partial class Foo",
+          "public int Id { get; set; }",
+          "public string Name { get; set; }",
+        ],
+      ],
+      ["Baz.cs", ["public partial class Baz : Foo"]],
+    ],
+  );
+});