[http-server-csharp] add arrayDeclarationContext (#10327)

In my work in [the openai-openapi-pr
repo](https://github.com/microsoft/openai-openapi-pr), the service
emitter would not compile and gave the following error message:
```
Error: Emit context must have a scope set in order to create references to declarations.
    at compilerAssert (asset-emitter.js)
    at invokeReference (asset-emitter.js)
    at CSharpCodeEmitter.collectionDeclaration (service.js)
    at CSharpCodeEmitter.arrayDeclaration (service.js)
```
I fixed the issue when I added `arrayDeclarationContext` to the local
`node_modules` in my repo. Here is the fix that I made as well as a test
that demonstrates the issue.

Author: sophia-ramsey

.chronus/changes/sramsey-csharp-service-arrayDeclarationContext-2026-3-9-13-37-52.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+add arrayDeclarationContext 

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

@@ -214,6 +214,15 @@ export async function $onEmit(context: EmitContext<CSharpServiceEmitterOptions>)
       return "TypeSpec.Service";
     }
 
+    arrayDeclarationContext(array: Model, name: string, elementType: Type) {
+      const arrayName = ensureCSharpIdentifier(this.emitter.getProgram(), array, name);
+      const arrayFile = this.emitter.createSourceFile(`generated/models/${arrayName}.cs`);
+      arrayFile.meta[this.#sourceTypeKey] = CSharpSourceType.Model;
+      arrayFile.meta["skipEmit"] = true;
+      const arrayNamespace = this.#getOrAddNamespace(array.namespace);
+      return this.#createModelContext(arrayNamespace, arrayFile, arrayName);
+    }
+
     arrayDeclaration(array: Model, name: string, elementType: Type): EmitterOutput<string> {
       return this.collectionDeclaration(elementType, array);
     }
@@ -1265,6 +1274,10 @@ export async function $onEmit(context: EmitContext<CSharpServiceEmitterOptions>)
         return sourceFile.meta.emitted;
       }
 
+      if (sourceFile.meta["skipEmit"]) {
+        return { path: sourceFile.path, contents: "" };
+      }
+
       const emittedSourceFile: EmittedSourceFile = {
         path: sourceFile.path,
         contents: "",
@@ -1426,7 +1439,7 @@ export async function $onEmit(context: EmitContext<CSharpServiceEmitterOptions>)
             break;
         }
       }
-      return super.writeOutput(emittedSourceFiles);
+      return super.writeOutput(emittedSourceFiles.filter((f) => !f.meta["skipEmit"]));
     }
   }
 

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

@@ -48,6 +48,11 @@ function assertFileDoesNotContain(
   );
 }
 
+function assertFileNotEmitted(fs: TestFileSystem, fileName: string): void {
+  const result = [...fs.fs.entries()].filter((e) => e[0].includes(`/${fileName}`));
+  assert.strictEqual(result.length, 0, `Expected ${fileName} to not be emitted, but it was`);
+}
+
 async function compileAndValidateSingleModel(
   tester: TesterInstance,
   code: string,
@@ -3396,6 +3401,61 @@ describe("collection type: defined as emitter option", () => {
   });
 });
 
+describe("arrayDeclarationContext", () => {
+  it("emits correct types for array model declarations", async () => {
+    const fs = await compileAndValidateMultiple(
+      tester,
+      `
+        model Tags is Array<string>;
+        @route("/tags") @get op getTags(): Tags;
+        `,
+      [["IContosoOperations.cs", ["Task<string[]> GetTagsAsync( )"]]],
+    );
+    assertFileNotEmitted(fs, "Tags.cs");
+  });
+
+  it("emits correct types for array model with custom namespace", async () => {
+    const fs = await compileAndValidateMultiple(
+      tester,
+      [
+        `
+        model Items is Array<int32>;
+        @route("/items") @get op getItems(): Items;
+        `,
+        "My.Custom.Ns",
+      ],
+      [["INsOperations.cs", ["Task<int[]> GetItemsAsync( )"]]],
+    );
+    assertFileNotEmitted(fs, "Items.cs");
+  });
+
+  it("emits correct types for array model with complex element type", async () => {
+    const fs = await compileAndValidateMultiple(
+      tester,
+      `
+        model Widget {
+          id: int32;
+          name: string;
+        }
+        model WidgetList is Array<Widget>;
+        @route("/widgets") @get op getWidgets(): WidgetList;
+        `,
+      [
+        [
+          "Widget.cs",
+          [
+            "public partial class Widget",
+            "public int Id { get; set; }",
+            "public string Name { get; set; }",
+          ],
+        ],
+        ["IContosoOperations.cs", ["Task<Widget[]> GetWidgetsAsync( )"]],
+      ],
+    );
+    assertFileNotEmitted(fs, "WidgetList.cs");
+  });
+});
+
 it("emits class for model extending another model with no additional properties", async () => {
   await compileAndValidateMultiple(
     tester,