Add @useSystemTextJsonConverter decorator for csharp

.chronus/changes/json-converter-2024-6-24-16-48-41.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Add `@hasJsonConverter` for csharp only to indicate if JSON converter is needed

docs/libraries/typespec-client-generator-core/reference/decorators.md

@@ -557,3 +557,30 @@ model Origin {
 @get
 op getModel(): Fish;
 ```
+
+## Azure.ClientGenerator.Core.CSharp
+
+### `@hasJsonConverter` {#@Azure.ClientGenerator.Core.CSharp.hasJsonConverter}
+
+Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+
+```typespec
+@Azure.ClientGenerator.Core.CSharp.hasJsonConverter
+```
+
+#### Target
+
+`Model`
+
+#### Parameters
+
+None
+
+#### Examples
+
+```typespec
+@hasJsonConverter
+model MyModel {
+  prop: string;
+}
+```

docs/libraries/typespec-client-generator-core/reference/index.mdx

@@ -51,3 +51,9 @@ npm install --save-peer @azure-tools/typespec-client-generator-core
 - [`@operationGroup`](./decorators.md#@Azure.ClientGenerator.Core.operationGroup)
 - [`@protocolAPI`](./decorators.md#@Azure.ClientGenerator.Core.protocolAPI)
 - [`@usage`](./decorators.md#@Azure.ClientGenerator.Core.usage)
+
+## Azure.ClientGenerator.Core.CSharp
+
+### Decorators
+
+- [`@hasJsonConverter`](./decorators.md#@Azure.ClientGenerator.Core.CSharp.hasJsonConverter)

packages/typespec-client-generator-core/README.md

@@ -564,3 +564,32 @@ model Origin {
 @get
 op getModel(): Fish;
 ```
+
+### Azure.ClientGenerator.Core.CSharp
+
+- [`@hasJsonConverter`](#@hasjsonconverter)
+
+#### `@hasJsonConverter`
+
+Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+
+```typespec
+@Azure.ClientGenerator.Core.CSharp.hasJsonConverter
+```
+
+##### Target
+
+`Model`
+
+##### Parameters
+
+None
+
+##### Examples
+
+```typespec
+@hasJsonConverter
+model MyModel {
+  prop: string;
+}
+```

packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.CSharp.ts

@@ -0,0 +1,14 @@
+import type { DecoratorContext, Model } from "@typespec/compiler";
+
+/**
+ * Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+ *
+ * @example
+ * ```typespec
+ * @hasJsonConverter
+ * model MyModel {
+ *   prop: string;
+ * }
+ * ```
+ */
+export type HasJsonConverterDecorator = (context: DecoratorContext, target: Model) => void;

packages/typespec-client-generator-core/generated-defs/Azure.ClientGenerator.Core.CSharp.ts-test.ts

@@ -0,0 +1,12 @@
+/** An error here would mean that the decorator is not exported or doesn't have the right name. */
+import { $hasJsonConverter } from "@azure-tools/typespec-client-generator-core";
+import type { HasJsonConverterDecorator } from "./Azure.ClientGenerator.Core.CSharp.js";
+
+type Decorators = {
+  $hasJsonConverter: HasJsonConverterDecorator;
+};
+
+/** An error here would mean that the exported decorator is not using the same signature. Make sure to have export const $decName: DecNameDecorator = (...) => ... */
+const _: Decorators = {
+  $hasJsonConverter,
+};

packages/typespec-client-generator-core/lib/decorators.tsp

@@ -419,3 +419,18 @@ extern dec access(
  */
 #deprecated "@flattenProperty decorator is not recommended to use."
 extern dec flattenProperty(target: ModelProperty, scope?: valueof string);
+
+namespace CSharp {
+  /**
+   * Whether a model needs the custom JSON converter, this is only used for backward compatibility for csharp.
+   *
+   * @example
+   * ```typespec
+   * @hasJsonConverter
+   * model MyModel {
+   *   prop: string;
+   * }
+   * ```
+   */
+  extern dec hasJsonConverter(target: Model);
+}

packages/typespec-client-generator-core/src/decorators.ts

@@ -26,6 +26,7 @@ import {
   isTemplateDeclarationOrInstance,
   listServices,
   projectProgram,
+  setTypeSpecNamespace,
   validateDecoratorUniqueOnNode,
 } from "@typespec/compiler";
 import { isHeader } from "@typespec/http";
@@ -1056,3 +1057,10 @@ export function getClientNameOverride(
 ): string | undefined {
   return getScopedDecoratorData(context, clientNameKey, entity, languageScope);
 }
+
+export const $hasJsonConverter: DecoratorFunction = (
+  context: DecoratorContext,
+  entity: Model
+) => {};
+
+setTypeSpecNamespace("CSharp", $hasJsonConverter);

packages/typespec-client-generator-core/test/test-host.ts

@@ -77,6 +77,7 @@ export async function createSdkTestRunner(
   const host = await createSdkTestHost(options);
   let autoUsings = [
     "Azure.ClientGenerator.Core",
+    "Azure.ClientGenerator.Core.CSharp",
     "TypeSpec.Rest",
     "TypeSpec.Http",
     "TypeSpec.Versioning",

packages/typespec-client-generator-core/test/types/general-decorators-list.test.ts

@@ -327,4 +327,29 @@ describe("typespec-client-generator-core: general decorators list", () => {
       ]);
     });
   });
+
+  describe("csharp only decorator", () => {
+    it("@hasJsonConverter", async function () {
+      runner = await createSdkTestRunner(
+        {},
+        { additionalDecorators: ["Azure\\.ClientGenerator\\.Core\\.CSharp\\.@hasJsonConverter"] }
+      );
+
+      await runner.compileWithBuiltInService(`
+        @hasJsonConverter
+        model A {
+          id: string;
+        }
+
+        op test(): A;
+      `);
+
+      const models = runner.context.sdkPackage.models;
+      strictEqual(models.length, 1);
+      deepStrictEqual(models[0].decorators, [
+        { name: "Azure.ClientGenerator.Core.CSharp.@hasJsonConverter", arguments: {} },
+      ]);
+      expectDiagnostics(runner.context.diagnostics, []);
+    });
+  });
 });