fix(csharp): make generated Serialize/Converter classes partial

packages/quicktype-core/src/language/CSharp/NewtonSoftCSharpRenderer.ts

@@ -408,7 +408,7 @@ export class NewtonsoftCSharpRenderer extends CSharpRenderer {
         this.emitType(
             undefined,
             AccessModifier.Public,
-            "static class",
+            "static partial class",
             "Serialize",
             undefined,
             () => {
@@ -471,7 +471,7 @@ export class NewtonsoftCSharpRenderer extends CSharpRenderer {
         this.emitType(
             undefined,
             AccessModifier.Internal,
-            "static class",
+            "static partial class",
             converterName,
             undefined,
             () => {

packages/quicktype-core/src/language/CSharp/SystemTextJsonCSharpRenderer.ts

@@ -413,7 +413,7 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
         this.emitType(
             undefined,
             AccessModifier.Public,
-            "static class",
+            "static partial class",
             "Serialize",
             undefined,
             () => {
@@ -484,7 +484,7 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
         this.emitType(
             undefined,
             AccessModifier.Internal,
-            "static class",
+            "static partial class",
             converterName,
             undefined,
             () => {

test/fixtures/csharp-SystemTextJson/Issue1520.cs

@@ -0,0 +1,6 @@
+namespace QuickType
+{
+    // Simulates partial helper declarations from another generated file.
+    public static partial class Serialize { }
+    internal static partial class Converter { }
+}

test/fixtures/csharp/Issue1520.cs

@@ -0,0 +1,6 @@
+namespace QuickType
+{
+    // Simulates partial helper declarations from another generated file.
+    public static partial class Serialize { }
+    internal static partial class Converter { }
+}

test/unit/csharp-partial-helpers.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, test } from "vitest";
+
+import {
+    InputData,
+    type RendererOptions,
+    jsonInputForTargetLanguage,
+    quicktype,
+} from "quicktype-core";
+
+async function cSharpFor(framework: string): Promise<string> {
+    const jsonInput = jsonInputForTargetLanguage("csharp");
+    await jsonInput.addSource({
+        name: "Something",
+        samples: ['{"some_property":"hello"}'],
+    });
+    const inputData = new InputData();
+    inputData.addInput(jsonInput);
+    const rendererOptions = { framework } as RendererOptions;
+    const result = await quicktype({
+        inputData,
+        lang: "csharp",
+        rendererOptions,
+    });
+    return result.lines.join("\n");
+}
+
+describe("C# helper classes", () => {
+    for (const framework of ["NewtonSoft", "SystemTextJson"]) {
+        test(`${framework} helpers are partial`, async () => {
+            const output = await cSharpFor(framework);
+            expect(output).toContain("public static partial class Serialize");
+            expect(output).toContain("internal static partial class Converter");
+        });
+    }
+});