fix(csharp): throw JsonException/NotSupportedException in SystemTextJson converters

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

@@ -375,8 +375,11 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
         this.emitLine("case JsonTokenType.", tokenType, ":");
     }
 
-    private emitThrow(message: Sourcelike): void {
-        this.emitLine("throw new Exception(", message, ");");
+    private emitThrow(
+        exceptionType: "JsonException" | "NotSupportedException",
+        message: Sourcelike,
+    ): void {
+        this.emitLine("throw new ", exceptionType, "(", message, ");");
     }
 
     private deserializeTypeCode(typeName: Sourcelike): Sourcelike {
@@ -1239,7 +1242,7 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
                         (v) => this.emitLine("return ", v, ";"),
                     );
                     if (!allHandled) {
-                        this.emitThrow([
+                        this.emitThrow("JsonException", [
                             '"Cannot unmarshal type ',
                             csType,
                             '"',
@@ -1266,7 +1269,7 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
                             () => this.emitLine("return;"),
                         );
                         if (!allHandled) {
-                            this.emitThrow([
+                            this.emitThrow("NotSupportedException", [
                                 '"Cannot marshal type ',
                                 csType,
                                 '"',

test/unit/csharp-system-text-json-exceptions.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, test } from "vitest";
+
+import {
+    InputData,
+    JSONSchemaInput,
+    quicktype,
+} from "../../packages/quicktype-core/src/index.js";
+
+async function renderSystemTextJsonCSharp(schema: object): Promise<string> {
+    const schemaInput = new JSONSchemaInput(undefined);
+    await schemaInput.addSource({
+        name: "TopLevel",
+        schema: JSON.stringify(schema),
+    });
+
+    const inputData = new InputData();
+    inputData.addInput(schemaInput);
+
+    const result = await quicktype({
+        inputData,
+        lang: "csharp",
+        rendererOptions: { framework: "SystemTextJson" },
+    });
+    return result.lines.join("\n");
+}
+
+describe("C# System.Text.Json converters", () => {
+    test("throw serializer-supported exceptions for union conversion failures", async () => {
+        const output = await renderSystemTextJsonCSharp({
+            type: "object",
+            properties: {
+                mixed: { oneOf: [{ type: "integer" }, { type: "string" }] },
+            },
+            required: ["mixed"],
+        });
+
+        expect(output).toContain(
+            'throw new JsonException("Cannot unmarshal type Mixed");',
+        );
+        expect(output).toContain(
+            'throw new NotSupportedException("Cannot marshal type Mixed");',
+        );
+        expect(output).not.toContain("throw new Exception(");
+    });
+});