MVC / Razor Pages: Verify union types work in model binding and responses

[DeagleGross]

Verify that C# union types work correctly in MVC controllers and Razor Pages for both input (model binding) and output (response serialization), and add test coverage.

Note: MVC and Razor Pages support for unions is lower priority for .NET 11

Expected behavior

Controller action with union parameter:

[ApiController]
[Route("api/orders")]
public class OrdersController : ControllerBase
{
    [HttpPost]
    public IActionResult Create([FromBody] Command command)
    {
        return Ok();
    }
}

Controller action returning union:

[HttpGet("{id}")]
public ApiResult Get(int id)
{
    if (id <= 0) return new ApiResult(new InProgressOrder(...));
    return new ApiResult(new CompletedOrder(...));
}

Razor Page with union model binding:

public class OrderModel : PageModel
{
    [BindProperty]
    public Command Command { get; set; } // union(CreateOrder, UpdateOrder)

    public IActionResult OnPost() { ... }
}

Areas to investigate

1. Input formatter (SystemTextJsonInputFormatter.cs):

   • Calls JsonSerializer.DeserializeAsync(stream, modelType, options) — should work via STJ's union converter
   • Verify no issues with modelType being a union type

2. Output formatter (SystemTextJsonOutputFormatter.cs):

   • Serialization should work transparently — STJ unpacks the union and writes the case value
   • Verify the declared return type doesn't cause the serializer to skip union-specific handling

3. Model validation:

   • If the union case value has [Required] or other validation attributes, does MVC's model validation reach into the union case?
   • Likely needs investigation — validation may only see the union wrapper, not the inner case value

4. API Explorer (DefaultApiDescriptionProvider):

   • Verify that response type descriptions correctly reflect union types
   • This may tie into MVC+OpenAPI: Support multiple Produces for same status code / content-type #65650 (multiple produces support)

[github-actions]

Triage Summary

Area: area-mvc ✅ (already labeled — issue is about MVC controllers, Razor Pages, model binding, output formatters, and API Explorer)
Type: feature-request — This is a verification/investigation task to ensure a new language feature (C# union types) works correctly with MVC infrastructure, with test coverage to be added. No existing bug is described; this is proactive work for .NET 11.

Potential Duplicates

• MVC+OpenAPI: Support multiple Produces for same status code / content-type #65650 — referenced in the issue body as related to multiple produces support / API Explorer handling of union return types (related, not a duplicate)
• No other duplicates found

Notes

• The issue already has the area-mvc label and .NET 11 Planning milestone set.
• Covers: SystemTextJsonInputFormatter, SystemTextJsonOutputFormatter, model validation reaching into union case values, and DefaultApiDescriptionProvider for API Explorer.
• The note in the issue body indicates this is lower priority for .NET 11 compared to other union type work.

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