Type disciminator is missing with OkResult

[vanillajonathan]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I am using polymorphic JSON which works when returning the class but not when returning Ok(data) because then the $type type discriminator is missing.

Works:

[HttpGet(Name = "Pet")]
public ActionResult<BaseModel> Get()
{
    return new Cat() { CatName = "Mizzy" };
}

Does not work:

[HttpGet(Name = "Pet")]
public ActionResult<BaseModel> Get()
{
    return Ok(new Cat() { CatName = "Mizzy" });
}

Also does not work:

[HttpGet(Name = "Pet")]
public ActionResult<BaseModel> Get()
{
    return Ok(new Cat() { CatName = "Mizzy" } as BaseModel);
}

Expected Behavior

When using the Ok method to return a OkObjectResult it should output a type discriminator.

Steps To Reproduce

using Microsoft.AspNetCore.Mvc;
using System.Text.Json.Serialization;

namespace WebApplication1.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class ExampleController : ControllerBase
    {
        [HttpGet(Name = "GetPet")]
        public ActionResult<BaseModel> Get()
        {
            return Ok(new Cat() { CatName = "Mizzy" } as BaseModel);
        }
    }

    [JsonPolymorphic]
    [JsonDerivedType(typeof(Cat), "cat")]
    [JsonDerivedType(typeof(Dog), "dog")]
    public class BaseModel
    {
        public string Name { get; set; }
    }

    public class Cat : BaseModel
    {
        public string CatName { get; set; }
    }

    class Dog : BaseModel
    {
        public string DogName { get; set; }
    }
}

Exceptions (if any)

No response

.NET Version

8.0.10

Anything else?

No response

[mikekistler]

Thank you for filing this issue and providing clear and complete information on how to reproduce it. I have reproduced it and done some investigation. I believe that this is happening because when the result value is wrapped in "Ok" the type information is lost / not conveyed to System.Text.Json, which treats is as just a regular object rather than a Cat.

I will continue investigating and post here when I have something concrete.