JSON polymorphic serialization does not add $type to single controller results

[fabsenet]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

dotnet/runtime#77044 added polymorphic serialization support to System.Text.Json and using it inside MVC controllers to return JSON with $type properties works in List<> cases but not if a single item is returned.

What I think happens under the covers after the controller action:

var json = JsonSerializer.Serialize<DerivedModel>(derivedModel); //will not write $type

what should happen:

var json = JsonSerializer.Serialize<BaseModel>(derivedModel); //will write $type !!!

Expected Behavior

$type gets written to the output in all cases if [JsonPolymorphic] is present.

Steps To Reproduce

dotnet new mvc -n JsonTypeMissingRepro

add controller file:

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

namespace JsonTypeMissingRepro.Controllers
{
    [JsonPolymorphic]
    [JsonDerivedType(typeof(DerivedModel), nameof(DerivedModel))]
    [JsonDerivedType(typeof(DerivedDerivedModel), nameof(DerivedDerivedModel))]
    public abstract record BaseModel
    {
        public required string Hello { get; init; }
    }

    public record DerivedModel : BaseModel
    {
        public required string World { get; init; }
    }


    public record DerivedDerivedModel : DerivedModel
    {
        public string? Foo { get; init; }
    }

    [ApiController]
    [Route("api/[controller]")]
    [Produces("application/json")]
    public class ReproController : Controller
    {
        [HttpGet]
        [Route("GetAll")]
        public ActionResult<IList<BaseModel>> GetAll()
        {
            return Ok(new List<BaseModel>() { new DerivedModel { Hello = "Hello1", World = "World2" } });
            // [{"$type":"DerivedModel","world":"World2","hello":"Hello1"}]
        }

        [HttpGet]
        [Route("GetOne")]
        public ActionResult<BaseModel> GetOne()
        {
            return Ok(new DerivedModel { Hello = "Hello1", World = "World2" });
            // {"world":"World2","hello":"Hello1"}
            // ^^^^^^ No $type!!!
        }
    }
}

Call /api/Repro/GetOne and compare it to /api/Repro/GetAll. You will see a missing $type property.

Exceptions (if any)

No response

.NET Version

7.0.100

Anything else?

.NET SDK:
Version: 7.0.100
Commit: e12b7af219

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19045
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\7.0.100\

Host:
Version: 7.0.0
Architecture: x64
Commit: d099f075e4

.NET SDKs installed:
3.1.425 [C:\Program Files\dotnet\sdk]
5.0.407 [C:\Program Files\dotnet\sdk]
5.0.408 [C:\Program Files\dotnet\sdk]
5.0.414 [C:\Program Files\dotnet\sdk]
7.0.100 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 3.1.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.25 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.31 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.24 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.25 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.31 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.24 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.25 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.31 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.10 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

[fabsenet]

using this workaround for now: dotnet/runtime#77532

[brunolins16]

@fabsenet Duplicate of # #44852 and we are working on a fix for this #45405