Type discriminator is lost when serializing a Result<T> using [TranslateResultToActionResult]

[uflowie]

When serializing a derived type as its base type, System.Text.Json will use an extra type discriminator field to communicate the actual type of the serialized data. Given a base class Base and a derived type Derived, I have observed the following behavior:

1. A controller endpoint returning Base will correctly include the type discriminator.
2. A controller endpoint returning ActionResult<Base> will include the type discriminator.
3. A controller endpoint returning Result<Base> will include the properties of the derived type but it will NOT include the type discriminator.

Minimal reproducible example: https://github.com/uflowie/ResultLostTypeDiscriminator

[ardalis]

Is this causing a real world problem? Your controllers should typically never return just an object and not an ActionRest, in order to support NotFound, etc.

[uflowie]

Our controllers aren't returning just an object, they are returning a Result. But since the type discriminator for T is lost (unlike if I returned T directly), I cannot have polymorphic endpoints returning Result

[ardalis]

Right but based on what you're writing above, if they returned ActionResult<Result<T>> everything would work correctly.

[uflowie]

ActionResult<Result<T>> does not work either, only ActionResult<T>

[marcelbarner]

I had a small look into it, the problem seems to be that in the ObjectResult the DeclaredType is null.

So when I add the DeclaredType to the ObjectResult I get the discriminator.

public ActionResult<WeatherForecast> Get(int id)
{
    var a = Result.Success<WeatherForecast>(new WeatherForecastDetails
    {
        Date = DateOnly.MaxValue,
        TemperatureC = Random.Shared.Next(-20, 55),
        Summary = "summ",
        RainingValue = id
    }).ToActionResult(this);
    ((ObjectResult)a.Result).DeclaredType = typeof(WeatherForecast);
    return a;
}