NSwag/NSwag

[Route("api/[controller]")]
[ApiController]
public class FruitController : ControllerBase
{
    [HttpGet]
    [Route("all")]
    public ActionResult<IEnumerable<Fruit>> GetAll()
    {
        return Ok(new List<Fruit>(){
            new Apple { Type = "apple", Color = "red" },
            new Apple { Type = "apple", Color = "green" },
            new Banana { Type = "banana", Age = 1 },
            new Banana { Type = "banana", Age = 2 }
        });
    }
}

public class Fruit
{
    public string Type { get; set; }
}

public class Banana : Fruit
{
    public int Age { get; set; }
}

public class Apple : Fruit
{
    public string Color { get; set; }
}

public class FruitConverter : JsonInheritanceConverter
{
    public FruitConverter(string name) : base(name)
    {

    }

    public override string GetDiscriminatorValue(Type type)
    {
        return type switch
        {
            { } when type == typeof(Banana) => "banana",
            { } when type == typeof(Apple) => "apple",
            _ => throw new NotImplementedException(),
        };
    }

    protected override Type GetDiscriminatorType(JObject jObject, Type objectType, string discriminatorValue)
    {
        return discriminatorValue switch
        {
            "banana" => typeof(Banana),
            "apple" => typeof(Apple)
        };
    }
}

That's the code I'm playing around with, yet the Banana and Apple are not added in the open API schema unless I add KnownType to Fruit. Also looks like even the converter will not add the discriminator in the open API unless via Attribute.

Using it like this will not work:

.AddNewtonsoftJson(options =>
{
    options.SerializerSettings.Converters.Add(new FruitConverter("type"));
})