Usage

1. Install the package

dotnet add package AspNetCore.Json.SinglePolymorphicResponse

2. Adjust your Program.cs and add

builder.Services.AddSinglePolymorphicJsonResponse();

Original Problem

Without this package it is not possible to identify the original response type by discriminator when only a single object is returned. This problem is also open as an issue in the aspnetcore repo, but it looks like that they will fix it only with .NET 8.

Defined response

[JsonDerivedType(typeof(SummayWeatherForecast), "summary")]
[JsonDerivedType(typeof(ExtendedWeatherForecast), "extended")]
public class WeatherForecast
{
    public DateOnly Date { get; set; }
}

public class SummayWeatherForecast : WeatherForecast
{
    public string Summary { get; set; }

    public SummayWeatherForecast(string summary)
    {
        Summary = summary;
    }
}

public class ExtendedWeatherForecast : SummayWeatherForecast
{
    public int TemperatureC { get; set; }

    public ExtendedWeatherForecast(string summary) : base(summary)
    {
    }
}

Defined API

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    [HttpGet()]
    public IEnumerable<WeatherForecast> Get()
    {
        var forecast1 = new ExtendedWeatherForecast("Freezing") { Date = DateOnly.Parse("2022-11-21"), TemperatureC = -15 };
        var forecast2 = new SummayWeatherForecast("Mild") { Date = DateOnly.Parse("2022-11-20") };
        return new WeatherForecast[] { forecast1, forecast2 };
    }

    [HttpGet("Summary")]
    public WeatherForecast GetSummary()
    {
        var forecast2 = new SummayWeatherForecast("Mild") { Date = DateOnly.Parse("2022-11-20") };
        return forecast2;
    }

    [HttpGet("Extended")]
    public WeatherForecast GetExtended()
    {
        var forecast1 = new ExtendedWeatherForecast("Freezing") { Date = DateOnly.Parse("2022-11-21"), TemperatureC = -15 };
        return forecast1;
    }
}

Current behaviour

Response of route '/WeatherForecast'

[
  {
    "$type": "extended",
    "temperatureC": -15,
    "summary": "Freezing",
    "date": "2022-11-21"
  },
  {
    "$type": "summary",
    "summary": "Mild",
    "date": "2022-11-20"
  }
]

Response of route '/WeatherForecast/summary'

{
  "summary": "Mild",
  "date": "2022-11-20"
}

Response of route '/WeatherForecast/extended'

{
  "temperatureC": -15,
  "summary": "Freezing",
  "date": "2022-11-21"
}

Expected behaviour

Response of route '/WeatherForecast'

[
  {
    "$type": "extended",
    "temperatureC": -15,
    "summary": "Freezing",
    "date": "2022-11-21"
  },
  {
    "$type": "summary",
    "summary": "Mild",
    "date": "2022-11-20"
  }
]

Response of route '/WeatherForecast/summary'

{
  "$type": "summary",
  "summary": "Mild",
  "date": "2022-11-20"
}

Response of route '/WeatherForecast/extended'

{
  "$type": "extended",
  "temperatureC": -15,
  "summary": "Freezing",
  "date": "2022-11-21"
}