Derived type's properties missing in JSON response from ASP.NET Core API

Question

The JSON response from my ASP.NET Core 3.1 API controller is missing properties. This happens when a property uses a derived type; any properties defined in the derived type but

Answer 1

The documentation shows how to serialize as the derived class when calling the serializer directly. The same technique can also be used in a custom converter that we then can tag our classes with.

First, create a custom converter

public class AsRuntimeTypeConverter : JsonConverter
{
    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return JsonSerializer.Deserialize(ref reader, options);
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        JsonSerializer.Serialize(writer, value, value?.GetType() ?? typeof(object), options);
    }
}

Then mark the relevant classes to be used with the new converter

[JsonConverter(typeof(AsRuntimeTypeConverter))]
public class MyBaseClass
{
   ...

Alternately, the converter can be registered in startup.cs instead

services
  .AddControllers(options =>
     .AddJsonOptions(options =>
            {
                options.JsonSerializerOptions.Converters.Add(new AsRuntimeTypeConverter());
            }));