The new ASP.NET Core 3.0 Json serializer is leaving out data - asp.net-core

Question

I'm porting a web application to ASP.NET Core 3, and after a bit of a battle, I'm almost at the finish line. Everything seems to work, but all of a sudden my JSON data returned from the api is missing some levels.
It seems the options.JsonSerializerOptions.MaxDepth is default at 64 levels, so it can be that. Some other places where an option can be playing tricks on me?
This is the code (and a quickview of the value):
And this is the JSON I get in the browser:
So the ParticipantGroups property/collection is completely missing in the generated output.
Any ideas where this happens?
EDIT:
I've added a repo on Github that showcases the issue. Standard ASP.NET Core 3.0 solution, created from the template, with a change to the result returned from the Weatherforecast controller:
https://github.com/steentottrup/systemtextjsonissue

Answer 1

For now I've gone back to using Newtonsoft.Json, with the Microsoft.AspNetCore.Mvc.NewtonsoftJson package. Then when I have some time, I'll try finding out what the solution is, without Newtonsoft.Json.

Answer 2

The problem seems to be an error in the new version 3.0. At least it seems like an error to me.
It seems System.Text.Json will convert the class mentioned in the hierarchy, not the actual class. So if you are using an abstract class in the hierarchy, you're in trouble. The second I removed the base class, and used the actual class I'm returning, the problem goes away it seems.
So this doesn't work:
public class SurveyReportResult {
public Guid Id { get; set; }
public String Name { get; set; }
public Int32 MemberCount { get; set; }
public IEnumerable<OrganisationalUnit> OrganisationalUnits { get; set; }
}
public abstract class OrganisationalUnit {
public Guid Id { get; set; }
public String Name { get; set; }
public Int32 MemberCount { get; set; }
}
public class OrganisationalUnitWithParticipantGroups : OrganisationalUnit {
public IEnumerable<ParticipantGroup> ParticipantGroups { get; set; }
}
public class ParticipantGroup {
public Guid Id { get; set; }
public String Name { get; set; }
public Int32 MemberCount { get; set; }
}
This will only return the properties of the OrganisationalUnit class, not the additional property of the OrganisationalUnitWithParticipantGroups.
This works:
public class SurveyReportResult {
public Guid Id { get; set; }
public String Name { get; set; }
public Int32 MemberCount { get; set; }
public IEnumerable<OrganisationalUnitWithParticipantGroups> OrganisationalUnits { get; set; }
}
public class OrganisationalUnitWithParticipantGroups /*: OrganisationalUnit*/ {
public Guid Id { get; set; }
public String Name { get; set; }
public Int32 MemberCount { get; set; }
public IEnumerable<ParticipantGroup> ParticipantGroups { get; set; }
}
public class ParticipantGroup {
public Guid Id { get; set; }
public String Name { get; set; }
public Int32 MemberCount { get; set; }
}