gistfile1.txt

public class InterfaceContractResolver : DefaultContractResolver
{
    private readonly Type[] _interfaceTypes;
 
    private readonly ConcurrentDictionary<Type, Type> _typeToSerializeMap;
 
    public InterfaceContractResolver(params Type[] interfaceTypes)
    {
        _interfaceTypes = interfaceTypes;
 
        _typeToSerializeMap = new ConcurrentDictionary<Type, Type>();
    }
 
    protected override IList<JsonProperty> CreateProperties(
        Type type,
        MemberSerialization memberSerialization)
    {
        var typeToSerialize = _typeToSerializeMap.GetOrAdd(
            type,
            t => _interfaceTypes.FirstOrDefault(
                it => it.IsAssignableFrom(t)) ?? t);
 
        return base.CreateProperties(typeToSerialize, memberSerialization);
    }
}

gistfile2.txt

JobHelper.SetSerializerSettings(new JsonSerializerSettings
{
    ContractResolver = new InterfaceContractResolver(typeof(IThingable))
});