using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Runtime.Serialization.Formatters;

using System.IO;

using System.Data;

using System.ComponentModel.DataAnnotations;

using System.Globalization;

using System.Reflection;

using System.Collections.ObjectModel;

​

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Converters;

using Newtonsoft.Json.Serialization;    

​

using NUnit.Framework; // Throws NUnit.Framework.AssertionException

​

//https://stackoverflow.com/questions/67775174/customising-json-net-serialisation-based-on-compile-time-type

​

    class MyConverter<TDeclared> : JsonConverter<TDeclared> where TDeclared : A

    {

        public override void WriteJson( JsonWriter writer,  TDeclared value, JsonSerializer serializer)

        { 

            // This is just a mockup, your question doesn't specify your requirements.

            writer.WriteStartObject();

            if (typeof(TDeclared) != value.GetType())

            {

                //By in the case of CA, MyConverter should tag what it serialises in some defined way

                writer.WritePropertyName("isB");

                writer.WriteValue(value.GetType() == typeof(B));

            }

            // Add whatever logic you need to serialize A

            // Add whatever additional logic you need to serialize B

            // And end the object

            writer.WriteEndObject();

        }

        public override TDeclared ReadJson(JsonReader reader, Type objectType, TDeclared existingValue, bool hasExistingValue, JsonSerializer serializer) => throw new NotImplementedException();

    }

​

    class CA {

        [JsonProperty] // This is only required because your question did not make this member public

        [JsonConverter(typeof(MyConverter<A>))]

        A _x;

        public CA(A x) { _x = x; }

    }

​

    class CB { 

        [JsonProperty] // This is only required because your question did not make this member public

        [JsonConverter(typeof(MyConverter<B>))]

        B _x;

        public CB(B x) { _x = x; }

    }

​

    [JsonConverter(typeof(MyConverter<A>))]

    class A {

    }

​

    [JsonConverter(typeof(MyConverter<B>))]

    class B : A {

    }

    class TestClass

    {

        public static void Test()

        {

            B b = new B();

            CA ca = new CA(b);

            CB cb = new CB(b);

​

            string caStr = JsonConvert.SerializeObject(ca);

            string cbStr = JsonConvert.SerializeObject(cb);

            Console.WriteLine("Result of serializing {0}:", ca);

            Console.WriteLine(caStr);

            Console.WriteLine("Result of serializing {0}:", cb);

            Console.WriteLine(cbStr);

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , GetNetCoreVersion());

            Console.WriteLine("{0} version: {1}", typeof(JsonSerializer).Assembly.GetName().Name,  typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

        public static string GetNetCoreVersion()

        {

            //https://techblog.dorogin.com/how-to-detect-net-core-version-in-runtime-ecd65ad695be

            //

            var assembly = typeof(System.Runtime.GCSettings).GetTypeInfo().Assembly;

            var assemblyPath = assembly.Location.Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);

            int netCoreAppIndex = Array.IndexOf(assemblyPath, "Microsoft.NETCore.App");

            if (netCoreAppIndex > 0 && netCoreAppIndex < assemblyPath.Length - 2)

                return assemblyPath[netCoreAppIndex + 1];

            return null;

        }

    }

​