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 System.Text.Json;

using System.Text.Json.Serialization;

using System.Text.Json.Serialization.Metadata;

using System.Text.Json.Nodes;

​

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

​

#nullable enable

​

//https://stackoverflow.com/questions/76446659/how-to-customize-system-text-json-serialization-of-certain-fields

​

    public interface I3rdParyObject

    {

        public string Id { get; }

        // Remainder not shown in your question

    }

​

    class Concrete3rdParyObject : I3rdParyObject

    {

        public string Id { get; init; }

        public string SomeMoreJunk { get; set; } = "this should not appear!";

    }

​

    class MyClass

    {

        public int Id;

        public I3rdParyObject Data;

    }

​

    public static class DataManager

    {

        public static I3rdParyObject GetDataById(string id) => new Concrete3rdParyObject { Id = id };

​

        public static string GetDataId(I3rdParyObject obj) => ((Concrete3rdParyObject)obj)?.Id ?? "";

    }

​

    public class I3rdParyObjectConverter : JsonConverter<I3rdParyObject>

    {

        public override void Write(Utf8JsonWriter writer, I3rdParyObject value, JsonSerializerOptions options) =>

            writer.WriteStringValue(DataManager.GetDataId(value));

        public override I3rdParyObject Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => 

            DataManager.GetDataById(reader.GetString()!);

    }

​

    class TestClass

    {

        public static void Test()

        {

            var myClass = new MyClass

            {

                Id = 101,

                Data = DataManager.GetDataById("abc"),

            };

            var options = new JsonSerializerOptions

            {

                Converters = { new I3rdParyObjectConverter() },

                // Other options as required

                IncludeFields = true,  // You must set this if MyClass.Id and MyClass.Data are really fields not properties.

            };

            var json = JsonSerializer.Serialize(myClass, options);

            Console.WriteLine(json);

            var myClass2 = JsonSerializer.Deserialize<MyClass>(json, options);

            Assert.AreEqual(myClass.Data.Id, myClass2?.Data.Id);

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1}), {2}", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);

            Console.WriteLine("System.Text.Json version: " + typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

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

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​