//https://stackoverflow.com/questions/65430420/how-to-use-default-serialization-in-a-custom-system-text-json-jsonconverter

using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Reflection;

​

using System.Text.Json;

using System.Text.Json.Serialization;

​

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

​

//How can I use default serialization inside a System.Text.Json JsonConverter?

    public record Person(string FirstName, string LastName);

​

    public sealed class PersonConverter : JsonConverter<Person>

    {

        record PersonDTO(string FirstName, string LastName, string Name); // A DTO with both the old and new properties.

        public override Person Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            var dto = JsonSerializer.Deserialize<PersonDTO>(ref reader, options);

            var oldNames = dto?.Name?.Split(' ', StringSplitOptions.RemoveEmptyEntries) ?? Enumerable.Empty<string>();

            return new Person(dto.FirstName ?? oldNames.FirstOrDefault(), dto.LastName ?? oldNames.LastOrDefault());

        }

​

        public override void Write(Utf8JsonWriter writer, Person person, JsonSerializerOptions options)

            => // What do I do here? I want to preserve other options such as options.PropertyNamingPolicy, which are lost by the following call

            JsonSerializer.Serialize(writer, person);

    }

​

    class TestClass

    {

        public static void Test()

        {

            foreach (var json in GetJson())

            {

                var options = new JsonSerializerOptions

                {

                    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                    Converters = { new PersonConverter() },

                };

                var person = JsonSerializer.Deserialize<Person>(json, options);

                var json2 = JsonSerializer.Serialize(person, options);

​

                Console.WriteLine(json);

                Console.WriteLine(person);

                Console.WriteLine(json2);

                Console.WriteLine();

                var person2 = JsonSerializer.Deserialize<Person>(json2, options);

                Assert.AreEqual(person, person2);

            }

        }   

        static IEnumerable<string> GetJson() =>

            new []

            {

                @"{""lastName"":""LastName"",""firstName"":""FirstName""}",

                @"{""name"":""Full Name""}",

            };

    }

​

    public class Program

    {

        public static void Main()

        {

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

            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;

            }

        }

        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;

        }

    }

​