//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 Name);

​

    public record Person(string FirstName, string LastName);

​

    public sealed class PersonConverter : DefaultConverterFactory<Person>

    {

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

​

        protected override Person Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions, JsonConverter<Person> defaultConverter)

        {

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

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

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

        }

    }

​

    public abstract class DefaultConverterFactory<T> : JsonConverterFactory

    {

        class DefaultConverter : JsonConverter<T>

        {

            readonly JsonSerializerOptions modifiedOptions;

            readonly DefaultConverterFactory<T> factory;

            readonly JsonConverter<T> defaultConverter;

            public DefaultConverter(JsonSerializerOptions options, DefaultConverterFactory<T> factory)

            {

                this.factory = factory;

                this.modifiedOptions = options.CopyAndRemoveConverter(factory.GetType());

                this.defaultConverter = (JsonConverter<T>)modifiedOptions.GetConverter(typeof(T));

            }

            public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options) => factory.Write(writer, value, modifiedOptions, defaultConverter);

​

            public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => factory.Read(ref reader, typeToConvert, modifiedOptions, defaultConverter);

        }

​

        protected virtual T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions, JsonConverter<T> defaultConverter)

            => defaultConverter.ReadOrSerialize<T>(ref reader, typeToConvert, modifiedOptions);

​

        protected virtual void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions modifiedOptions, JsonConverter<T> defaultConverter) 

            => defaultConverter.WriteOrSerialize(writer, value, modifiedOptions);

​

        public override bool CanConvert(Type typeToConvert) => typeof(T) == typeToConvert;

        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) => new DefaultConverter(options, this);

    }

​

    public static class JsonSerializerExtensions

    {

        public static JsonSerializerOptions CopyAndRemoveConverter(this JsonSerializerOptions options, Type converterType)

        {

            var copy = new JsonSerializerOptions(options);

            for (var i = copy.Converters.Count - 1; i >= 0; i--)

                if (copy.Converters[i].GetType() == converterType)

                    copy.Converters.RemoveAt(i);

            return copy;

        }

        public static void WriteOrSerialize<T>(this JsonConverter<T> converter, Utf8JsonWriter writer, T value, JsonSerializerOptions options)

        {

            if (converter != null)

                converter.Write(writer, value, options);

            else

                JsonSerializer.Serialize(writer, value, options);

        }

        public static T ReadOrSerialize<T>(this JsonConverter<T> converter, ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            if (converter != null)

                return converter.Read(ref reader, typeToConvert, options);

            else

                return (T)JsonSerializer.Deserialize(ref reader, typeToConvert, options);

        }

    }

​

    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<List<Person>>(json, options);

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

​

                Console.WriteLine(json);

                person.ForEach(p => Console.WriteLine("  " + p));

                Console.WriteLine(json2);

                Console.WriteLine();

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

                Assert.That(person.SequenceEqual(person2));

            }

        }   

        static IEnumerable<string> GetJson() =>

            new []

            {

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

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

                @"[{""name"":""Full Name""},{""name"":""Full Name""},{""lastName"":""LastName"",""firstName"":""FirstName""}]",

            };

    }

​

    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;

        }

    }

​