//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.Text.Json;

using System.Text.Json.Serialization;

using System.Text.Json.Nodes;

​

using Newtonsoft.Json.Linq;

​

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

​

//https://stackoverflow.com/questions/74856371/c-sharp-jsonserializer-deserialize-fails-if-property-has-null-value-despite-json

​

    public class WebResponse

    {     

        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]   

        [JsonConverter(typeof(NullTextValueForNullObjectConverter<Config>))]

        public Config Config { get; set; }  

        public string Message { get; set; } 

    }

​

    public class Config

    {

        public string C1 { get; set; }

        public string C2 { get; set; }

    }

​

    public sealed class NullTextValueForNullObjectConverter<T> : DefaultConverterFactory<T> where T : class

    {

        const string NullValue = "null";

        protected override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions) 

        {

            if (reader.TokenType == JsonTokenType.String)

            {

                var s = reader.GetString();

                if (string.Equals(s, NullValue, StringComparison.OrdinalIgnoreCase)) // Or use StringComparison.Ordinal if you are sure the text "null" will always be lowercase

                    return null;

            }

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

        }

    }

​

    public abstract class DefaultConverterFactory<T> : JsonConverterFactory

    {

        class DefaultConverter : JsonConverter<T>

        {

            readonly JsonSerializerOptions modifiedOptions;

            readonly DefaultConverterFactory<T> factory;

​

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

            {

                this.factory = factory;

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

            }

​

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

​

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

        }

​

        protected virtual T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions)

            => (T)JsonSerializer.Deserialize(ref reader, typeToConvert, modifiedOptions);

​

        protected virtual void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions modifiedOptions) 

            => JsonSerializer.Serialize(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;

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            foreach (var jsonWithConfig in GetJson())

            {

                Console.WriteLine(jsonWithConfig);

​

                var options = new JsonSerializerOptions

                {

                    Converters = { new NullTextValueForNullObjectConverter<Config>() },

                    // Other options as required

                    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,

                    PropertyNameCaseInsensitive = true

                };

                var webResponseWithConfig = JsonSerializer.Deserialize<WebResponse>(jsonWithConfig, options);

                webResponseWithConfig.Dump();

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

​

                Console.WriteLine(json2);

                Console.WriteLine();

                // Assert that webResponseWithConfig was properly populated.

                var jsonWithConfigNode = JsonNode.Parse(jsonWithConfig);

                var configOriginalJson = JsonSerializer.Serialize(jsonWithConfigNode["config"]);

                Assert.AreEqual(webResponseWithConfig.Message, jsonWithConfigNode["message"].ToString());

                if (webResponseWithConfig.Config == null)

                {

                    Assert.That(configOriginalJson == "null" || configOriginalJson == "\"null\"");

                }

                else if (webResponseWithConfig.Config != null)

                {

                    var options2 = new JsonSerializerOptions

                    {

                        Converters = { new NullTextValueForNullObjectConverter<Config>() },

                        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,

                        PropertyNamingPolicy = JsonNamingPolicy.CamelCase, // The incoming JSON was actually camel case

                    };

                    Assert.AreEqual(configOriginalJson, JsonSerializer.Serialize(webResponseWithConfig.Config, options2));

                }

            }

        }   

        static IEnumerable<string> GetJson() =>

            new []

            {

                """

                {"config":{"c1":"All","c2":"is peachy"},"message":"We found a config object"}

                """,

                """

                {"config":"null","message":"Config is \"null\""}

                """

​

            };

    }

​

    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;

            }

        }

    }

​