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

​

    //[JsonConverter(typeof(EntityConverter))] -- this won't work

    public class EventDefinition

    {

        public int EventDefinitionId { get; set; }

    }

​

//https://stackoverflow.com/questions/65744572/custom-serialization-of-null-values-in-asp-net-core-webapi

​

    public class EntityConverter : DefaultConverterFactory<EventDefinition>

    {

        protected override bool HandleNull => true;

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

        {

            var result = base.Read(ref reader, typeToConvert, modifiedOptions, defaultConverter);

            return result?.EventDefinitionId == 0 ? null : result;

        }

        protected override void Write(Utf8JsonWriter writer, EventDefinition value, JsonSerializerOptions modifiedOptions, JsonConverter<EventDefinition> defaultConverter) 

        {

            value ??= new EventDefinition { EventDefinitionId = 0 };

            base.Write(writer, value, modifiedOptions, defaultConverter);

        }

    }

​

    public abstract class DefaultConverterFactory<T> : JsonConverterFactory

    {

        class NullHandlingDefaultConverter : DefaultConverter

        {

            public NullHandlingDefaultConverter(JsonSerializerOptions options, DefaultConverterFactory<T> factory) : base(options, factory) { }

            public override bool HandleNull => true;

        }

        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 ?? throw new ArgumentNullException();

                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);

​

        protected virtual bool HandleNull => false;

​

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

​

        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) => HandleNull ? new NullHandlingDefaultConverter(options, this) : 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()

        {

            var events = new List<EventDefinition>

            {

                new () { EventDefinitionId = 101 },

                null,

                new () { EventDefinitionId = 202 }

            };

            var options = new JsonSerializerOptions

            {

                Converters = { new EntityConverter() },

                WriteIndented = true,

            };

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

            Console.WriteLine("Serializing the {0} list with {1}:", nameof(EventDefinition), nameof(EntityConverter));

            Console.WriteLine(json);

            var events2 = JsonSerializer.Deserialize<List<EventDefinition>>(json, options);

            Assert.IsTrue(events.Select(e => e?.EventDefinitionId).SequenceEqual(events2.Select(e => e?.EventDefinitionId)));

            Console.WriteLine("Serializing the {0} list without {1}:", nameof(EventDefinition), nameof(EntityConverter));

            var json2 = JsonSerializer.Serialize(events2, new JsonSerializerOptions { WriteIndented = true });

            Console.WriteLine(json2);

        }   

    }

​

    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;

        }

    }

​