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

​

//https://stackoverflow.com/questions/63418549/custom-json-serializer-for-optional-property-with-system-text-json

​

    public readonly struct Optional<T>

    {

        public Optional(T value)

        {

            this.HasValue = true;

            this.Value = value;

        }

​

        public bool HasValue { get; }

        public T Value { get; }

        public static implicit operator Optional<T>(T value) => new Optional<T>(value);

        public override string ToString() => this.HasValue ? (this.Value?.ToString() ?? "null") : "unspecified";

    }

​

    public class CustomType

    {

        [JsonPropertyName("foo")]

        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]

        public Optional<int?> Foo { get; set; }

​

        [JsonPropertyName("bar")]

        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]

        public Optional<int?> Bar { get; set; }

​

        [JsonPropertyName("baz")]

        [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]

        public Optional<int?> Baz { get; set; }

    }

​

    public class OptionalConverter : JsonConverterFactory

    {

        public override bool CanConvert(Type typeToConvert)

        {

            if (!typeToConvert.IsGenericType) { return false; }

            if (typeToConvert.GetGenericTypeDefinition() != typeof(Optional<>)) { return false; }

            return true;

        }

​

        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)

        {

            Type valueType = typeToConvert.GetGenericArguments()[0];

​

            return (JsonConverter)Activator.CreateInstance(

                type: typeof(OptionalConverterInner<>).MakeGenericType(new Type[] { valueType }),

                bindingAttr: BindingFlags.Instance | BindingFlags.Public,

                binder: null,

                args: null,

                culture: null

            );

        }

​

        private class OptionalConverterInner<T> : JsonConverter<Optional<T>>

        {

            public override Optional<T> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

            {

                T value = JsonSerializer.Deserialize<T>(ref reader, options);

                return new Optional<T>(value);

            }

​

            public override void Write(Utf8JsonWriter writer, Optional<T> value, JsonSerializerOptions options) =>

                JsonSerializer.Serialize(writer, value.Value, options);

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            var options = new JsonSerializerOptions();

            options.Converters.Add(new OptionalConverter());

​

            string json = @"{""foo"":0,""bar"":null}";

            CustomType parsed = JsonSerializer.Deserialize<CustomType>(json, options);

            string roundtrippedJson = JsonSerializer.Serialize(parsed, options);

​

            // json and roundtrippedJson should be equivalent

            Console.WriteLine("json:             " + json);

            Console.WriteLine("roundtrippedJson: " + roundtrippedJson);

            Assert.AreEqual(json, roundtrippedJson);

        }   

    }

​

    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;

        }

    }

​