using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Reflection;

using System.Diagnostics;

​

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Converters;

using Newtonsoft.Json.Serialization;    

​

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

​

#nullable enable

​

//https://stackoverflow.com/questions/76221332/differentiate-between-skipped-value-and-null-value-in-json?noredirect=1#comment134416276_76221332

​

    public interface IHasValue

    {

        bool HasValue { get; }

        object? GetValue();

    }

​

    [JsonConverter(typeof(OptionalConverter))]

    public readonly struct Optional<T> : IHasValue

    {

        //Taken from https://stackoverflow.com/q/63418549/3744182

        //By https://stackoverflow.com/users/547733/maxime-rossini

        public Optional(T value) => (this.HasValue, this.Value) = (true, value);

        public bool HasValue { get; }

        public T Value { get; }

        object? IHasValue.GetValue() => Value;

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

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

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

    }

​

    class OptionalConverter : JsonConverter

    {

        static Type? GetValueType(Type objectType) =>

            objectType.IsGenericType && objectType.GetGenericTypeDefinition() == typeof(Optional<>) ? objectType.GetGenericArguments()[0] : null;

        public override bool CanConvert(Type objectType) => GetValueType(objectType) != null;

​

        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)

        {

            var valueType = GetValueType(objectType) ?? throw new ArgumentException(objectType.ToString());

            var value = serializer.Deserialize(reader, valueType);

            return Activator.CreateInstance(objectType, value);

        }

​

        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)

            => serializer.Serialize(writer, ((IHasValue?)value)?.GetValue());

    }

​

    public class Tmp2

    {

        [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)] 

        public Optional<int> A { get; set; }

        [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]

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

        [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]

        public Optional<string> SomeString { get; set; }

    } 

​

    namespace V2

    {

        public record Tmp2(Optional<int> A, Optional<int?> B);

    }

​

    public class OptionalResolver : DefaultContractResolver

    {

        protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)

        {

            var property = base.CreateProperty(member, memberSerialization);

            if (property.ValueProvider != null && property.Readable && (typeof(IHasValue).IsAssignableFrom(property.PropertyType) || typeof(IHasValue).IsAssignableTo(property.PropertyType)))

            {

                var old = property.ShouldSerialize;

                Predicate<object> shouldSerialize = (o) => property.ValueProvider.GetValue(o) is IHasValue v ? v.HasValue : true;

                property.ShouldSerialize = (old == null ? shouldSerialize : (o) => old(o) && shouldSerialize(o));

            }

            return property;

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            Assert.That(!JsonConvert.DeserializeObject<Tmp2>("{}")!.A.HasValue);

            Assert.That(JsonConvert.SerializeObject(new Tmp2 { B = 32 }) == "{\"B\":32}");

            // Cache this statically somewhere for best performance

            IContractResolver resolver = new OptionalResolver

            {

                // Configure as required, e.g.:

                NamingStrategy = new CamelCaseNamingStrategy(),

            };

            var settings = new JsonSerializerSettings { ContractResolver = resolver };

​

            var inJson = "{\"b\" : null}";

            var tmp = JsonConvert.DeserializeObject<Tmp2>(inJson, settings);

​

            Console.WriteLine("Dumping result of deserializing JSON {0}", inJson);

            tmp.Dump();

            var json2 = JsonConvert.SerializeObject(tmp, settings);

            Console.WriteLine(json2);

            Assert.AreEqual("{}", JsonConvert.SerializeObject(JsonConvert.DeserializeObject<Tmp2>("{}"), settings));

            TestRoundTrip<Tmp2>("{}", settings);

            TestRoundTrip<Tmp2>("{\"b\":null}", settings);

            TestRoundTrip<Tmp2>("{\"b\":32}", settings);

            TestRoundTrip<Tmp2>("{\"a\":32}", settings);

            TestRoundTrip<Tmp2>("{\"someString\":\"foo\"}", settings);

​

            TestRoundTrip<V2.Tmp2>("{}", settings);

            TestRoundTrip<V2.Tmp2>("{\"b\":null}", settings);

            TestRoundTrip<V2.Tmp2>("{\"b\":32}", settings);

            TestRoundTrip<V2.Tmp2>("{\"a\":32}", settings);

        }   

        static void TestRoundTrip<Tmp2>(string json, JsonSerializerSettings settings)

        {

            var tmp = JsonConvert.DeserializeObject<Tmp2>(json, settings);

            var newJson = JsonConvert.SerializeObject(tmp, settings);

            Assert.That(JToken.DeepEquals(JToken.Parse(json), JToken.Parse(newJson)));

        }

    }

​

    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("{0} version: {1}", typeof(JsonSerializer).Namespace, typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​