using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.IO;

using System.Collections.ObjectModel;

using System.Reflection;

using System.Runtime.Serialization;

using System.Data;

using System.Xml;

​

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Converters;

using Newtonsoft.Json.Serialization;

​

​

namespace Question42274847

{

    // https://stackoverflow.com/questions/42274847/how-to-return-null-from-jsonconvert-deserializeobject

    public class ReturnNullConverter<T> : JsonConverter

    {

        public override bool CanConvert(Type objectType)

        {

            return typeof(T).IsAssignableFrom(objectType);

        }

​

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

        {

            if (reader.TokenType == JsonToken.Null)

                return null;

​

            // Get the contract.

            var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(objectType);

​

            // Allocate the object (it must have a parameterless constructor)

            existingValue = existingValue ?? contract.DefaultCreator();

​

            // Populate the values.

            serializer.Populate(reader, existingValue);

​

            // This checks for all properties being null.  Value types are never null, however the question

            // doesn't specify a requirement in such a case.  An alternative would be to check equality

            // with p.DefaultValue

            // http://www.newtonsoft.com/json/help/html/P_Newtonsoft_Json_Serialization_JsonProperty_DefaultValue.htm

            if (contract.Properties

                .Where(p => p.Readable)

                .All(p => object.ReferenceEquals(p.ValueProvider.GetValue(existingValue), null)))

                return null;

​

            return existingValue;

        }

​

        public override bool CanWrite { get { return false; } }

​

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

        {

            throw new NotImplementedException();

        }

    }

​

    public class CustomResponse

    {

        [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]

        public string Message { get; set; }

        [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]

        public string Details { get; set; }

    }

​

    public class CustomResponseSubClass : CustomResponse

    {

        public int SomeInt { get; set; }

    }

​

    internal class TestClass

    {

        internal static void Test()

        {

            Test<CustomResponse>("{}", true);

            Test<CustomResponse>("{Message: null}", true);

            Test<CustomResponse>("{Details: null}", true);

            Test<CustomResponse>(@"{""DocumentId"":""123321"", ""DocumentNumber"":""ABC123""}", true);

            Test<CustomResponse>("{Message: \"a\"}", false);

            Test<CustomResponse>("{Details: \"a\", Message: null}", false);

            Test<CustomResponseSubClass>("{}", false);

​

            Console.WriteLine("All tests passed");

        }

​

        static void Test<T>(string jsonString, bool shouldBeNull) where T : CustomResponse

        {

            var settings = new JsonSerializerSettings

            {

                NullValueHandling = NullValueHandling.Ignore,

                MissingMemberHandling = MissingMemberHandling.Ignore,

                Converters = { new ReturnNullConverter<CustomResponse>() },

            };

            var customResponse = JsonConvert.DeserializeObject<T>(jsonString, settings);

            Assert.IsTrue((customResponse == null) == shouldBeNull);

​

            Console.WriteLine("Correctly deserialized \"{0}\" to {1}", jsonString, shouldBeNull ? "null" : "a non-null object of type " + typeof(T).Name);

        }

    }

​

    public class AssertionFailedException : System.Exception

    {

        public AssertionFailedException() : base() { }

​

        public AssertionFailedException(string s) : base(s) { }

    }

​

    public static class Assert

    {

        public static void IsTrue(bool value)

        {

            if (value == false)

                throw new AssertionFailedException("failed");

        }

    }

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Json.NET version: " + typeof(JsonSerializer).Assembly.FullName);

        Console.WriteLine("");

        Question42274847.TestClass.Test();

    }

}