using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Data;

​

using System.ComponentModel.DataAnnotations;

​

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using Newtonsoft.Json.Converters;

using Newtonsoft.Json.Serialization;    

​

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

​

//https://stackoverflow.com/questions/76558333/jsonconvert-deserializeobject-throwing-error-after-adding-addnewtonsoftjson

​

    public class ValidationResultConverter : JsonConverter<ValidationResult>

    {

        record DTO(string ErrorMessage, IEnumerable<string> MemberNames);       

        public override ValidationResult ReadJson(JsonReader reader, Type objectType, ValidationResult existingValue, bool hasExistingValue, JsonSerializer serializer) =>

            serializer.Deserialize<DTO>(reader) is {} dto

                ? new ValidationResult(dto.ErrorMessage, dto.MemberNames)

                : null;

        public override void WriteJson(JsonWriter writer, ValidationResult value, JsonSerializer serializer) => 

            serializer.Serialize(writer, new DTO(value.ErrorMessage, value.MemberNames));

    }

​

    public class Mymodel

    {

        public List<ValidationResult> ValidationResults { get; } = new ();

    }

​

    class TestClass

    {

        public static void Test()

        {

            var model = new Mymodel

            {

                ValidationResults = 

                {

                    new ("message1"),

                    new ("message2", new [] { "member name 2.1", "member name 2.2" }),

                }

            };

            var settings = new JsonSerializerSettings

            {

                Converters = { new ValidationResultConverter() },

            };

            var res = JsonConvert.SerializeObject(model, Formatting.Indented, settings);

            Console.WriteLine(res);

            var validationResponse = JsonConvert.DeserializeObject<Mymodel>(res, settings);

​

            Assert.That(model.ValidationResults.Select(v => v.ErrorMessage).SequenceEqual(validationResponse.ValidationResults.Select(v => v.ErrorMessage)));

            Assert.That(model.ValidationResults.SelectMany(v => v.MemberNames).SequenceEqual(validationResponse.ValidationResults.SelectMany(v => v.MemberNames)));

            Assert.That(res == JsonConvert.SerializeObject(validationResponse, Formatting.Indented, settings));

        }   

    }

​

    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;

            }

        }

    }

​