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 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/61798964/serialize-single-keyvaluepair-to-json

​

    public class MyDto

    {

        [MyJsonExtensionData]

        public Dictionary<string, object> Identifier { get; set; }

​

        public double Value1 { get; set; }

​

        public string Value2 { get; set; }

    }

​

    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false)]

    public class MyJsonExtensionDataAttribute : Attribute

    {

    }

​

    public class MyContractResolver : DefaultContractResolver

    {

        protected override JsonObjectContract CreateObjectContract(Type objectType)

        {

            var contract = base.CreateObjectContract(objectType);

            if (contract.ExtensionDataGetter == null && contract.ExtensionDataSetter == null)

            {

                var dictionaryProperty = contract.Properties

                    .Where(p => typeof(IDictionary<string, object>).IsAssignableFrom(p.PropertyType) && p.Readable && p.Writable)

                    .Where(p => p.AttributeProvider.GetAttributes(typeof(MyJsonExtensionDataAttribute), false).Any())

                    .SingleOrDefault();

                if (dictionaryProperty != null)

                {

                    dictionaryProperty.Ignored = true;

                    contract.ExtensionDataGetter = o => 

                        ((IDictionary<string, object>)dictionaryProperty.ValueProvider.GetValue(o)).Select(p => new KeyValuePair<object, object>(p.Key, p.Value));

                    contract.ExtensionDataSetter = (o, key, value) =>

                        {

                            var dictionary = (IDictionary<string, object>)dictionaryProperty.ValueProvider.GetValue(o);

                            if (dictionary == null)

                            {

                                dictionary = (IDictionary<string, object>)this.ResolveContract(dictionaryProperty.PropertyType).DefaultCreator();

                                dictionaryProperty.ValueProvider.SetValue(o, dictionary);

                            }

                            dictionary.Add(key, value);

                        };

                    }

                    contract.ExtensionDataValueType = typeof(object);

                    // TODO set contract.ExtensionDataNameResolver

            }

            return contract;

        }

    }

​

    class TestClass

    {

        static IContractResolver resolver = new MyContractResolver();

        public static void Test()

        {

            var results = new List<MyDto> 

                          {

                            new MyDto{

                              Identifier = new Dictionary<string, object> { { "Car", "Ford" } },

                              Value1 = 13,

                              Value2 = "A"

                            },

                            new MyDto{

                              Identifier = new Dictionary<string, object> { { "Train", "Bombardier" } },

                              Value1 = 14,

                              Value2 = "B"

                            },

                          };            

            var settings = new JsonSerializerSettings

            {

                ContractResolver = resolver,

            };

            var json = JsonConvert.SerializeObject(results, Formatting.Indented, settings);

            Console.WriteLine(json);

            var results2 = JsonConvert.DeserializeAnonymousType(json, results, settings);

            var json2 = JsonConvert.SerializeObject(results2, Formatting.Indented, settings);

​

            Console.WriteLine("Round-tripped {0}:", results2);

            Console.WriteLine(json2);

            Assert.AreEqual(json, json2);

            Assert.IsTrue(JToken.DeepEquals(JToken.Parse(json), JToken.Parse(GetExpectedJson())));

        }   

        static string GetExpectedJson()

        {

            // Note -- I added decimal points to the Value1 values to make the comparison work.

            return @"[

  {

    ""Car"": ""Ford"",

    ""Value1"": 13.0,

    ""Value2"": ""A""

  },

  {

    ""Train"": ""Bombardier"",

    ""Value1"": 14.0,

    ""Value2"": ""B""

  }

]";

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , GetNetCoreVersion());

            Console.WriteLine("{0} version: {1}", typeof(JsonSerializer).Assembly.GetName().Name,  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.CodeBase.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;

        }

    }

​