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 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/57152212/creating-a-custom-converter-to-write-json-as-a-dictionarystring-iobject-from

​

    public class PDItem : IDictionaryObject<List<IItem>>

    {

        [JsonProperty]

        public string Key { get; set; }

​

        [JsonProperty]

        public List<IItem> Value { get; set; }

    }

​

    public interface IDictionaryObject<T>

    {

        string Key { get; set; }

        T Value { get; set; }

    }

​

    //[JsonConverter(typeof(IItemConverter))]

    //This converter was missing from the question so I commented it out.

    public interface IItem: IElement

    {

    }

​

    [JsonConverter(typeof(ElementConverter))]

    public interface IElement

    {

        string elementType { get; }

    }

​

    // The following type was missing from the question so I auto-generated it from the JSON.

    public class ConcreteTypeA : IItem

    {

        public string elementType { get; set; }

        public string inputType { get; set; }

        public string name { get; set; }

        public string label { get; set; }

        public bool @checked { get; set; }

    }

​

    public class ElementConverter : JsonCreationConverter<IElement>

    {

        protected override IElement Create(Type objectType, JObject jObject)

        {

            // The referenced constants and types for this method were missing from the question, so I replaced

            // the method with a stub that returns a hardcoded FirstType

            return new ConcreteTypeA();

    #if false

            //TODO:  Add objects to ElementConverter as they come online.

            switch (jObject["elementType"].Value<string>())

            {

                case ElementTypeDescriptions.FirstType:

                    return new FirstType();

                case ElementTypeDescriptions.SecondType:

                    return new SecondType();

                case ElementTypeDescriptions.ThirdType:

                    return new ThirdType();

                case ElementTypeDescriptions.FourthType:

                case ElementTypeDescriptions.FifthType:

                default:

                    throw new NotImplementedException("This object type is not yet implemented.");

            }

    #endif

        }

    }

​

    public class PDItemConverter: JsonConverter<PDItem>

    {

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

        {

            // Convert to a dictionary DTO and serialize

            serializer.Serialize(writer, new Dictionary<string, List<IItem>> { { value.Key, value.Value } });

        }

​

        public override PDItem ReadJson(JsonReader reader, Type objectType, PDItem existingValue,

            bool hasExistingValue, JsonSerializer serializer)

        {

            // Deserialize as a dictionary DTO and map to a PDItem

            var dto = serializer.Deserialize<Dictionary<string, List<IItem>>>(reader);

            if (dto == null)

                return null;

            if (dto.Count != 1)

                throw new JsonSerializationException(string.Format("Incorrect number of dictionary keys: {0}", dto.Count));

            var pair = dto.First();

            existingValue = hasExistingValue ? existingValue : new PDItem();

            existingValue.Key = pair.Key;

            existingValue.Value = pair.Value;

            return existingValue;

        }

    }

​

    public abstract class JsonCreationConverter<T> : JsonConverter

    {

        // Override CanWrite and return false

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

​

        protected abstract T Create(Type objectType, JObject jObject);

​

        public override bool CanConvert(Type objectType)

        {

            return typeof(T) == objectType;

        }

​

        public override object ReadJson(JsonReader reader, Type objectType,

            object existingValue, JsonSerializer serializer)

        {

            var jObject = JObject.Load(reader);

            var target = Create(objectType, jObject);

            serializer.Populate(jObject.CreateReader(), target);

            return target;

        }

​

        public override void WriteJson(JsonWriter writer, object value,

            JsonSerializer serializer)

        {

            throw new NotImplementedException();

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            TestRoundTrip();

            TestSerialize();

        }

        static void TestRoundTrip()

        {

            var json = GetJson();

            var settings = new JsonSerializerSettings

            {

                Converters = { new PDItemConverter() },

            };

            var item = JsonConvert.DeserializeObject<PDItem>(json, settings);

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

            Console.WriteLine("\nRound-tripped JSON: ");

            Console.WriteLine(json2);

            Assert.IsTrue(JToken.DeepEquals(JToken.Parse(json), JToken.Parse(json2)));

        }   

        static void TestSerialize()

        {

            PDItem item = new PDItem() {

                Key = "checked",

                Value = new List<IItem>() {

                    new ConcreteTypeA () {

                        elementType = "input",

                        inputType = "checkbox",

                        name = "another_checkbox",

                        label = "Another checkbox",

                        @checked = true

                    }

                }

            };  

            var settings = new JsonSerializerSettings

            {

                Converters = { new PDItemConverter() },

            };

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

            Console.WriteLine("\nSerialized {0}", item);

            Console.WriteLine(json);

        }

        static string GetJson()

        {

            var json = @"{

    ""checked"": [

            {

                ""elementType"": ""input"",

                ""inputType"": ""checkbox"",

                ""name"": ""another_checkbox"",

                ""label"": ""Another checkbox"",

                ""checked"": true

            }

    ]

}

";

            return json;

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: " + Environment.Version);

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

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

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

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​