//https://stackoverflow.com/questions/65430420/how-to-use-default-serialization-in-a-custom-system-text-json-jsonconverter

using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Reflection;

​

​

using System.Text.Json;

using System.Text.Json.Serialization;

​

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

​

//https://stackoverflow.com/questions/71675067/have-a-jsonconverter-that-is-only-used-during-deserialization

​

    public class JsonDictionaryConverterFactory : JsonConverterFactory

    {

        public override bool CanConvert(Type typeToConvert) => TryGetDictionaryType(typeToConvert, out _, out _, out  _);

        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)

        {

            TryGetDictionaryType(typeToConvert, out var dictionaryType, out var keyType, out var valueType);        

            var converterType = typeof(JsonDictionaryConverter<,,>).MakeGenericType(dictionaryType, keyType, valueType);

            return (JsonConverter)Activator.CreateInstance(converterType, options, this);

        }

        static bool TryGetDictionaryType(Type typeToConvert, out Type dictionaryType, out Type keyType, out Type valueType)

        {

            var keyValueTypes = typeToConvert.GetDictionaryKeyValueType();

            if (keyValueTypes != null && keyValueTypes.Length == 2)

            {

                // Todo; maybe put a restriction here on keyType, such as keyType == typeof(string).

                if (typeToConvert.IsInterface)

                {

                    var concreteType = typeof(Dictionary<,>).MakeGenericType(keyValueTypes);

                    if (typeToConvert.IsAssignableFrom(concreteType))

                    {

                        dictionaryType = concreteType;

                        keyType = keyValueTypes[0];

                        valueType = keyValueTypes[1];

                        return true;

                    }

                }

                else

                {

                    if (typeToConvert.GetConstructor(Type.EmptyTypes) != null)

                    {

                        dictionaryType = typeToConvert;

                        keyType = keyValueTypes[0];

                        valueType = keyValueTypes[1];

                        return true;

                    }

                }

            }

            dictionaryType = keyType = valueType = null;

            return false;

        }

        class JsonDictionaryConverter<TDictionary, TKey, TValue> : JsonConverter<TDictionary> where TDictionary : IDictionary<TKey, TValue>, new()

        {

            readonly JsonSerializerOptions modifiedOptions;

            public JsonDictionaryConverter(JsonSerializerOptions options, JsonDictionaryConverterFactory factory) => this.modifiedOptions = options.CopyAndRemoveConverter(factory.GetType());

            public override void Write(Utf8JsonWriter writer, TDictionary value, JsonSerializerOptions options) =>  JsonSerializer.Serialize(writer, value, modifiedOptions);

            public override TDictionary Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) 

            {

                switch (reader.TokenType)

                {

                    case JsonTokenType.StartObject:

                        return (TDictionary)JsonSerializer.Deserialize(ref reader, typeToConvert, modifiedOptions);

                    case JsonTokenType.StartArray:

                        var list = JsonSerializer.Deserialize<List<KeyValuePair<TKey, TValue>>>(ref reader, modifiedOptions);

                        var dictionary = new TDictionary();

                        foreach (var item in list)

                            dictionary.Add(item);

                        return dictionary;

                    default:

                        throw new JsonException();

                }

            }

        }

    }

    public static class JsonExtensions

    {

        public static JsonSerializerOptions CopyAndRemoveConverter(this JsonSerializerOptions options, Type converterType)

        {

            var copy = new JsonSerializerOptions(options);

            for (var i = copy.Converters.Count - 1; i >= 0; i--)

                if (copy.Converters[i].GetType() == converterType)

                    copy.Converters.RemoveAt(i);

            return copy;

        }

​

        public static IEnumerable<Type> GetInterfacesAndSelf(this Type type)

            => (type ?? throw new ArgumentNullException()).IsInterface ? new[] { type }.Concat(type.GetInterfaces()) : type.GetInterfaces();

​

        public static IEnumerable<Type []> GetDictionaryKeyValueTypes(this Type type)

            => type.GetInterfacesAndSelf().Where(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(IDictionary<,>)).Select(t => t.GetGenericArguments());

​

        public static Type [] GetDictionaryKeyValueType(this Type type)

            => type.GetDictionaryKeyValueTypes().SingleOrDefaultIfMultiple();

​

        // Copied from this answer https://stackoverflow.com/a/25319572

        // By https://stackoverflow.com/users/3542863/sean-rose

        // To https://stackoverflow.com/questions/3185067/singleordefault-throws-an-exception-on-more-than-one-element

        public static TSource SingleOrDefaultIfMultiple<TSource>(this IEnumerable<TSource> source)

        {

            var elements = source.Take(2).ToArray();

            return (elements.Length == 1) ? elements[0] : default(TSource);

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            foreach (var json in GetJson())

            {

                var options = new JsonSerializerOptions

                {

                    Converters = { new JsonDictionaryConverterFactory() },

                    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                    WriteIndented = true,

                };

                var dictionary = JsonSerializer.Deserialize<IDictionary<string, long>>(json, options);

​

                var newJson = JsonSerializer.Serialize(dictionary, options);

​

                Console.WriteLine("Deserializing JSON:");

                Console.WriteLine(json);

                Console.WriteLine("Re-serialized {0}:", dictionary);

                Console.WriteLine(newJson);

            }

        }

        static IEnumerable<string> GetJson() =>

            new []

            {

                @"[

  {

    ""key"": ""foo"",

    ""value"": 42

  }

]",

                @"{""foo"":42}",

        };

    }

​

    public class Program

    {

        public static void Main()

        {

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

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

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

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

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​