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 System.Text.Json.Serialization;

using System.Text.Json;

​

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

​

//https://stackoverflow.com/questions/68435043/what-is-the-purpose-of-typetoconvert-in-jsonconvertert-read

//https://stackoverflow.com/questions/59430728/how-to-handle-both-a-single-item-and-an-array-for-the-same-property-using-system

​

    public class SingleOrArrayCollectionConverter<TItem> : JsonConverter<Collection<TItem>>

    {

        public SingleOrArrayCollectionConverter() : this(true) { }

        public SingleOrArrayCollectionConverter(bool canWrite) => CanWrite = canWrite;

        public bool CanWrite { get; }

        public override bool CanConvert(Type typeToConvert) => typeof(Collection<TItem>).IsAssignableFrom(typeToConvert);

        public override Collection<TItem> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

        {

            var list = typeToConvert == typeof(Collection<TItem>) ? new Collection<TItem>() : (Collection<TItem>)Activator.CreateInstance(typeToConvert);

            switch (reader.TokenType)

            {

                case JsonTokenType.StartArray:

                    while (reader.Read())

                    {

                        if (reader.TokenType == JsonTokenType.EndArray)

                            break;

                        list.Add(JsonSerializer.Deserialize<TItem>(ref reader, options));

                    }

                    break;

                default:

                    list.Add(JsonSerializer.Deserialize<TItem>(ref reader, options));

                    break;

            }

            return list;

        }

        public override void Write(Utf8JsonWriter writer, Collection<TItem> value, JsonSerializerOptions options)

        {

            if (CanWrite && value.Count == 1)

                JsonSerializer.Serialize(writer, value.First(), options);

            else

            {

                writer.WriteStartArray();

                foreach (var item in value)

                    JsonSerializer.Serialize(writer, item, options);

                writer.WriteEndArray();

            }

        }

    }

​

    class Item

    {

        public Collection<string> Values { get; set; }

        public ObservableCollection<string> ObservableValues { get; set; }

    }

​

    class TestClass

    {

        public static void Test()

        {

            TestSimple();

            TestItem();

        }

​

        public static void TestSimple()

        {

            var json = @"""hello"""; // A JSON string primitive "hello"

            var options = new JsonSerializerOptions

            {

                Converters = { new SingleOrArrayCollectionConverter<string>() },

            };

            var collection = JsonSerializer.Deserialize<ObservableCollection<string>>(json, options);

            collection.Dump();

            Assert.IsTrue(collection.SequenceEqual(new [] { "hello" }));

        }

        public static void TestItem()

        {

            var json = GetJson();

​

            var options = new JsonSerializerOptions

            {

                Converters = { new SingleOrArrayCollectionConverter<string>() },

            };

            var item = JsonSerializer.Deserialize<Item>(json, options);

            item.Dump();

            var json2 = JsonSerializer.Serialize(item, options);

            AssertEqualsJson(json, json2);

        }

        static void AssertEqualsJson(string json1, string json2)

        {

            Assert.IsTrue(Newtonsoft.Json.Linq.JToken.DeepEquals(

                Newtonsoft.Json.Linq.JToken.Parse(json1, new Newtonsoft.Json.Linq.JsonLoadSettings { CommentHandling = Newtonsoft.Json.Linq.CommentHandling.Ignore }),

                Newtonsoft.Json.Linq.JToken.Parse(json2, new Newtonsoft.Json.Linq.JsonLoadSettings { CommentHandling = Newtonsoft.Json.Linq.CommentHandling.Ignore })

            ));

        }

        static string GetJson()

        {

            var json = @"{

    ""Values"" : ""hello"",

    ""ObservableValues"" : ""hello""

}";

            return json;

        }

    }

​

    public class Program

    {

        public static void Main()

        {

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

            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;

            }

        }

        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.Location.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;

        }

    }

​