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/59430728/how-to-handle-both-a-single-item-and-an-array-for-the-same-property-using-system

​

    public class SingleOrArrayConverter<TItem> : SingleOrArrayConverter<List<TItem>, TItem>

    {

        public SingleOrArrayConverter() : this(true) { }

        public SingleOrArrayConverter(bool canWrite) : base(canWrite) { }

    }

​

    public class SingleOrArrayConverterFactory : JsonConverterFactory

    {

        public bool CanWrite { get; }

        public SingleOrArrayConverterFactory() : this(true) { }

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

        public override bool CanConvert(Type typeToConvert)

        {

            var itemType = GetItemType(typeToConvert);

​

            if (itemType == null)

                return false;

            if (itemType != typeof(string) && typeof(IEnumerable).IsAssignableFrom(itemType))

                return false;

​

            if (typeToConvert.GetConstructor(Type.EmptyTypes) == null || typeToConvert.IsValueType)

                return false;

​

            return true;

        }

        public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)

        {

            var itemType = GetItemType(typeToConvert);

            var converterType = typeof(SingleOrArrayConverter<,>).MakeGenericType(typeToConvert, itemType);

            return (JsonConverter)Activator.CreateInstance(converterType, new object [] { CanWrite });

        }

        static Type GetItemType(Type type)

        {

            // Quick reject for performance

            if (type.IsPrimitive || type.IsArray || type == typeof(string))

                return null;

            while (type != null)

            {

                if (type.IsGenericType)

                {

                    var genType = type.GetGenericTypeDefinition();

                    if (genType == typeof(List<>))

                        return type.GetGenericArguments()[0];

                    // Add here other generic collection types as required, e.g. HashSet<> or ObservableCollection<> or etc.

                }

                type = type.BaseType;

            }

            return null;

        }

    }

​

    public class SingleOrArrayConverter<TCollection, TItem> : JsonConverter<TCollection> where TCollection : class, ICollection<TItem>, new()

    {

        public SingleOrArrayConverter() : this(true) { }

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

        public bool CanWrite { get; }

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

        {

            switch (reader.TokenType)

            {

                case JsonTokenType.Null:

                    return null;

                case JsonTokenType.StartArray:

                    var list = new TCollection();

                    while (reader.Read())

                    {

                        if (reader.TokenType == JsonTokenType.EndArray)

                            break;

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

                    }

                    return list;

                default:

                    return new TCollection { JsonSerializer.Deserialize<TItem>(ref reader, options) };

            }

        }

        public override void Write(Utf8JsonWriter writer, TCollection 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 string Email { get; set; }

        public int Timestamp { get; set; }

        public string Event { get; set; }

​

        public List<string> Category { get; set; }

    }

​

    namespace ConverterAppliedDirectly

    {

        class Item

        {

            public string Email { get; set; }

            public int Timestamp { get; set; }

            public string Event { get; set; }

​

            [JsonConverter(typeof(SingleOrArrayConverter<string>))]

            public List<string> Category { get; set; }

        }

    }

​

    namespace ObservableCollection

    {

        class Item

        {

            public string Email { get; set; }

            public int Timestamp { get; set; }

            public string Event { get; set; }

​

            [JsonConverter(typeof(SingleOrArrayConverter<ObservableCollection<string>, string>))]

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

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            TestFactory();

            TestTruncatedJson();

            TestObservableCollection();

            TestConverterAppliedDirectly();

            TestMiscellaneousCollections();

            TestInitial();

            Assert.IsTrue(new SingleOrArrayConverterFactory().CanConvert(typeof(List<string>)));

            Assert.IsTrue(!new SingleOrArrayConverterFactory().CanConvert(typeof(List<List<string>>)));

        }   

​

        static void TestTruncatedJson()

        {

            var json = GetJson();

​

            var options = new JsonSerializerOptions

            {

                Converters = { new SingleOrArrayConverterFactory() },

                // Other    

                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                WriteIndented = true,

                ReadCommentHandling = JsonCommentHandling.Skip,

            };

​

            for (int i = 2; i < json.Length-1; i++)

            {

                var badJson = json.Substring(0, json.Length - i);

                Assert.Throws<JsonException>(() => JsonSerializer .Deserialize<List<Item>>(badJson, options));

            }

            var sampleBadJson = @"[{ ""category"": [ ""newuser"",";

            Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<List<Item>>(sampleBadJson, options));

        }   

        static void TestObservableCollection()

        {

            var json = GetJson();

            var options = new JsonSerializerOptions

            {

                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                //

                WriteIndented = true,

                ReadCommentHandling = JsonCommentHandling.Skip,

                AllowTrailingCommas = true,

            };

​

            var list = JsonSerializer.Deserialize<List<ObservableCollection.Item>>(json, options);

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

​

            AssertEqualsJson(json, json2);

        }

        static void TestConverterAppliedDirectly()

        {

            var json = GetJson();

            var options = new JsonSerializerOptions

            {

                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                //

                WriteIndented = true,

                ReadCommentHandling = JsonCommentHandling.Skip,

                AllowTrailingCommas = true,

            };

​

            var list = JsonSerializer.Deserialize<List<ConverterAppliedDirectly.Item>>(json, options);

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

​

            AssertEqualsJson(json, json2);

        }

        static void TestInitial()

        {

            var json = GetJson();

​

            Assert.Throws<JsonException>(() => {

                var options = new JsonSerializerOptions

                {

                    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                };

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

​

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

            });

        }

        static void TestMiscellaneousCollections()

        {

            Assert.IsTrue(

                JsonSerializer.Deserialize<HashSet<string>>(

                    "\"item\"", // A single JSON string value

                    new JsonSerializerOptions { Converters = { new SingleOrArrayConverter<HashSet<string>, string>() } })

                .Count == 1);

            Assert.IsTrue(

                JsonSerializer.Deserialize<Collection<string>>(

                    "\"item\"", // A single JSON string value

                    new JsonSerializerOptions { Converters = { new SingleOrArrayConverter<Collection<string>, string>() } })

                .Count == 1);

            Assert.IsTrue(

                JsonSerializer.Deserialize<ObservableCollection<string>>(

                    "\"item\"", // A single JSON string value

                    new JsonSerializerOptions { Converters = { new SingleOrArrayConverter<ObservableCollection<string>, string>() } })

                .Count == 1);

        }

        static void TestFactory()

        {

            var json = GetJson();

            var options = new JsonSerializerOptions

            {

                Converters = { new SingleOrArrayConverterFactory() },

                // Other    

                PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

                WriteIndented = true,

                ReadCommentHandling = JsonCommentHandling.Skip,

                AllowTrailingCommas = true,

            };

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

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

​

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

            Console.WriteLine(json2);

​

            AssertEqualsJson(json, json2);

            foreach (var item in list)

            {

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

                var itemList = JsonSerializer.Deserialize<List<Item>>(itemJson, options);

                Assert.IsTrue(itemList.Count == 1, "itemList.Count == 1");

                var itemJson2 = JsonSerializer.Serialize(itemList, options);

                AssertEqualsJson(itemJson, itemJson2);

            }

        }

        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 = @"[

  {

    ""email"": ""john.doe@sendgrid.com"",

    ""timestamp"": 1337966815,

    ""category"": [

      ""newuser"", 

      ""transactional""

    ],

    ""event"": ""open""

  },

  /*Comment */{

    ""email"": ""jane.doe@sendgrid.com"",

    ""timestamp"": 1337966815,

    ""category"": ""olduser"",

    ""event"": ""open""

  },

  {

    ""email"": ""nul@sendgrid.com"",

    ""timestamp"": 1337966815,  

    ""category"": null 

    , 

    ""event"": ""open""

  },// Testing AllowTrailingCommas = true,

​

]

";

            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.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;

        }

    }

​