using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

​

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Text.Json.Serialization.Metadata;

using System.Text.Json.Nodes;

​

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

​

public abstract class ClassC

{

    public string PropertyC { get; set; }

    public string TypeDiscriminator => this.GetType().FullName;

}

​

public class ClassB : ClassC

{

    public string PropertyB { get; set; }

}

​

public class ClassA : ClassB

{

    public string PropertyA { get; set; }

}

​

#nullable enable

​

public class ClassCJsonConverter : JsonConverter<ClassC>

{

    public override bool CanConvert(Type typeToConvert) => typeToConvert.IsAssignableFrom(typeof(ClassC)) && typeToConvert.IsAbstract;

​

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

    {

        using (var jsonDoc = JsonDocument.ParseValue(ref reader))

        {

            var root = jsonDoc.RootElement;

            var typeDiscriminator = root.GetProperty("TypeDiscriminator").GetString();

​

            var type = typeDiscriminator == null ? null : Type.GetType(typeDiscriminator);

            if (type == null || CanConvert(type)) // Here we guarantee there is no infinite recursion.

                throw new JsonException($"Type {typeDiscriminator} could not be found or is invalid.");

​

            return (ClassC?)root.Deserialize(type, options);

        }

    }

​

    public override void Write(Utf8JsonWriter writer, ClassC value, JsonSerializerOptions options) =>

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

}

​

    class TestClass

    {

        public static void Test()

        {

            var options = new JsonSerializerOptions

            {

                Converters = { new ClassCJsonConverter() },

                WriteIndented = true

            };

​

            ClassA classA = new ClassA { PropertyA = "ValueA", PropertyB = "ValueB", PropertyC = "ValueC" };

​

            string json = JsonSerializer.Serialize<ClassC>(classA, options);

            ClassC deserialized = JsonSerializer.Deserialize<ClassC>(json, options)!;

​

            Console.WriteLine(json);

            Console.WriteLine(deserialized is ClassA); // Should output 'True'

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1}), {2}", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);

            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;

            }

        }

    }

​