using System;

using System.ComponentModel.DataAnnotations;

using System.Linq;

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Collections.Generic;

​

public class Program

{

    [JsonConverter(typeof(MyJsonConverter))]

    public abstract class A {

        [Required]

        public string Id { get; set; }

    }

​

    public class B : A {

        [Required]

        public string F { get; set; }

    }

​

    public class C : A {

        [Required]

        public string G { get; set; }

    }

    public class MyJson {

        public List<A> Items { get; set; }

    }

    public class MyJsonConverter : JsonConverter<A>

    {

        public override bool CanConvert(Type type)

        {

            return type.IsAssignableFrom(typeof(A));

        }

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

        {

            Console.WriteLine("Read Json");

​

            if (JsonDocument.TryParseValue(ref reader, out var doc))

            {

                if (doc.RootElement.TryGetProperty("type", out var type))

                {

                    var typeValue = type.GetInt16();

                    var rootElement = doc.RootElement.GetRawText();

                    if (typeValue == 0) {

                        return JsonSerializer.Deserialize<B>(rootElement, options);

                    } else if (typeValue == 1) {

                        return JsonSerializer.Deserialize<C>(rootElement, options);

                    } else {

                        throw new JsonException("{typeValue} has not been mapped to a custom type yet!");

                    }

                }

​

                throw new JsonException("Failed to extract type property, it might be missing?");

            }

​

            throw new JsonException("Failed to parse JsonDocument");

        }

​

        public override void Write(Utf8JsonWriter writer, A value, JsonSerializerOptions options)

        {

            // We don't care about writing JSON.

            throw new NotImplementedException();

        }

    }

    public static void Main()

    {

        var json = @" {

            ""Items"": [

                {   

                    ""Id"": ""dsdasd"",

                    ""type"": 0,

                    ""F"": ""aa""

                },

                { 

                    ""type"": 1,

                    ""G"": ""bb""

                }

            ]

        }";

        var result = JsonSerializer.Deserialize<MyJson>(json);

​

        Console.WriteLine(result.Items.First().Id);

    }

}