using System;

using System.Linq;

using System.Data;

using System.Reflection;

​

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Text.Json.Serialization.Metadata;

​

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

​

//https://stackoverflow.com/questions/77809445/custom-typeinforesolver-to-manage-polimorphic-deserialization-in-system-text-jso

​

#nullable enable

​

    public static class JsonExtensions

    {

        public static void AddNativePolymorphicTypInfo(JsonTypeInfo jsonTypeInfo)

        {

            Type baseValueObjectType = typeof(ValueObject);

            if (jsonTypeInfo.Type == baseValueObjectType) {

                jsonTypeInfo.PolymorphismOptions = new JsonPolymorphismOptions {

                    TypeDiscriminatorPropertyName = "$mytype",

                    IgnoreUnrecognizedTypeDiscriminators = true,

                    UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization,

                };

                var types = Assembly

                    .GetExecutingAssembly()

                    .GetTypes()

                    .Where(t => t.IsSubclassOf(typeof(ValueObject))); // Fixed MyBaseClass => ValueObject

                foreach (var t in types.Select(t => new JsonDerivedType(t, t.Name.ToLowerInvariant()))) // Fixed ToLower() => ToLowerInvariant

                    jsonTypeInfo.PolymorphismOptions.DerivedTypes.Add(t);

            }

        }

    }

​

public class ValueObject;

​

public class MyValueObject : ValueObject {

    public int Counter { get; set; }

}

​

public class RootObjectOne {

    public ValueObject? ValueObject { get; set; }

}

​

public class RootObjectTwo { 

    // No polymorphic type identifier is emitted here because the property is declaed as MyValueObject not ValueObject.

    [JsonIgnore]

    public MyValueObject? ValueObject { get; set; }

    // You could force one e.g. by using a private property like so, or by modifying AddNativePolymorphicTypInfo() to add the required type identifiers for derived types:

    [JsonInclude, JsonPropertyName("valueObject")]

    ValueObject? SerializedValueObject { get => ValueObject; set => ValueObject = (MyValueObject?)value; }

}

​

class TestClass {

    private static JsonSerializerOptions BuildSerializerOptions() {

        return new JsonSerializerOptions {

            TypeInfoResolver = new DefaultJsonTypeInfoResolver() 

                .WithAddedModifier(JsonExtensions.AddNativePolymorphicTypInfo),

            // Add other options as required, e.g.

            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

        };

    }

​

    public static void Test1_Serialize_Only_MyValueObject_Declared_AsSelf() {

        Console.WriteLine("TEST 1");

        MyValueObject rootObject = new MyValueObject() { Counter = 5 }; 

        var json = JsonSerializer.Serialize<ValueObject>(rootObject, BuildSerializerOptions()); // FIXED made declared type be ValueObject.

        Console.WriteLine(json);

        var expectedJson = "{\"$mytype\":\"myvalueobject\",\"counter\":5}";

        Assert.That(expectedJson == json);

    }   

​

    public static void Test2_Serialize_Only_MyValueObject_Declared_AsBase() {

        Console.WriteLine("TEST 2");

        ValueObject rootObject = new MyValueObject() { Counter = 5 };                   

        var json = JsonSerializer.Serialize(rootObject, BuildSerializerOptions());

        Console.WriteLine(json);

        var expectedJson = "{\"$mytype\":\"myvalueobject\",\"counter\":5}";

        Assert.That(expectedJson == json);

    }   

​

    public static void Test3_Serialize_RootObjectOne_With_ValueObject_Declared_AsBase() {

        Console.WriteLine("TEST 3");

        var rootObject = new RootObjectOne { ValueObject = new MyValueObject() { Counter = 5 } };           

        var json = JsonSerializer.Serialize(rootObject, BuildSerializerOptions());

        Console.WriteLine(json);            

        var expectedJson = "{\"valueObject\":{\"$mytype\":\"myvalueobject\",\"counter\":5}}";

        Assert.That(expectedJson == json);

    }   

​

    public static void Test4_Serialize_RootObjectTwo_With_ValueObject_Declared_AsSelf() {

        Console.WriteLine("TEST 4");

        var rootObject = new RootObjectTwo { ValueObject = new MyValueObject() { Counter = 5 } };           

        var json = JsonSerializer.Serialize(rootObject, BuildSerializerOptions());

        Console.WriteLine(json);

        var expectedJson = "{\"valueObject\":{\"$mytype\":\"myvalueobject\",\"counter\":5}}";

        Assert.That(expectedJson == json);

    }   

}

​

    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.Test1_Serialize_Only_MyValueObject_Declared_AsSelf();

            }

            catch (Exception ex) {

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

                Console.WriteLine(ex);

            }

            try {

                TestClass.Test2_Serialize_Only_MyValueObject_Declared_AsBase();

            }

            catch (Exception ex) {

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

                Console.WriteLine(ex);

            }

            try {

                TestClass.Test3_Serialize_RootObjectOne_With_ValueObject_Declared_AsBase();

            }

            catch (Exception ex) {

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

                Console.WriteLine(ex);

            }

            try {

                TestClass.Test4_Serialize_RootObjectTwo_With_ValueObject_Declared_AsSelf();

            }

            catch (Exception ex) {

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

                Console.WriteLine(ex);

            }

        }

    }

​