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;

using System.Text.Json.Serialization;

using System.Text.Json.Serialization.Metadata;

using System.Text.Json.Nodes;

​

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

​

#nullable enable

​

public static partial class JsonExtensions

{

    public static Action<JsonTypeInfo> SetupUnknownDerivedTypeHandling(bool objectsOnly = false) =>

        (typeInfo) =>

    {

        // Polymorphism using type discriminators is only supported for type hierarchies that use the default converters for objects, collections, and dictionary types.

        if (typeInfo.Kind == JsonTypeInfoKind.None || typeInfo.Type.IsSealed || typeInfo.Type == typeof(object))

            return;

        if (objectsOnly && typeInfo.Kind != JsonTypeInfoKind.Object)

            return;

        // TODO: decide whether to fail serialization even for types marked with

        // [JsonPolymorphic(UnknownDerivedTypeHandling = FallBackToBaseType or FallBackToNearestAncestor)]

        if (typeInfo.PolymorphismOptions == null)

        {

            typeInfo.PolymorphismOptions = new() { DerivedTypes = { new(typeInfo.Type) } };

            // TODO: decide whether to fail serialization even for types marked with

            // [JsonPolymorphic(UnknownDerivedTypeHandling = FallBackToBaseType or FallBackToNearestAncestor)]

            // If so, move the setting of UnknownDerivedTypeHandling out of this if statement.

            typeInfo.PolymorphismOptions.UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FailSerialization;

        };

    };

}

​

public class Root<T>

{

    public T? Data { get; set; }

}

​

public class Foo;

public class SubFoo : Foo;

public class SubSubFoo : SubFoo;

​

public class FooList : List<Foo>;

public class SpecialFooList : FooList;

​

public class FooDictionary : Dictionary<string, Foo>;

public class SpecialFooDictionary : FooDictionary;

​

    class TestClass

    {

        public static void Test()

        {

            Assert.DoesNotThrow(() => Serialize<Foo, Foo>(false));

            Assert.DoesNotThrow(() => Serialize<Foo, Foo>(true));

            Assert.Throws(Is.InstanceOf(typeof(Exception)), () => Serialize<Foo, SubFoo>(false));

            Assert.Throws(Is.InstanceOf(typeof(Exception)), () => Serialize<Foo, SubFoo>(true));

​

            Assert.DoesNotThrow(() => Serialize<FooList, FooList>(false));

            Assert.DoesNotThrow(() => Serialize<FooList, FooList>(true));

            Assert.Throws(Is.InstanceOf(typeof(Exception)), () => Serialize<FooList, SpecialFooList>(false));

            Assert.DoesNotThrow(() => Serialize<FooList, SpecialFooList>(true));

​

            Assert.DoesNotThrow(() => Serialize<FooDictionary, FooDictionary>(false));

            Assert.DoesNotThrow(() => Serialize<FooDictionary, FooDictionary>(true));

            Assert.Throws(Is.InstanceOf(typeof(Exception)), () => Serialize<FooDictionary, SpecialFooDictionary>(false));

            Assert.DoesNotThrow(() => Serialize<FooDictionary, SpecialFooDictionary>(true));

​

            Assert.DoesNotThrow(() => Serialize<object, int>(false));

        }   

        static void Serialize<TBase, TDerived>(bool objectsOnly) where TDerived : TBase, new()

        {

            var options = new JsonSerializerOptions()

            {

                // Configure either a DefaultJsonTypeInfoResolver or some JsonSerializerContext and add the required modifier:

                TypeInfoResolver = new DefaultJsonTypeInfoResolver() 

                    .WithAddedModifier(JsonExtensions.SetupUnknownDerivedTypeHandling(objectsOnly)),

                // Set other options as required.

            };

            var json = JsonSerializer.Serialize<TBase>(new TDerived(), options);

            Console.WriteLine(json);

        }

    }

​

    public static class ObjectExtensions

    {

        public static T ThrowOnNull<T>(this T? value) where T : class => value ?? throw new ArgumentNullException();

    }

​

    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;

            }

        }

    }

​