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

​

//https://stackoverflow.com/questions/77969530/net-7-8-system-text-json-deserialize-polymorphic-parameter

// Failing demo: https://dotnetfiddle.net/kiptzX

​

[JsonDerivedType(typeof(Apple),typeDiscriminator: "apple")]

public abstract class Fruit

{

    public Seed seed { get; }

​

    protected Fruit(Seed seed)

    {

        this.seed = seed; // FIXED COMPILATION -- added ; at end.

    }

}

​

public sealed class Apple : Fruit

{

    [JsonConstructor]

    public Apple(AppleSeed seed) : base(seed) { }

    public new AppleSeed seed => (AppleSeed)base.seed;

}

​

public sealed class AppleSeed : Seed

{

    [JsonConstructor]

    public AppleSeed() {}

}

​

[JsonDerivedType(typeof(AppleSeed),typeDiscriminator: "apple")]

public abstract class Seed { }

​

#nullable enable

​

    class TestClass

    {

        public static void Test()

        {

            Fruit fruit = new Apple(new AppleSeed());

            var json = JsonSerializer.Serialize(fruit);

            Console.WriteLine(json);

            var fruit2 = JsonSerializer.Deserialize<Fruit>(json);

            var json2 = JsonSerializer.Serialize(fruit2);

            Assert.That(fruit.GetType() == fruit2?.GetType());

            Assert.That(fruit.seed.GetType() == fruit2?.seed?.GetType());

            Assert.That(json == json2);

        }   

    }

​

    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;

            }

        }

    }

​