using System;

//using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var obj = new MyDerivedClass() {

            Name = "name",

            DerivedName = "derived"

        };

        SerializeNewton(obj);   

        SerializeMicrosoft(obj);

    }

    public static void SerializeNewton(MyBase obj)

    {

        var value = Newtonsoft.Json.JsonConvert.SerializeObject(obj);

        Console.WriteLine($"newton : {value}");

    }

​

    public static void SerializeMicrosoft(MyBase obj)

    {

        var value = System.Text.Json.JsonSerializer.Serialize(obj);

        Console.WriteLine($"ms : {value}");

    }

}

​

public class MyBase 

{

    public string Name { get; set;}

}

​

public class MyDerivedClass : MyBase

{

    public string DerivedName { get; set;}

}

​