using System;

public class Program

{

    public static void Main()

    {       

       IName p = new Person {Age = 4, Name = "Waldo"};

       var s1 = System.Text.Json.JsonSerializer.Serialize(p); // --> {"Name":"Waldo"}

       var s2 = Newtonsoft.Json.JsonConvert.SerializeObject(p); // --> {"Name":"Waldo","Age":4

       var s3 = System.Text.Json.JsonSerializer.Serialize(p, p.GetType());

       var s4 = System.Text.Json.JsonSerializer.Serialize((object)p);

        Console.WriteLine("System.Text: " + s1);

        Console.WriteLine("Newtonsoft.Json: " + s2);

        Console.WriteLine("System.Text explicit type: " + s3);

        Console.WriteLine("System.Text object: " + s4);

    }   

}

​

public interface IName

{

    string Name { get; set; }

​

}

​

public class Person : IName

{

    public string Name { get; set; }

    public int Age { get; set; }

}

​