using System;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var bases = new Base[]{ new Foo(), new Bar() };

        var json = JsonSerializer.Serialize<object>(bases);

        Console.WriteLine(json);

    }

    public class Base

    {

        public string PropertyBase{ get; set; }= "Base";

    }

    public class Foo : Base

    {

        public string PropertyFoo{ get; set; } = "Foo";

    }

​

    public class Bar : Base

    {

        public string PropertyBar{ get; set; } = "Bar";

    }

}