using System;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

​

        var json = JsonSerializer.Serialize<Base>(new Base());

        Console.WriteLine(json);

    }

    public class Base

    {

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

        public Foo foo { get;set;} = new();

    }

    public class Foo

    {

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

    }

}