public class A
    {
        public B PropertyB{ get; set; }
    }

    public class B
    {
        public A PropertyA { get; set; }
        
    }
    public class HomeController : Controller
    {
        public JsonResult Index()
        {
            var a = new A
                        {
                            PropertyB = new B
                                            {
                                                PropertyA = new A()
                                            }
                        };

            return this.Json(a,JsonRequestBehavior.AllowGet);
        }
    }