问输入隐藏显示不正确的Id (Guid.Empty)

EN

@Html.HiddenFor(model => model.Id)

<input id="Id" name="Id" type="hidden" value="00000000-0000-0000-0000-000000000000" />

<input type="hidden" id="Id" name="Id" value="@Model.Id" />

@using MvcApplication4.Models
@model Foo

@using (Html.BeginForm())
{
    @Html.HiddenFor(model => model.Id)
    @Model.Id

    <input type="submit" value="ok" />
}

namespace MvcApplication4.Controllers
{
    public class HomeController : Controller
    {
        [HttpGet]
        public ActionResult Index()
        {
            Foo foo = new Foo();
            return View(foo);
        }  

        [HttpPost]
        public ActionResult Index(Foo foo)
        {
            foo.Id = Guid.NewGuid();
            return View(foo);
        } 
    }
}

using System;

namespace MvcApplication4.Models 
{
    public class Foo
    {
         public Guid? Id { get; set; }
    }
}