如何动态预选ASP.NET MVC中的html.DropDownlist中的项目

public ActionResult Index()
{
    ViewBag.Items = new List<SelectListItem>
    {
        new SelectListItem { Text = "Item 1", Value = "1" },
        new SelectListItem { Text = "Item 2", Value = "2" },
        new SelectListItem { Text = "Item 3", Value = "3" },
    };
    return View();
}

@Html.DropDownListFor(m => m.SelectedItem, (IEnumerable<SelectListItem>)ViewBag.Items, "Select an item")

public ActionResult Index()
{
    ViewBag.Items = new List<SelectListItem>
    {
        new SelectListItem { Text = "Item 1", Value = "1" },
        new SelectListItem { Text = "Item 2", Value = "2" },
        new SelectListItem { Text = "Item 3", Value = "3" },
    };
    var model = new MyModel();
    model.SelectedItem = "2";
    return View(model);
}