如何从控制器中的DropDownListFor获取MvcHtmlString

如何解决如何从控制器中的DropDownListFor获取MvcHtmlString

我已经尝试过了：

public ActionResult RebuildDropDown(string selectedItem)
{
    bool error = false;
    MvcHtmlString drop = null;

    try
    {
        var helper = new HtmlHelper<Asset>(new ViewContext(),new ViewPage());

        //SelectList made from List<SelectListItem>
        var selectList = Asset.CriarSelectList(selectedItem);

        //x.Id is string
        drop = helper.DropDownListFor(x => x.Id,selectList);
    }
    catch (System.Exception e)
    {
        error = true;
    }

    return Json(new { error,drop });
}

在此行中始终获取空引用：

drop = helper.DropDownListFor(x => x.Id,selectList);