MVC dropdowlistfor 默认值无效（Selected 无效）

下面是设置无效的例子

@Html.DropDownListFor(model => model.City, ViewBag.City as IEnumerable<SelectListItem>, "==选择==", new { @class = "form-control dropdown-toggle" })

@Html.DropDownListFor(model => model.City, ViewBag.City as SelectList, "==选择==", new { @class = "form-control dropdown-toggle" })

下面是设置成功的例子
@Html.DropDownListFor(model => model.City, ViewBag.City as List<SelectListItem>, "==选择==", new { @class = "form-control dropdown-toggle" })

后端代码都是一样的，究其原因就是要把界面获取的列表转为List