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
MVC dropdowlistfor 默认值无效（Selected 无效）
原文：http://www.cnblogs.com/zglyzi/p/4519226.html