验证 asp.net mvc 中的下拉列表

//in controller
ViewBag.Categories = categoryRepository.GetAllCategories().ToList();

//in view
 @Html.DropDownList("Cat", new SelectList(ViewBag.Categories,"ID", "CategoryName"))

我怎样才能使它默认显示“-选择类别-”

并验证以检查某些内容是否已选择（客户端和模型上）

Thanks

我简直不敢相信有人还在 ASP.NET MVC 3 中使用 ViewData/ViewBag，而不是强类型视图和视图模型：

public class MyViewModel
{
    [Required]
    public string CategoryId { get; set; }

    public IEnumerable<Category> Categories { get; set; }
}

在你的控制器中：

public class HomeController: Controller
{
    public ActionResult Index()
    {
        var model = new MyViewModel
        {
            Categories = Repository.GetCategories()
        }
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        if (!ModelState.IsValid)
        {
            // there was a validation error =>
            // rebind categories and redisplay view
            model.Categories = Repository.GetCategories();
            return View(model);
        }
        // At this stage the model is OK => do something with the selected category
        return RedirectToAction("Success");
    }
}

然后在你的强类型视图中：

@Html.DropDownListFor(
    x => x.CategoryId, 
    new SelectList(Model.Categories, "ID", "CategoryName"), 
    "-- Please select a category --"
)
@Html.ValidationMessageFor(x => x.CategoryId)

另外，如果您想要客户端验证，请不要忘记引用必要的脚本：

<script src="@Url.Content("~/Scripts/jquery.validate.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.js")" type="text/javascript"></script>