validate a dropdownlist in asp.net mvc

问题:

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

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

How can I make it so that by default it says "-Select Category-"

And validate to check something is selected (client and on the model)

Thanks

回答1:

I just can't believe that there are people still using ViewData/ViewBag in ASP.NET MVC 3 instead of having strongly typed views and view models:

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

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

and in your controller:

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");
    }
}

and then in your strongly typed view:

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

Also if you want client side validation don't forget to reference the necessary scripts:

<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>

回答2:

There is an overload with 3 arguments. Html.DropdownList(name, selectList, optionLabel) Update: there was a typo in the below code snippet.

@Html.DropDownList("Cat", new SelectList(ViewBag.Categories,"ID", "CategoryName"), "-Select Category-")

For the validator use

@Html.ValidationMessage("Cat")

回答3:

For ListBox / DropDown in MVC5 - i've found this to work for me sofar:

in Model:

[Required(ErrorMessage = "- Select item -")]
 public List<string> SelectedItem { get; set; }
 public List<SelectListItem> AvailableItemsList { get; set; }

in View:

@Html.ListBoxFor(model => model.SelectedItem, Model.AvailableItemsList)
@Html.ValidationMessageFor(model => model.SelectedItem, "", new { @class = "text-danger" })

回答4:

Example from MVC 4 for dropdownlist validation on Submit using Dataannotation and ViewBag (less line of code)

Models:

namespace Project.Models
{
    public class EmployeeReferral : Person
    {

        public int EmployeeReferralId { get; set; }


        //Company District
        //List                
        [Required(ErrorMessage = "Required.")]
        [Display(Name = "Employee District:")]
        public int? DistrictId { get; set; }

    public virtual District District { get; set; }       
}


namespace Project.Models
{
    public class District
    {
        public int? DistrictId { get; set; }

        [Display(Name = "Employee District:")]
        public string DistrictName { get; set; }
    }
}

EmployeeReferral Controller:

namespace Project.Controllers
{
    public class EmployeeReferralController : Controller
    {
        private ProjDbContext db = new ProjDbContext();

        //
        // GET: /EmployeeReferral/

        public ActionResult Index()
        {
            return View();
        }

 public ActionResult Create()
        {
            ViewBag.Districts = db.Districts;            
            return View();
        }

View:

<td>
                    <div class="editor-label">
                        @Html.LabelFor(model => model.DistrictId, "District")
                    </div>
                </td>
                <td>
                    <div class="editor-field">
                        @*@Html.DropDownList("DistrictId", "----Select ---")*@
                        @Html.DropDownListFor(model => model.DistrictId, new SelectList(ViewBag.Districts, "DistrictId", "DistrictName"), "--- Select ---")
                        @Html.ValidationMessageFor(model => model.DistrictId)                                                
                    </div>
                </td>

Why can't we use ViewBag for populating dropdownlists that can be validated with Annotations. It is less lines of code.