ASP.NET MVC ValidationMessage 未显示下拉列表

Question

I have set a Required Validationmessage for the Role field in the ViewModel . The dropdownlist supposedly reflects the Role field in the ViewModel . However, when I did not select any values from the dropdownlist , it does not show any error at all even though I already set the Required validation for that field..any idea why?

The ViewModel:

public class RegisterViewModel
{
    [Required]
    [EmailAddress]
    [Display(Name = "Email")]
    public string Email { get; set; }

    [Required]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }

    [Required(ErrorMessage = "Please select a user type")]
    [Display(Name = "Select user type:")]
    public string Role { get; set; }
 }

The View:

<div class="form-group">
    @Html.LabelFor(m => m.Role, new { @class = "col-md-2 control-label"})
    <div class="col-md-10">
        @Html.DropDownList("Role", null, "Select user type", new { @class = "form-control" })
        @Html.ValidationMessage("Role")
    </div> 
</div>

Answer 1

You cannot get client side validation when you use an overload of DropDownList() when the first parameter is IEnumerable<SelectListItem> (ie when using the same name for the SelectList as the property your binding to). If you inspect the html your generating, you will see that there are no data-val-* attributes generated for the <select> element.

It would work using if you were to use (say)

ViewBag.RoleList = new SelectList(...);

in the controller and

@Html.DropDownList("Role", (IEnumerable<SelectListItem>),ViewBag.RoleList "Select user type", new { @class = "form-control" })

in the view, however since you have a view model (which is best practice), then add a property for the SelectList to the model

public class RegisterViewModel
{
    ....
    [Required(ErrorMessage = "Please select a user type")]
    [Display(Name = "Select user type:")]
    public string Role { get; set; }
    public IEnumerable<SelectListItem> RoleList { get; set; }
 }

and in the GET method, populate the collection before you pass the model to the view

RegisterViewModel model = new RegisterViewModel()
{
    RoleList = .... // your query to generate the SelectList
};
return View(model);

and in the view, use the strongly typed HtmlHelper methods

@Html.DropDownListFor(m => m.Role, Model.RoleList "Select user type", new { @class = "form-control" })
@Html.ValidationMessageFor(m => m.Role)

Refer also this DotNetFiddle for further examples of why the property your binding to and the name of the ViewBag for the SelectList should not be the same.