How to use data annotations for drop-down lists?

Question

In MVC3 data annotations can be used to speed up the UI development and validations; ie.

    [Required]
    [StringLength(100, ErrorMessage = \"The {0} must          


        

Answer 1

If you want to enforce the selection of an element in the DropDown use the [Required] attribute on the field you are binding to:

public class MyViewModel
{
    [Required]
    [Display(Name = "")]
    public string Continent { get; set; }

    public IEnumerable<SelectListItem> Continents { get; set; }
}

and in your view:

@Html.DropDownListFor(
    x => x.Continent, 
    Model.Continents, 
    "-- Select a continent --"
)
@Html.ValidationMessageFor(x => x.Continent)

Answer 2

Change your ViewModel like this

public class RegisterViewModel
{
   //Other Properties

   [Required]
   [Display(Name = "Continent")]
   public string SelectedContinent { set; get; }
   public IEnumerable<SelectListItem> Continents{ set; get; }

}

and in your GET Action method, Set the Get the Data from your DB and set the Continents Collection property of your ViewModel

public ActionResult DoThatStep()
{
  var vm=new RegisterViewModel();
  //The below code is hardcoded for demo. you may replace with DB data.
  vm.Continents= new[]
  {
    new SelectListItem { Value = "1", Text = "Prodcer A" },
    new SelectListItem { Value = "2", Text = "Prodcer B" },
    new SelectListItem { Value = "3", Text = "Prodcer C" }
  }; 
  return View(vm);
}

and in your View (DoThatStep.cshtml) use this

@model RegisterViewModel
@using(Html.BeginForm())
{
  @Html.ValidationSummary()

  @Html.DropDownListFor(m => m.SelectedContinent, 
               new SelectList(Model.Continents, "Value", "Text"), "Select")

   <input type="submit" />
}

Now this will make your DropDown Required field.