Validating Dropdownlist in MVC

public class DeviceModel
   {
       [Required(ErrorMessage = "Select a value")]
       public SelectList DeviceListModel { get; set; }

       [Required(ErrorMessage = "Select a value")]
       public SelectList DeviceType { get; set; }

   }

public ActionResult Index()
      {
          List<Example> mod = (from data in obj.Examples select data).ToList();
          Example exp = new Example();
          exp.Name = "--Select--";
          exp.ID = 0;
          mod.Insert(0, exp);
          SelectList modeldata = new SelectList(mod, "ID", "Name", 0);
          DeviceModel objmodel = new DeviceModel();
          objmodel.DeviceListModel = modeldata;
          List<Gearhead_Type> types = (from data in obj.Gearhead_Type select data).ToList();
          Gearhead_Type GTypre = new Gearhead_Type();
          GTypre.Name = "--Select--";
          GTypre.ID = 0;
          types.Insert(0, GTypre);
          SelectList asd = new SelectList(types, "ID", "Name", 0);
          objmodel.DeviceType = asd;

          return View(objmodel);

      }

<div class="form-group">
               @Html.Label("Gearhead Model and Type  ", new { @class = "col-md-6 control-label ", @style = "padding-left:200px" })
               @Html.DropDownListFor(s=>s.DeviceListModel,  Model.DeviceListModel)
               @Html.ValidationMessageFor(s => s.DeviceListModel)
           </div>