how to use viewbag to create a dropdownlist?

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C#

MVC3

ASP.NET4

actually i want to use viewbag to create a dropdownlist

in controller
--

public ActionResult Index()
      {
          var par = (from d in _db.DTypes
                     select new ArticleRepository {  dtype = d }).First();
          var query = _db.DTypes.Select(c => c.DataType);
          //var query = from tbl in _db.DTypes
          //            join tb in _db.ParamDtypes on tbl.DataId=tbl.DataId
          //            select new SelectListItem(tbl.DataType).;
          ViewBag.DTypes = new SelectList(query.AsEnumerable());
          return View();
      }

in view page

@Html.DropDownListFor(model => model.ddlStateId,IEnumerable<SelectListItem>>ViewBag.DTypes,"")
    @Html.ValidationMessageFor(model => model.ddlStateId)

now what i want to do is when i select the text as cat in dropdownlist then i have to pass id of that cat?
how can i do that ?
help