下拉列表在MVC3 Razor中设置选定的值

这是我的模型：

public class NewsCategoriesModel {
    public int NewsCategoriesID { get; set; }        
    public string NewsCategoriesName { get; set; }
}

我的控制器：

public ActionResult NewsEdit(int ID, dms_New dsn) {
    dsn = (from a in dc.dms_News where a.NewsID == ID select a).FirstOrDefault();
    var categories = (from b in dc.dms_NewsCategories select b).ToList();
    var selectedValue = dsn.NewsCategoriesID;
    SelectList ListCategories = new SelectList(categories, "NewsCategoriesID", "NewsCategoriesName",selectedValue);
    // ViewBag.NewsCategoriesID = new SelectList(categories as IEnumerable, "NewsCategoriesID", "NewsCategoriesName", dsn.NewsCategoriesID);
    ViewBag.NewsCategoriesID = ListCategories;
    return View(dsn);
}

然后是我的视图：

@Html.DropDownList("NewsCategoriesID", (SelectList)ViewBag.NewsCategoriesID)

当我运行时，`DropDownList` 没有选择我设置的值。它总是选择第一个选项。