Razor - cast IEnumerable in dropdownlist

How to cast in MVC 3 an object from ViewBag to IEnumerable ?

i would like to do soethnig like that:

@Html.DropDownListFor(model => model.CategoryID, @(IEnumerable<SelectListItem>)ViewBag.CategoriesList)

the dropdownlist takes IEnumerable as an argument, but I don't know how to cast it properly. The above code returns an error.

@Html.DropDownListFor(
    model => model.CategoryID, 
    (IEnumerable<SelectListItem>)ViewBag.CategoriesList
)

@Html.DropDownList("categoryId",
            new SelectList(ViewBag.Categories as System.Collections.IEnumerable,
            "categoryId", "categoryName", Model.categoryId))