@Html.DropDownListFor(x => x.SelectedCategoryId,
        Model.AllCategoreis.Select(x => 
                 new SelectListItem {
                        Text = x.Name,
                        Value = x.ID.ToString(),
                        Selected = Model.SelectedValue == x.ID
                 }
         )
)

var list = new[] {   
    new Person { Id = 1, Name = "Name1" }, 
    new Person { Id = 2, Name = "Name2" }, 
    new Person { Id = 3, Name = "Name3" } 
};

var selectList = new SelectList(list, "Id", "Name", 2);
ViewData["People"] = selectList;

Html.DropDownList("PeopleClass", (SelectList)ViewData["People"])