如何在mvc中的@ Html.DropdownList中选择默认值

controller :

List<SelectListItem> li = new List<SelectListItem>();

DataSet ds = new DataSet();
li.Add(new SelectListItem { Text = "--Select--", Value = "-1" });
foreach (DataRow item in ds.Tables[0].Rows) {
    if (item["Name"].ToString() == abc) {
        li.Add(new SelectListItem { Text = item["Name"].ToString(), Value = Convert.ToString(item["id"]), Selected = true });

    } else {
        li.Add(new SelectListItem { Text = item["Name"].ToString(), Value = Convert.ToString(item["id"]) });
    }
}
List<SelectListItem> li = new Li