public ActionResult SomeAction(int id)
{
  //从数据库获取Domain Model
  var domainModel = ModelService.LoadEntities(m => m.ID == id).FirstOrDefault<Model>();
 
  //通过某个方法获取List<SelectListItem>类型对象实例
  List<SelectListItem> items = SomeMethod();
 
  //遍历集合，如果当前Domain model的某个属性与SelectListItem的Value属性相等，把SelectListItem的Selected属性设置为true
  foreach(SelectListItem item in items)
  {
    if(item.Value == Convert.ToString(domainModel.某属性))
    {
      item.Selected = true;
    }
  }
 
  //把List<SelectListItem>集合对象实例放到ViewData中
  ViewData["somekey"] = items;
 
  //可能涉及到把Domain Model转换成View Model
 
  return PartialView(domainModel);
}

public class Department
{
  public int Id {get;set;}
  public string Name {get;set;}
  public bool IsSelected {get;set;}
}

public ActionResult Index()
{
 SampleDbContext db = new SampleDbContext();
 List<SelectListItem> selectListItems = new List<SelectListItem>();
 
 //遍历Department的集合
 foreach(Department department in db.Departments)
 {
  SelectListItem = new SelectListItem
  {
   Text = department.Name,
   Value = department.Id.ToString(),
   Selected = department.IsSelected.HasValue ? department.IsSelected.Value : false
  }
  selectListItems.Add(selectListItem);
 }
 ViewBag.Departments = selectListItems;
 return View();
}

public ActionResult Index(FormCollection collection)
     {
       IList<Project> li = Utility.SqlHelper.getProjectList();
       SelectList selec = new SelectList(li, "ID", "Name");
   
       if (collection["drop"] != null)
       {
         string projectID = collection["drop"];
         selec = new SelectList(li, "ID", "Name", projectID);//根据返回的选中项值设置选中项  
        ViewData["ruturned"] = collection["drop"];
       }
       ViewData["drop"] = selec;
      return View();
    }