MVC 3 Html.DropDownList error

Try to avoid dynamic variables like ViewBag and ViewData. It will make your code unreadable and painful to maintain in future as it grows. ViewBag is like Magic strings !

Switch to the ViewModel approach.

Example, If you are creating a View to Create a Book, Create a Viewmodel (it is just a plain Class) for that like this

public class BookViewModel
{
  public int BookId { set;get;}
  public string BookName {set;get;}
  public IEnumerable<SelectListItem> Attributes{ get; set; }
  public int SelectedAttribute{ get; set; }

}

Now in your GET Action, Simply create an object of this class, Set the BookAttribbutes proeprties to your Dropdown items and pass this ViewModel object to the View

public ActionResult Create()
{
  BookViewModel vm=new BookViewModel();
  //The below code is hardcoded for demo. you mat replace with DB data.
  vm.Attributes=new[]
  {
    new SelectListItem { Value = "1", Text = "F Cookie" },
    new SelectListItem { Value = "2", Text = "Title" },
  }
  return View(vm);
}

Now in We will make our view strongly typed to this ViewModel class

@model BookViewModel
@using(Html.BeginForm())
{
 @Html.TextBoxFor(x=>x.BookName)
 @Html.DropDownListFor(x => x.SelectedAttribute, 
      new SelectList(Model.Attributes, "Value", "Text"), "Select Attribute")

 <input type="submit" value="Save" />
}

Now you will get the Selected Dropdown value and the textbox value in your HttpPost action by accessing the corresponding properties of your ViewModel

[HttpPost]
public ActionResult Create(BookViewModel model)
{
  if(ModelState.IsValid)
  { 
    //check for model.BookName / model.SelectedAttribute
  }
  //validation failed.TO DO : reload dropdown here
  return View(model);
} 

Fully agree with View Model approach response to you (and its me who selected it as useful). However, if you don't want to switch, but remain as is I bet your answer lays in this article.

Hope this help you.