Persist SelectList in model on Post

问题:

In MVC4:

I have the following property in my model used for a dropdown list:

public SelectList Subjects { get; set; }

I set the Subjects property in my Index() Action on page load and return the model.

The dropdown gets populated just fine with the SelectListItems.

@Html.DropDownListFor(x => x.Subject, new SelectList(Model.Subjects, "Text", "Text", "Other"))

When I submit the form the Subjects SelectList in the model has changed to null. There has to be a simple way to persist this on HttpPost. I assume I want to submit and post this SelectList as well, along with all the form fields? How would I do this?

回答1:

It is commonly accepted that you re-populate a SelectList after the Post action. Just extract it inside a method and call it in the Get and Post action.

Posting it back again to the controller is not the way to go. You can cache the items in the SelectList so you won't have to make a query to the data store twice.

Example:

public ActionResult Create()
{
    var model = new SubjectModel();
    PopulateSubjectList(model);
    return View(model);
}

[HttpPost]
public ActionResult Create(SubjectModel model)
{
    if (ModelState.IsValid)
    {
        // Save item..
    }
    // Something went wrong.
    PopulateSubjectList(model);
    return View(model);
}

private void PopulateSubjectList(SubjectModel model)
{
    if (MemoryCache.Default.Contains("SubjectList"))
    {
        // The SubjectList already exists in the cache,
        model.Subjects = (List<Subject>)MemoryCache.Default.Get("SubjectList");
    }
    else
    {
        // The select list does not yet exists in the cache, fetch items from the data store.
        List<Subject> selectList = _db.Subjects.ToList();

        // Cache the list in memory for 15 minutes.
        MemoryCache.Default.Add("SubjectList", selectList, DateTime.Now.AddMinutes(15));
        model.Subjects = selectList;
    }
}

Note: MemoryCache uses the System.Runtime.Caching namespace. See: System.Runtime.Caching namespace.

Also, caching should be in a seperate layer between your controller (or business layer) and the data access layer, this is just for clarity.

回答2:

Browsers only post back the selected values on the form elements. Also, its not a good idea to post back the values which can be retrieved from the data store. You would have to pull the items in the list just like you did while populating the list.

Also, MVC does not maintain the state of the page like .NET webpages as it does not have a view state. Developers are fully responsible for managing states of pages between the post backs, which is the essence of MVC design pattern.