Why is ListBoxFor not selecting items, but ListBox is?

Question

I have the following code in my view:

<%= Html.ListBoxFor(c => c.Project.Categories,
        new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

<%= Html.ListBox("MultiSelectList", 
        new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

The only difference is that the first helper is strongly typed (ListBoxFor), and it fails to show the selected items (1,2), even though the items appear in the list, etc. The simpler ListBox is working as expected.

I'm obviously missing something here. I can use the second approach, but this is really bugging me and I'd like to figure it out.

For reference, my model is:

public class ProjectEditModel
{
    public Project Project { get; set; }
    public IEnumerable<Project> Projects { get; set; }
    public IEnumerable<Client> Clients { get; set; }
    public IEnumerable<Category> Categories { get; set; }
    public IEnumerable<Tag> Tags { get; set; }
    public ProjectSlide SelectedSlide { get; set; }
}

Update

I just changed the ListBox name to Project.Categories (matching my model) and it now FAILS to select the item.

<%= Html.ListBox("Project.Categories",
        new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

I'm obviously not understanding the magic that is happening here.

Update 2

Ok, this is purely naming, for example, this works...

<%= Html.ListBox("Project_Tags",
new MultiSelectList(Model.Tags, "Id", "Name", Model.Project.Tags.Select(t => t.Id)))%>

...because the field name is Project_Tags, not Project.Tags, in fact, anything other than Tags or Project.Tags will work. I don't get why this would cause a problem (other than that it matches the entity name), and I'm not good enough at this to be able to dig in and find out.

Answer 1

I've stumbled across this problem myself, finally I realized that the problem was a naming convention.

You cannot name the ViewBag or ViewData poperty containing the SelectList or MultiSelectList to the same name your property model containing the selected items. At least not if you're using the ListBoxFor or DropDownListFor helper.

Here's an example:

    public class Person
    {
          public List<int> Cars { get; set; }
    }

    [HttpGet]
    public ActionResult Create()
    {
          //wont work
          ViewBag.Cars = new SelectList(carsList, "CarId", "Name"); 

          //will work due to different name than the property.
          ViewBag.CarsList = new SelectList(carsList, "CarId", "Name"); 

          return View();
    }

    //View
    @Html.ListBoxFor(model => model.Cars, ViewBag.CarsList as SelectList)

I'm sure theres plenty of other ways doing this, but it solved my problem, hope it will help someone!