Question

I have the following error when I click save:

the ViewData item that has the key ‘SelectedCategoryId’ is of type ‘System.Int32’ but must be of type ‘IEnumerable’?

my controller:

public ActionResult IndexTwee()
    {

        var listCategories = new List<SelectListItem>();

        listCategories.Add(new SelectListItem() {Text="foo",Value="1" });
        listCategories.Add(new SelectListItem() { Text = "bar", Value = "2" });

        MyViewModelTwee model = new MyViewModelTwee() { };

        model.Categories = listCategories;
        model.SelectedCategoryId = 2;

        return View(model);
    }


    [HttpPost]
    public ActionResult IndexTwee(MyViewModelTwee Model)
    {

        return View(Model);

    }

my model:

  public class MyViewModelTwee
{
    public int SelectedCategoryId { get; set; }
    public IEnumerable<SelectListItem> Categories { get; set; }
}

my view:

@model mvc3DropDown.Models.MyViewModelTwee
@using (Html.BeginForm())
{
    @Html.DropDownListFor(
    x => x.SelectedCategoryId,
    Model.Categories
)
    <button>Save</button>

}

Answer 1

Don’t forget to rebind the list in your POST action:

[HttpPost]
public ActionResult Index(MyViewModelTwee Model)
{
    var listCategories = new List<SelectListItem>();
    listCategories.Add(new SelectListItem() { Text = "foo", Value = "1" });
    listCategories.Add(new SelectListItem() { Text = "bar", Value = "2" });
    Model.Categories = listCategories;
    return View(Model);
}

Remember that only the selected value is sent when you submit the html <form>. The other values are lost so you need to refetch them from wherever you fetched them in the GET action. Of course you will externalize this code into a repository layer so that your code now looks like this:

public ActionResult IndexTwee()
{
    var model = new MyViewModelTwee
    {
        SelectedCategoryId = 2,
        Categories = _repository.GetCategories()
    };
    return View(model);
}

[HttpPost]
public ActionResult IndexTwee(MyViewModelTwee Model)
{
    Model.Categories = _repository.GetCategories();
    return View(Model);
}