C# – The ViewData item that has the key ‘XXX’ is of type ‘System.Int32’ but must be of type ‘IEnumerable

asp.net-mvcc++

I have the following view model

public class ProjectVM
{
    ....
    [Display(Name = "Category")]
    [Required(ErrorMessage = "Please select a category")]
    public int CategoryID { get; set; }
    public IEnumerable<SelectListItem> CategoryList { get; set; }
    ....
}

and the following controller method to create a new Project and assign a Category

public ActionResult Create()
{
    ProjectVM model = new ProjectVM
    {
        CategoryList = new SelectList(db.Categories, "ID", "Name")
    }
    return View(model);
}

public ActionResult Create(ProjectVM model)
{
    if (!ModelState.IsValid)
    {
        return View(model);
    }
    // Save and redirect
}

and in the view

@model ProjectVM
....
@using (Html.BeginForm())
{
    ....
    @Html.LabelFor(m => m.CategoryID)
    @Html.DropDownListFor(m => m.CategoryID, Model.CategoryList, "-Please select-")
    @Html.ValidationMessageFor(m => m.CategoryID)
    ....
    <input type="submit" value="Create" />
}

The view displays correctly but when submitting the form, I get the following error message

InvalidOperationException: The ViewData item that has the key 'CategoryID' is of type 'System.Int32' but must be of type 'IEnumerable<SelectListItem>'.

The same error occurs using the @Html.DropDownList() method, and if I pass the SelectList using a ViewBag or ViewData.

Best Solution

The error means that the value of CategoryList is null (and as a result the DropDownListFor() method expects that the first parameter is of type IEnumerable<SelectListItem>).

You are not generating an input for each property of each SelectListItem in CategoryList (and nor should you) so no values for the SelectList are posted to the controller method, and therefore the value of model.CategoryList in the POST method is null. If you return the view, you must first reassign the value of CategoryList, just as you did in the GET method.

public ActionResult Create(ProjectVM model)
{
    if (!ModelState.IsValid)
    {
        model.CategoryList = new SelectList(db.Categories, "ID", "Name"); // add this
        return View(model);
    }
    // Save and redirect
}

To explain the inner workings (the source code can be seen here)

Each overload of DropDownList() and DropDownListFor() eventually calls the following method

private static MvcHtmlString SelectInternal(this HtmlHelper htmlHelper, ModelMetadata metadata,
  string optionLabel, string name, IEnumerable<SelectListItem> selectList, bool allowMultiple,
  IDictionary<string, object> htmlAttributes)

which checks if the selectList (the second parameter of @Html.DropDownListFor()) is null

// If we got a null selectList, try to use ViewData to get the list of items.
if (selectList == null)
{
    selectList = htmlHelper.GetSelectData(name);
    usedViewData = true;
}

which in turn calls

private static IEnumerable<SelectListItem> GetSelectData(this HtmlHelper htmlHelper, string name)

which evaluates the the first parameter of @Html.DropDownListFor() (in this case CategoryID)

....
o = htmlHelper.ViewData.Eval(name);
....
IEnumerable<SelectListItem> selectList = o as IEnumerable<SelectListItem>;
if (selectList == null)
{
    throw new InvalidOperationException(String.Format(CultureInfo.CurrentCulture, 
        MvcResources.HtmlHelper_WrongSelectDataType,
        name, o.GetType().FullName, "IEnumerable<SelectListItem>"));
}

Because property CategoryID is typeof int, it cannot be cast to IEnumerable<SelectListItem> and the exception is thrown (which is defined in the MvcResources.resx file as)

<data name="HtmlHelper_WrongSelectDataType" xml:space="preserve">
    <value>The ViewData item that has the key '{0}' is of type '{1}' but must be of type '{2}'.</value>
</data>
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