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

Question

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>

according to stephens (user3559349) answer, this can be useful:

@Html.DropDownListFor(m => m.CategoryID, Model.CategoryList ?? new List<SelectListItem>(), "-Please select-")

or in ProjectVM:

public class ProjectVM
{
    public ProjectVM()
    {
        CategoryList = new List<SelectListItem>();
    }
    ...
}

Most Likely Caused some sort of error redirecting to your page and you not initializing your model's drop down lists again.

Make sure that you initialize your drop downs in either the model's constructor or every time before you send said model to the page.

Otherwise you will need to maintain the state of the drop down lists either through the view bag or through the hidden value helpers.

OK, the poster's canned answer neatly explained why the error occurred, but not how to get it to work. I'm not sure that's really an answer, but it did point me in the right direction.

I ran into the same issue and found a slick way to resolve it. I'll try to capture that here. Disclaimer - I work on web pages once a year or so and really don't know what I'm doing most of the time. This answer should in no way be considered an "expert" answer, but it does the job with little work...

Given that I have some data object (most likely a Data Transfer Object) that I want to use a drop-down list to supply valid values for a field, like so:

public class MyDataObject
{
  public int id;
  public string StrValue;
}

Then the ViewModel looks like this:

public class MyDataObjectVM
{
  public int id;

  public string StrValue;
  public List<SectListItem> strValues;
}

The real problem here, as @Stephen so eloquently described above, is the select list isn't populated on the POST method in the controller. So your controller methods would look like this:

// GET
public ActionResult Create()
{
  var dataObjectVM = GetNewMyDataObjectVM();
  return View(dataObjectVM); // I use T4MVC, don't you?
}

private MyDataObjectVM GetNewMyDataObjectVM(MyDataObjectVM model = null)
{
  return new MyDataObjectVM
  {
    int id = model?.Id ?? 0,
    string StrValue = model?.StrValue ?? "", 
    var strValues = new List<SelectListItem> 
      { 
        new SelectListItem {Text = "Select", Value = ""},
        new SelectListITem {Text = "Item1", Value = "Item1"},
        new SelectListItem {Text = "Item2", Value = "Item2"}
      };
  };
}

// POST
public ActionResult Create(FormCollection formValues)
{
  var dataObject = new MyDataObject();

  try
  {
    UpdateModel(dataObject, formValues);
    AddObjectToObjectStore(dataObject);

    return RedirectToAction(Actions.Index);
  }
  catch (Exception ex)
  {
    // fill in the drop-down list for the view model
    var dataObjectVM = GetNewMyDataObjectVM();
    ModelState.AddModelError("", ex.Message);

    return View(dataObjectVM);
  )
}

There you have it. This is NOT working code, I copy/pasted and edited to make it simple, but you get the idea. If the data members in both the original data model and the derived view model have the same name, UpdateModel() does an awesome job of filling in just the right data for you from the FormCollection values.

I'm posting this here so I can find the answer when I inevitably run into this issue again -- hopefully it will help someone else out as well.