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

Question

I am trying to populate a dropdown list from a database mapped with Linq 2 SQL, using ASP.NET MVC 2, and keep getting this error.

I am so confused because I am declaring a variable with the type IEnumerable<SelectListItem> on the second line, but the error makes me think this is not the case. I feel like this should be very simple, but I am struggling. Any help is appreciated.

Here are the interesting bits of my controller:

public ActionResult Create()
    {
        var db = new DB();
        IEnumerable<SelectListItem> basetypes = db.Basetypes.Select(b => new SelectListItem { Value = b.basetype, Text = b.basetype });
        ViewData["basetype"] = basetypes;
        return View();
    }

And here are the interesting bits of my view:

            <div class="editor-label">
            <%: Html.LabelFor(model => model.basetype) %>
        </div>
        <div class="editor-field">
            <%: Html.DropDownList("basetype") %>
            <%: Html.ValidationMessageFor(model => model.basetype) %>
        </div>

Here is the Post action when submitting the Form

        // POST: /Meals/Create

    [HttpPost]
    public ActionResult Create(Meal meal)
    {
        if (ModelState.IsValid)
        {
            try
            {
                // TODO: Add insert logic here
                var db = new DB();
                db.Meals.InsertOnSubmit(meal);
                db.SubmitChanges();
                return RedirectToAction("Index");
            }

            catch
            {
                return View(meal);
            }
        }

        else
        {
            return View(meal);
        }
    }

Thanks.

Answer 1

You are setting the collection as an item in ViewData dictionary and trying to retreive it as property on the model. A simple fix would be to reference it the same way as you set it:

    <%var basetype = ViewData["basetype"] as IEnumerable<SelectListItem>;%>
    <div class="editor-label">
        <%: Html.Label("basetype") %>
    </div>
    <div class="editor-field">
        <%: Html.DropDownList("basetype", basetype) %>
        <%: Html.ValidationMessage("basetype") %>
    </div>

Alternatively, the below code uses a strongly typed view:

public class ViewModel {
   //Model properties
   public IEnumerable<SelectListItem> basetype {get;set;}
}

public ActionResult Create()
    {
        var db = new DB();
        IEnumerable<SelectListItem> basetypes = db.Basetypes.Select(b => new SelectListItem { Value = b.basetype, Text = b.basetype });
        return View(new ViewModel { basetype=basetypes });
    }

Then, in your strongly typed view:

    <div class="editor-label">
        <%: Html.LabelFor(model => model.basetype) %>
    </div>
    <div class="editor-field">
        <%: Html.DropDownListFor(model=>model.basetype) %>
        <%: Html.ValidationMessageFor(model => model.basetype) %>
    </div>

Answer 2

Try adding a string for the name of your dropdown list as the first parameter, and get the item out of your viewdata:

 <%= Html.DropDownList("SomeDropdownName", (IEnumerable<SelectListItem>)ViewData["basetype"]) %>

Here is also an extension method you can use so the dropdown list is set up in a similar style to how you have done your other controls:

        public static string DropDownList<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<SelectListItem> selectList, string optionLabel)
        where TModel : class
    {
        string inputName = ExpressionHelper.GetInputName(expression);
        return htmlHelper.DropDownList(inputName, selectList, optionLabel);
    }

For example

<%= Html.DropDownList(x => x.BaseType, (IEnumerable<SelectListItem>)ViewData["basetype"], "")%>