ASP.NET MVC 2 - DropDownFor -- Force user to select a value for a non-nullable type?

Question

Hey all,

I have (or so I think!) a simple problem.

I will greatly simplify my model to speed this along.

I have a model class lets call it item:

public class Item    
{
    [Required(ErrorMessage="Must indicate if product is to be tracked by serial number.")]
    public bool TrackedBySerial { get; set; }
}

I have an "Add" view where I create a DropDownList like so:

<%= Html.DropDownListFor(model=>model.TrackedBySerial, new SelectList(new List<object> {null,true,false},null),"Select One") %>
<%= Html.ValidationMessageFor(model => model.TrackedBySerial) %>

My problem is that unless I create my boolean value in the model to be a nullable type, I can't force the blank default value.

If I use the Html.DropDownList() instead of DropDownListFor() , is there any way for me to use ModelState.IsValid -- or do I need to mix my own custom validation in my controller as well?

Update: So I got the functionality I was looking for, its just a bit more verbose than I'd have liked. Is there a better way to do this?

Controller:

  [HttpPost]
    public ActionResult Add(InventoryItem newItem)
    {
        try
        {

            //get the selected form value
            string formVal = Request.Form["TrackBySerial"];

            //convert to true, false, or null
            bool? selectedValue = TryParseNullable.TryParseNullableBool(formVal);

            //if there is no value, add an error
            if (!selectedValue.HasValue)
            {
                ModelState.AddModelError("TrackBySerial", "You must indicate whether or not this part will be tracked by serial number.");
            }
            else
            {//otherwise add it to the model

                newItem.TrackedBySerial = selectedValue.Value;
            }         


            if (ModelState.IsValid)
            {
                //add part into inventory
                newItem.ID = inventory.AddNewProduct(newItem).ToString();

                //log the action
                logger.LogAction(ActionTypes.ProductCatalog, Request.LogonUserIdentity.Name, "Added new product: " + newItem.PartNumber);

                //put into QA queue here?

                //redirect to edit screen to add suppliers
                return RedirectToAction("Edit", new { id = newItem.ID });


            }

            ViewData["TrackBySerial"] = new SelectList(new object[] { true, false },selectedValue);
            return View(newItem);
        }}


 /// <summary>
    /// Form for adding new catalog entries
    /// </summary>
    /// <returns></returns>
    /// GET: /Purchasing/Catalog/Add
    public ActionResult Add()
    {

        InventoryItem newItem = new InventoryItem();



        ViewData["TrackBySerial"] = new SelectList(new object[] { true, false });

        return View(newItem);
    }

view:

    <div class="editor-field">

            <%=Html.DropDownList("TrackBySerial","- Select One- ") %>                
            <%=Html.ValidationMessage("TrackBySerial") %>


            <%= Html.ValidationMessageFor(model => model.TrackedBySerial) %>
        </div>