DropDownListFor & Navigation Properties

Question

I'm running into an issue trying to use @Html.DropDownListFor().

I have a model with a navigation property on it:

public class Thing {
  ...
  public virtual Vendor Vendor { get; set; }
}

In the controller I'm grabbing the vendor list to throw into the ViewBag:

public ActionResult Create() {
  ViewBag.Vendors = Vendor.GetVendors(SessionHelper.CurrentUser.Unit_Id);
  return View();
}

The html item in the view looks like this:

@Html.DropDownListFor(model => model.Vendor, new SelectList(ViewBag.Vendors, "Id", "Name"), "---- Select vendor ----")
@Html.ValidationMessageFor(model => model.Vendor)

The dropdown list is being rendered, and everything seems fine until I submit the form. The HttpPost Create method is returning false on the ModelState.IsValid and throwing a Model Error: The parameter conversion from type 'System.String' to type '...Models.Vendor' failed because no type converter can convert between these types.

If I let the page post through, I end up with a server error: Exception Details: System.ArgumentNullException: Value cannot be null. Parameter name: items

After searching high and low I haven't been able to find a reason that the @Html.DropDownListFor() isn't properly auto-binding a Vendor object to the navigation property.

Any help would be greatly appreciated.

EDIT: I ended up having to explicitly set the ForeignKey attributes so that I could directly access "Vendor_Id" then I changed the DropDownListFor to point to "Vendor_Id" instead of the navigation property. That seems to work.

TheGwa TheGwa · Accepted Answer · 2011-10-06T11:20:03

I have found that the best way to do this is as follows. Change the controller to create the SelectListItems.

   public ActionResult Create() {
       ViewBag.Vendors = Vendor.GetVendors(SessionHelper.CurrentUser.Unit_Id)
                               .Select(option => new SelectListItem
                               {
                                  Text = (option == null ? "None" : option.Name),
                                  Value = option.Id.ToString()
                               });
       return View();
    }

Then modify the view as follows:

@Html.DropDownListFor(model => model.Vendor, (IEnumerable<SelectListItem>)ViewBag.Vendors,    "---- Select vendor ----")
@Html.ValidationMessageFor(model => model.Vendor)

You have to cast the ViewBag.Vendors as (IEnumerable).

This keeps the views nice and neat. You could also move the code that gets the SelectListItems to your repo and put it in a method called something like GetVendorsList().

public IEnumerable<SelectListItem> GetVendorsList(int unitId){
    return Vendor.GetVendors(unitId)
                               .Select(option => new SelectListItem
                               {
                                  Text = (option == null ? "None" : option.Name),
                                  Value = option.Id.ToString()
                               });
}

This would separate concerns nicely and keep your controller tidy.

Good luck

DropDownListFor & Navigation Properties

2 Answers