DropDownListFor no ViewData item of type IEnumerable

Question

Model:

public class Category
{
    [Key]
    public int ID { get; set; }

    [Required(ErrorMessage = "Kategoria jest wymagana!")]
    public string Kategoria { get; set; }

    [Required(ErrorMessage = "Kod jest wymagany!")]
    public string Kod { get; set; }
}

Controller:

public ActionResult DodajPrzedmiot()
{
    if (StaticFunctions.IfLogged())
    {
        using (var db = new DatabaseContext())
        {
            var cats = from b in db.Categories
                       select new { b.Kategoria };

            var x = cats.ToList().Select(c => new SelectListItem
            {
                Text = c.Kategoria,
                Value = c.Kategoria
            }).ToList();

            //List<SelectListItem> catList = new List<SelectListItem>();
            //foreach (var t in cats)
            //{
            //    SelectListItem s = new SelectListItem();
            //    s.Text = t.ToString();
            //    s.Value = t.ToString();
            //    catList.Add(s);
            //}
            ViewBag.Kategoria = x;
        }
        return View();
    }
    else
    {
        return RedirectToAction("Logowanie", "User");
    }
}

View:

<div class="form-group">
        @Html.LabelFor(model => model.Kategoria, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownListFor(model => model.Kategoria, null)
        </div>
    </div>  

The problem is that this list shows values that I selected from database, when I add an item with selected e.g. "Biurowe" category it adds informations to database but in the same time it gives me error:

An exception of type 'System.InvalidOperationException' occurred in System.Web.Mvc.dll but was not handled in user code

Additional information: There is no ViewData item of type 'IEnumerable' that has the key 'Kategoria'.

What should I change here?

Answer 1

The second parameter in @Html.DropDownListFor(model => model.Kategoria, null) should be the list of items which would be ViewBag.Kategoria in your case. I would even suggest that you use a different name like ViewBag.KategoriaList. I've run into problems sometimes when using the same name for list in ViewBag as that of the property being bound to DropDownList.

Try this code:

public ActionResult DodajPrzedmiot()
    {
        if (StaticFunctions.IfLogged())
        {
            using (var db = new DatabaseContext())
            {
                var cats = from b in db.Categories
                           select new { b.Kategoria };

                var x = cats.ToList().Select(c => new SelectListItem
                {
                    Text = c.Kategoria,
                    Value = c.Kategoria
                }).ToList();

                ViewBag.KategoriaList = x;
            }
            return View();
        }
        else
        {
            return RedirectToAction("Logowanie", "User");
        }
    }

In the View:

<div class="form-group">
        @Html.LabelFor(model => model.Kategoria, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownListFor(model => model.Kategoria, ViewBag.KategoriaList as IEnumerable<SelectListItem>)
        </div>
    </div>  

You have to also populate the ViewBag in POST action. This is what your POST action should be:

[HttpPost]
    public ActionResult DodajPrzedmiot(Item itm)
    {
        if (ModelState.IsValid)
        {
            try
            {
                using (var db = new DatabaseContext())
                {
                    // try putting this code in a separate function and use that function in both actions to populate the ViewBag.
                    var cats = from b in db.Categories
                               select new { b.Kategoria };

                    var x = cats.ToList().Select(c => new SelectListItem
                    {
                        Text = c.Kategoria,
                        Value = c.Kategoria
                    }).ToList();

                    ViewBag.KategoriaList = x;

                    if (itm.Ilosc_zakupiona - itm.Ilosc_wypozyczona < 0) throw new ArgumentOutOfRangeException();
                    itm.Ilosc_magazynowa = itm.Ilosc_zakupiona - itm.Ilosc_wypozyczona;
                    db.Items.Add(itm);
                    db.SaveChanges();
                }
            }
            catch (System.Data.Entity.Infrastructure.DbUpdateException)
            {
                ViewBag.ErrorMessage = "Istnieje już przedmiot o takiej nazwie i/lub kodzie!";
                return View();
            }
            catch (ArgumentOutOfRangeException)
            {
                ViewBag.ErrorMessage = "Ilość zakupiona i/lub wypożyczona nie mogą być mniejsze od zera!";
                return View();
            }
        }
        return View();
    }