"No parameterless constructor defined for this object" error when submitting a form with a DropDownList

Question

When I click on the submit button in the example below, I get an error message like this one:

No parameterless constructor defined for this object.

After getting the error message I am unable to continue using the application. I tried to debug but I don't get any useful information to help in fixing the issue.

Controller:

[MvcApplication.SessionExpire]
    [HttpGet]
    public ActionResult Index()
    {
        DataLinqDB db = new DataLinqDB();

        OpgaverPage model = new OpgaverPage();

        var random = new Random();
        var antalopgaver = db.opgavers.Count();
        var number = random.Next(antalopgaver);

        List<opgaver> List = db.opgavers.Skip(number).Take(1).ToList();
        var A = db.opgavers.Skip(number).Take(1).FirstOrDefault();
        model.Overskift = A.overskift;

        model.OpgaveValueList = new SelectList(
                                new List<SelectListItem>
                                {
                                    new SelectListItem { Selected = true, Text = A.svar1, Value = "A"},
                                    new SelectListItem { Selected = false, Text = A.svar2, Value = "B"},
                                    new SelectListItem { Selected = false, Text = A.svar3, Value = "C"},
                                }, "Value", "Text", 1);

        //angiver id til opgaven
        model.HiddenId = A.Id;

        //Fremvis hvornår man har fået point
        //List<pointantal> PointValue = db.pointantals.Where(x => x.omrade == o && x.brugerid == brugerId).OrderByDescending(i => i.Id).Take(20).ToList();
        //model.ListPoint = PointValue.ToList();

        return View(model);
    }

    //error her
    [MvcApplication.SessionExpire]
    [HttpPost]
    public ActionResult index(OpgaverPage opgavervalue)
    {
        if (ModelState.IsValid)
        {
            DataLinqDB db = new DataLinqDB();
            var opgaver = db.opgavers.FirstOrDefault(i => i.Id == opgavervalue.HiddenId);
            if (opgaver != null)
            {
                if (new SelectList(opgavervalue.OpgaveValueList).SelectedValue.ToString() == "A" 
                    || new SelectList(opgavervalue.OpgaveValueList).SelectedValue.ToString() == "B" 
                    || new SelectList(opgavervalue.OpgaveValueList).SelectedValue.ToString() == "C")
                {
                    Value();
                }
                else
                {
                    //error
                    ModelState.AddModelError("", "Desværre dit svar forkert");
                }
            }
            else
            {
                return RedirectToAction("index", opgavervalue);
            }
        }
        return View();
    }

Model:

public class OpgaverPage
{
    public string Overskift
    {
        get; set;
    }

    public int HiddenId
    {
        get; set;
    }

    [Display(Name ="Spørgsmål")]
    public string Svar
    {
        get; set;
    }

    public SelectList OpgaveValueList
    {
        get; set;
    }

    public List<opgaver> ListPoint
    {
        get; set;
    }
}

View:

@using (Html.BeginForm("index", "spil"))
{
    @Html.AntiForgeryToken()
    @Html.ValidationSummary(true)

    @Html.HiddenFor(i => i.HiddenId)

    <div class="form-group">
        @Html.LabelFor(x => x.Svar)<br />
        @Html.DropDownListFor(x => x.OpgaveValueList, Model.OpgaveValueList, new
   {
       @class = "form-control"
   })
    </div>


    <button type="submit" class="btn btn-effect-ripple btn-success"><i class="fa fa-check"></i> Tjek mit svar</button>
}

I tried to follow the answer here but I was unable to fix the issue: ASP.NET MVC Dropdown List From SelectList

What is causing this issue?

make sure your model `OpgaverPage` has a parameterless constructor — JamieD77, Dec 01 '15 at 20:12
@JamieD77 Good suggestion, but I guess `OpgaverPage` has a parameterless constructor - he is creating an instance of it using a default constructor in `Index()` (the GET variant) and does not mention any compilation issues. — Lasse Christiansen, Dec 01 '15 at 20:15

score 0 · Accepted Answer · answered Dec 01 '15 at 20:34

0

You cannot bind a <select> to a SelectList. A <select> element post s back a single value. Your model needs a property to bind to, say

public string SelectedOpgaveValue { get; set; }

and then in the view

 @Html.DropDownListFor(x => x.SelectedOpgaveValue, Model.OpgaveValueList, new { @class = "form-control" })

The reason you get the error is that your attempting to bind to a property which is type of SelectList which has no parameterless constructor.

Also you should modify the code for generating the select list to

public IEnumerable<SelectListItem> OpgaveValueList { get; set; }

model.OpgaveValueList = new List<SelectListItem>
{
    new SelectListItem { Text = A.svar1, Value = "A"},
    new SelectListItem { Text = A.svar2, Value = "B"},
    new SelectListItem { Text = A.svar3, Value = "C"},
};

There is no point creating a second select list from the first one. And there is no point setting the Selected property of SelectListItem when your binding to a property because its the value of the property that determines what is selected - if the value of SelectedOpgaveValue is "B", then the second option will be selected irrespective of what values you give the Selected property (the Selected is simply ignored by the HtmlHelper)

answered Dec 01 '15 at 20:34

The problem right now is when I click http post, so when the like must reload the page. then there's no content into the dropdown list. – Jesper Petersen Dec 02 '15 at 07:44
That because if you return the view, you need to repopulate the `SelectList` property just as you did in the GET method – Dec 02 '15 at 07:45
And you need to remove all that `if (new SelectList(opgavervalue.OpgaveValueList).SelectedValue.ToString() == "A" ....` nonsense in your POST method. The value of the property `SelectedOpgaveValue` will contain the selected value (either "A", "B", or "C"). – Dec 02 '15 at 07:51
What do you mean _which one is right_? What are you referring to? – Dec 02 '15 at 08:27
If I choose answers, how do I find out. – Jesper Petersen Dec 02 '15 at 08:36
What _answers_? Is that what `OpgaveValueList` is - a list of possible answers? As per my answer, you add a property to your model - e.g. `public string SelectedOpgaveValue { get; set; }`and you bind the dropdownlist to it. Then when you post your model, the value of the property `SelectedOpgaveValue` will be either `"A", "B" or "C"` depending on which option the user chose. – Dec 02 '15 at 08:39

"No parameterless constructor defined for this object" error when submitting a form with a DropDownList

1 Answers1