Value cannot be null. Parameter name: items on DropDownListFor

Question

I'm trying to return a list of emails from a model using DropDownListFor. I have found a few topics on this issue, so it seems to be common, and I presume the list needs to be repopulated, but I can't find a way that works (or a way that I understand against the other examples).

I always get the null error regardless of what I do. I'm thinking that maybe actually it is not returning any emails in the first place, but I'm not understanding the way dropdownlist works with a model.

Is there a different issue causing the null that I can't see right now?

I define the list in a class:

public class User {
    [Required]
    [Display(Name = "Email Address")]
    public string Email { get; set; }
    public IEnumerable<SelectListItem> EmailList { get; set; }
}

Controller:

[HttpGet]
public ActionResult ChangeDetails()
{
    var u = new User();
    ViewBag.DropDownList = new SelectList(u.EmailList, "Email", "Email");
    return View(u);
}

View:

@Html.DropDownListFor(model => model.Email, (IEnumerable<SelectListItem>)ViewBag.DropDownList, "---Select a value---", new { htmlAttributes = new { @class = "form-control" } })

Error message:

Message: Value cannot be null. Parameter name: items

at System.Web.Mvc.MultiSelectList..ctor(IEnumerable items, String dataValueField, String dataTextField, String dataGroupField, IEnumerable selectedValues, IEnumerable disabledValues, IEnumerable disabledGroups) ....

try passing "ViewBag.DropDownList" in the SelectList inside your razor view, rather than Model.EmailList — Nirman
– Nirman, Commented Sep 18, 2017 at 10:29
@Nirman I actually had that in and changed it to new SelectList ... trial and error really. Edited the main post to add the ViewBag in razor. — kamil1995b
– kamil1995b, Commented Sep 18, 2017 at 10:31
Because u.EmailList is null (you did not initialize it, let alone populate it with any items) — user3559349
– user3559349, Commented Sep 18, 2017 at 12:24
@Stephen Muecke How can I populate it with items? Been trying to wrap the answer (lst) below in a foreach and doing SelectListItem item in user.EmailList, but to no avail. I feel like I'm not understanding something. — kamil1995b
– kamil1995b, Commented Sep 18, 2017 at 13:13
But all you have done is var u = new User(); which just initializes a new User which wont have any items. And you do not even need ViewBag.DropDownList = ... The model already contains a property IEnumerable<SelectListItem> EmailList so in the view its just @Html.DropDownListFor(model => model.Email, Model.EmailList, "---Select a value---", new { ... }) (except that you must first initialize EmailList so its not null) — user3559349
– user3559349, Commented Sep 18, 2017 at 13:26

Willy David Jr · Accepted Answer · 2017-09-18 11:10:17Z

3

Try this as SelectListItem:

IList<SelectListItem> lst = new List<SelectListItem>();
        lst.Add(new SelectListItem()
            {
               Value = "Hello",
                Text = "World"
            });        
ViewBag.DropDownList = lst;

Or:

var u = new User();
ViewBag.DropDownList = u.EmailList //You need to populate your EmailList first before you declare it here.
return View(u);

Another option is this:

IList<SelectListItem> lst = new List<SelectListItem>();
        lst.Add(new SelectListItem()
            {
               Value = "Email",
               Text = "Email"
            });            
ViewBag.DropDownList = new SelectList(p.EmailList, "Value", "Text");

and on your View:

@Html.DropDownListFor(model => model.Email, (SelectList)ViewBag.DropDownList, "---Select a value---", new { htmlAttributes = new { @class = "form-control" } })

edited Sep 18, 2017 at 11:10

answered Sep 18, 2017 at 10:47

Willy David Jr

9,2017 gold badges51 silver badges62 bronze badges

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2 Comments

kamil1995b Over a year ago

The first solution just returns one record World whereas it should return a list of emails. Have I missed the part where I should be looking/querying for them? I thought the model is able to resolve it automatically.

Willy David Jr Over a year ago

Of course it's just my example, you need to populate that with your desired record. I just give you lead on how you it should work and prevent the error.

rikobgrff · Accepted Answer · 2023-03-16 20:27:04Z

0

ensure that you are passing your selectlist in both get and post methods

answered Mar 16, 2023 at 20:27

rikobgrff

194 bronze badges

1 Comment

KiynL Over a year ago

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