ASP.NET MVC cannot get dropdownlist to set selected value

Question

In my ASP.NET MVC application, I have a drop-down list in a form on a page, where the dropdown list item that should be selected when the page loads don't get selected.

The ViewModel for the page has a property for the list of SelectListItems that should be used to populate the drop-down list:

See below for the View model and illustration of the "Index" class, which has HeaderItemID as one of it's properties:

public class IndexItemResult
{
    public Index FocalIndex { get; set; }
    public string FocalIndexHeaderItemText { get; set; }
    public List<Item> IndexItems { get; set; }
    public List<SelectListItem> ItemSelectList { get; set; }
    public List<Item> ItemList { get; set; }

    public IndexItemResult() { }
}

public class Index
{
    public int IndexID { get; set; }
    public string IndexName { get; set; }
    public int IndexHeaderItemID { get; set; }
}

I set the "text", "value", and "selected" properties of items in the SelectListItems from a database LINQ query in my controller. This method also sets the "FocalIndex" property of the ViewModel. As HeaderItemID is a property of FocalIndex, I therefore assumed that "HeaderItemID" is available as a field that can be bound to.

"HeaderItemID" is the item that should be selected. When I look at the list of SelectListItems that gets generated, everything looks ok - the "text" and "value" properties are all fine, and the correct item is specified as "selected".

private IndexItemResult GetIndexItemResult(int? indexID)
{
    IndexItemResult indexItemResult = new IndexItemResult();
    Index focalIndex = db.Indexes.Find(indexID);
    indexItemResult.FocalIndex = focalIndex;

    int HeaderItemID = focalIndex.IndexHeaderItemID;

    indexItemResult.ItemSelectList = db.ItemTexts.
        Where(it => it.LanguageID == 1).
        Select(iText =>
            new SelectListItem
            {
                Selected =
                (
                    iText.ItemID == HeaderItemID ? true : false
                ),
                Value = iText.ItemID.ToString(),
                Text = String.Concat(iText.ID.ToString(), " ", iText.Text)
            }).ToList();

    return indexItemResult;
}

I then have a form on the page, where I specify the dropdown list with the @Html.DropDownList helper. I pass the list of SelectListItems from the ViewModel in the second parameter.

@Html.DropDownList("IndexHeaderItemID", Model.ItemSelectList,
  htmlAttributes: new { @class = "form-control", @id = "HeaderItemDDL" })

The drop-down list renders with all the items as expected, but the item that should be set at "selected" is not selected. Instead, the first item is selected. Any suggestions for what I'm missing here?

Chris Pratt · Accepted Answer · 2015-10-12 13:54:07Z

1

I see no place where you're actually setting anything in ModelState named IndexHeaderItemID. Since that is the name you're binding to, then one of Request["IndexHeaderItemID"], ViewData["IndexHeaderItemID"], ViewBag.IndexHeaderItemID or Model.IndexHeaderItemID must be set to the value you expect to be selected.

answered Oct 12, 2015 at 13:54

Chris Pratt

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5 Comments

Proposition Joe Over a year ago

Thanks for the suggestion - I updated the question with some additional information. Basically, "HeaderItemID" is a property of ViewModel.FocalIndex.

Chris Pratt Over a year ago

Then you would need to bind to to "FocalIndex.HeaderItemID" to have it populate properly. This is why you should use the *For helpers, as you just pass the property you want to bind to, and then Razor will figure out what the field name should be to bind it properly.

Proposition Joe Over a year ago

Great! Thanks a lot! I didn't realise that the field name parameter in the helper method is what always drives which item in the list is selected. So it looks like setting the "selected" property on the item that is meant to be selected in the SelectListItem list is superfluous? In the code above, I tried commenting out the " Selected = (iText.ItemID == HeaderItemID ? true : false) " code in the controller method. And it still works. A bit confusing... Do we ever need to set the "selected" property on the SelectList items then?

Chris Pratt Over a year ago

Not really, no. Razor will select the appropriate value based on ModelState, and as you've seen, will actually override your explicitly set Selected value. In fact, you don't even need a SelectList, just an IEnumerable<SelectListItem>.

Proposition Joe Over a year ago

OK, really useful clarification. Thanks!

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