没有类型为“ IEnumable < SelectListItem >”且键为“ xxx”的 ViewData 项

Ok, so the answer was derived from some other posts about this problem and it is:

If your ViewData contains a SelectList with the same name as your DropDownList i.e. "submarket_0", the Html helper will automatically populate your DropDownList with that data if you don't specify the 2nd parameter which in this case is the source SelectList.

What happened with my error was:

Because the table containing the drop down lists was in a partial view and the ViewData had been changed and no longer contained the SelectList I had referenced, the HtmlHelper (instead of throwing an error) tried to find the SelectList called "submarket_0" in the ViewData (GRRRR!!!) which it STILL couldnt find, and then threw an error on that :)

Please correct me if im wrong

The problem is because of post back happens on submit button click. So while posting data on submit click again write before returning View()

ViewData["Submarkets"] = new SelectList(submarketRep.AllOrdered(), "id", "name");

I had same error, I think the problem is that the error text is confusing, because its giving a false key name.

In your case It should say "There is no ViewData item of type 'IEnumerable' that has the key "Submarkets"".

My error was a misspelling in the view code (your "Submarkets"), but the error text made me go crazy.

I post this answer because I want to say people looking for this error, like I was, that the problem is that its not finding the IENumerable, but in the var that its supposed to look for it ("Submarkets" in this case), not in the one showed in error ("submarket_0").

Accepted answer is very interesting, but as you said the convention is applied if you dont specify the 2nd parameter, in this case it was specified, but the var was not found (in your case because the viewdata had not it, in my case because I misspelled the var name)

Hope this helps!

Check the Namespace.

You might assign System.Web.Webpages.Html.SelectListItem in the Controller, instead of System.Web.Mvc.SelectListItem.

Old question, but here's another explanation of the problem. You'll get this error even if you have strongly typed views and aren't using ViewData to create your dropdown list. The reason for the error can becomes clear when you look at the MVC source:

// If we got a null selectList, try to use ViewData to get the list of items.
if (selectList == null)
{
selectList = htmlHelper.GetSelectData(name);
usedViewData = true;
}

So if you have something like:

@Html.DropDownList("MyList", Model.DropDownData, "")

And Model.DropDownData is null, MVC looks through your ViewData for something named MyList and throws an error if there's no object in ViewData with that name.

In my case there was a conflict in the namespaces , I have:

using System.Web.Mvc;

and

using System.Collections.Generic;

I explicitly want to use the Mvc one so I declared it as :

new System.Web.Mvc.SelectList(...)

This is OK too; For example:
==> In "NumberController" file:

public ActionResult Create([Bind(Include = "NumberId,Number1,Number2,OperatorId")] Number number)
{
if (ModelState.IsValid)
{
...
...
return RedirectToAction("Index");
}
ViewBag.OperatorId = new SelectList(db.Operators, "OperatorId",
"OperatorSign", number.OperatorId);
return View();
}

==> In View file (Create.cshtml):

<div class="form-group">
@Html.LabelFor(model => model.Number1, htmlAttributes: new { @class =
"control-label col-md-2" })
<div class="col-md-10">
@Html.EditorFor(model => model.Number1, new { htmlAttributes = new {
@class = "form-control" } })
@Html.ValidationMessageFor(model => model.Number1, "", new { @class =
"text-danger" })
</div>
</div>

Now if we remove this statement:

ViewBag.OperatorId = new SelectList(db.Operators, "OperatorId", "OperatorSign", number.OperatorId);

from back of the following statement (in our controller) :

return View();

we will see this error:

There is no ViewData item of type 'IEnumerable' that has the key 'OperatorId'.

* So be sure of the existing of these statements. *

For me, the problem that caused this error arose when I was saving a new row to the database, but a field was null. In the database table design, that field is NOT NULL. So when I tried to save a new row with a null value for not-null field, Visual Studio threw this error. Thus, I made sure that the field was assigned a value, and the problem was fixed.

In my case, I found that I set the post method as private mistakenly. after changing private to public.

[HttpPost]
private async Task<ActionResult> OnPostRemoveForecasting(){}

change to

[HttpPost]
public async Task<ActionResult> OnPostRemoveForecasting(){}

Now works fine.

The cause isn't contrary to syntax rather than inappropriate usage of objects. Life Cycle of objects in ViewData, ViewBag, & View Life Cycle is shorter than in the session. Data defined in the formers will be lost after a request-response(if try to access after a request-response, you will get exceptions). So the formers are appropriate for passing data between View & Controller while the latter for storing temporary data. The temporary data should store in the session so that can be accessed many times.