MVC 3 Dropdown 使用 ViewBag 丢失所选值

Question

I am having an issue with dropdowns and the viewbag. I am setting a dropdown using the following code in my controller:

        applicationGuid = new Guid(form["applicationList"]);

        var applications = _applicationRepository.List();

        ViewBag.applicationList = new SelectList(applications, "Id", "Name", applicationGuid);

and in the view, this works perfectly and returns the previously selected value (applicationGuid):

 @Html.DropDownList("applicationList", "")

Now I want to use the following code as I want to add some attributes to the dropdown:

 @Html.DropDownList("applicationList", ViewBag.applicationList as SelectList, "", new { rel = "0", @class = "required" }) 

but for some reason, the selected value is not rendered (even though it is passed to the view and I can see selected = "true" against the correct item in the ViewBag SelectList).

The two examples above render as (this one has the selected="selected"):

<select id="applicationList" name="applicationList"><option value=""></option><option selected="selected" value="2f666645-9b28-406f-bd9f-9ecc009346a6">app1</option><option value="898cbbb5-5dff-4378-b15a-9ecc00b8242f">app2</option></select>

and like so (selected is gone:!):

<select class="required" id="applicationList" name="applicationList" rel="0"><option value=""></option><option value="2f666645-9b28-406f-bd9f-9ecc009346a6">app1</option><option value="898cbbb5-5dff-4378-b15a-9ecc00b8242f">app2</option></select> 

Can anyone explain what I'm doing wrong here and why it is losing the selected value? I have found a number of posts that go into how the names of view data items cannot clash etc but I have stripped this right down with random names and nothing seems to work? Is this a problem in MVC3?

Answer 1

The problem seems to be that when you pass the SelectList as a parameter to Html.DropDownList() , it doesn't like it to have the same name as the actual dropdown list.

I copied your code and encountered the same problem as you.

But as soon as I changed

@Html.DropDownList("applicationList", ViewBag.applicationList as SelectList, "", new { rel = "0", @class = "required" })

to

@Html.DropDownList("applicationListX", ViewBag.applicationList as SelectList, "", new { rel = "0", @class = "required" })

It produced the working output:

<select class="required" id="applicationListX" name="applicationListX" rel="0"><option value=""></option>
<option selected="selected" value="2f666645-9b28-406f-bd9f-9ecc009346a6">app1</option>
<option value="898cbbb5-5dff-4378-b15a-9ecc00b8242f">app2</option>
</select>

I don't know why that's the case, but there's your workaround.

Answer 2

After days of hacking around and not wanting to change the name of the Select List (because I didn't want to break autobinding of the model on post back), the alternate solution at the end of this article showed me the way. Don't use ViewBag. Use ViewData instead, with the same name as the model field's name, and either don't pass anything else to DropDownList, or pass null as the second parameter in case you want the third parameter for HTML attributes (I did).