Question

I’m new to MVC, but I’ve been all over this, read all the documentation and all the questions and all the blog posts I can find, and all I’m doing is getting completely wrapped around the axle.

I’m trying to make a “create” Action and View. My data entry is relatively straight forward, and common: I have a drop down list and a text box. In my case, I’m creating a user contact channel, and the drop down box chooses between email and textmsg, and the text box then enters the relevant contact information, either a well formed email address, or a mobile phone number.

Here’s a (slightly simplified form of) my View page:

 <tr>
     <td><%= Html.DropDownList("ChannelDescription", Model.ChannelDescription, "Select a Channel", new { id = "ChannelDDL", onchange="ChannelDDLChanged()" })%>
         <br />
         <%= Html.ValidationMessage("ChannelDescription", "Please Select a Channel") %>
      </td>
      <td>
           <%= Html.TextBox("SubscriberNotificationAddr") %> <br />
           <%= Html.ValidationMessage("SubscriberNotificationAddr", "Please enter a contact address or number") %>
       </td>
  </tr>

I’m using a strongly typed ViewData model, rather than using the ViewDataDictionary. The ChannelDescription element is a SelectList, which is initialized with the list of choices and no selection.

The initial display of the form, the data entry into the form, and the extraction of the data from the form by the controller goes fine.

My problem is if the data contains an error, such as a mal-formed email address or cell phone number, and I have to return to the view, I have not been successful in getting the drop down list selection redisplayed. The ChannelDescription element is recreated in the controller with the user’s choice as the selected item. I have set breakpoints on that line of the View, and verified that the selected element of the list of items has the Selected property set to true, but it still displays the default “Select a Channel”.

This seems like it would be a very common situation, and shouldn’t be this hard. What am I doing wrong?

FYI, this is with MVC 1.0 (Release), Windows 7, and VS 2008, running under Firefox 3.5.2.

Answer 1

I had some discussions with Brad Wilson, from the MVC team, and he explained to me that I was misunderstanding how the DropDownList helper method should be used (a misunderstanding that I think might be fairly common, from what I’ve read).

Basically, EITHER give it the SelectList in the named parameter of the ViewModel, and let it build the drop down list from that with the appropriate items selected, OR give it the SelectList as a separate parameter and let the named parameter of the ViewModel be just the value strings for the selected item(s). If you give it a SelectList parameter, then it expects the named value to be a string or list of strings, NOT a SelectList.

So, now your ViewModel ends up having two elements for one conceptual item in the view (the dropdown list). Thus, you might have a model that has

string SelectedValue {get; set;}
SelectList DropDownElements { get; set;}

Then you can pre-populate the DropDownElements with the choices, but in your model view binding, you just need to deal with SelectedValue element. It seems to work pretty well for me when I do it that way.