将SelectList“ SelectedValue”传递给控制器​​操作方法

Question

I have a registration form which displays a users Name (textbox), Email (textbox) and Division (SelectList). The Name and Email are pre-populated (I'm using Windows Authentication, Intranet app), and I want to send the SelectedValue from the DropDown to my controller as an Int32, I don't want to send the entire SelectList back. This list is small now, but will grow to considerable size.

I a class called RegistrationViewModel , it contains public properties for these fields. However, when I use SelectList for the DivisionList, I receive this error: No parameterless constructor defined for this object. .

If i change the Type, it works no problem, but Division is null or 0. Is there a way to pass the SelectedValue from a DropDown to a Controller Action method as a Int32?

Edit 1:

I'm not really sure what I'm doing, I've been using MVC for about 48 hours, watched the PDF, TechEd, and TechDays videos.

My apologies, here is my controller code:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Register(RegistrationViewModel rvm)
{
    IApplicationContext context = ContextRegistry.GetContext();
    IValidationErrors errors = new ValidationErrors();

    IValidator validator = (IValidator)context.GetObject("RegistrationValidator");
    bool valid = validator.Validate(rvm, errors);

    if (valid)
        repo.SaveRegistration();
    else
        ViewData["DivisionList"] = repo.GetDivisions();

    return View(rvm);
}

RegistrationViewModel Class

public class RegistrationViewModel
{
    public string Name { get; set; }
    public string Email { get; set; }
    //public SelectList DivisionList { get; private set; }
    public int Division { get; set; }    
}

Here's the view

<%@ Page Language="C#" 
MasterPageFile="~/Views/Shared/Site.Master" 
Inherits="System.Web.Mvc.ViewPage<RegistrationViewModel>" %>

<%@ Import Namespace="Project1.Entities"%>
<%@ Import Namespace="Project1.Models"%>

<asp:Content ID="registerTitle" ContentPlaceHolderID="TitleContent" runat="server">
    Register
</asp:Content>

<asp:Content ID="registerContent" ContentPlaceHolderID="MainContent" runat="server">
...
    <% using (Html.BeginForm())
       { %>
        <div>
            <fieldset>
                <legend>Account Information</legend>
                <p>
                    <label for="name">Name:</label>
                    <%= Html.TextBox("Name", User.Identity.Name.GetDisplayName()) %>
                    <%= Html.ValidationMessage("username") %>
                </p>
                <p>
                    <label for="email">Email:</label>
                    <%= Html.TextBox("email", User.Identity.Name.GetEmailFromLogin()) %>
                    <%= Html.ValidationMessage("email") %>
                </p>              
                <p>
                    <label for="division">Division:</label>
                    <%= Html.DropDownList("DivisionList", ViewData["DivisionList"] as SelectList)%>
                    <%= Html.ValidationMessage("confirmPassword") %>
                </p>
                <p>                
                    <input type="submit" value="Register" />
                </p>
            </fieldset>
        </div>
    <% } %>
</asp:Content>

Edit 2:

Eilon: Here is what I changed it too:

Controller:

public ActionResult Register()
{
  ViewData["DivisionList"] = repo.GetDivisions();
  return View();
}

View:

<%= Html.DropDownList("DivisionValue", ViewData["DivisionList"] as SelectList)%>

I recieve this exception: There is no ViewData item with the key 'DivisionValue' of type 'IEnumerable'.

When I updated the View to this:

<%= Html.DropDownList("DivisionList", ViewData["DivisionList"] as SelectList)%>

It works just great! It only seems to work if all the "Division" items named identically. If I change the name the View crashes or the ViewModel "Division" property is sent as 0.

Why is that?

Answer 1

The RegistrationViewModel type should contain a simple-typed property such as:

public string DivisionValue { get; set; }

Or change the type to int, DateTime, or whatever the appropriate type is.

In HTML and HTTP the only thing that gets posted back for a drop down list is the name of the field and the selected value.

To get everything to match up you also need to change the view to render a different input name for the drop down list:

<%= Html.DropDownList("DivisionValue", ViewData["DivisionList"] as SelectList)%>

Notice that I'm using "DivisionValue" is the value of the list, and DivisionList as the list of all available items.

Answer 2

I'd just be more explicit with the SelectList type. I'd suggest creating the SelectList in the controller action and forget about casting it in the view. My code works like this (CRUD Edit page):

..in the Action:

            ViewData["WorkType.ID"] = new SelectList(this._vacancySvc.GetVacancyWorkTypes(),
            "ID", "Name", ViewData["WorkType.ID"] ?? vacancy.WorkType.ID);

..and in the view:

<p><% =Html.Encode("Work Type:") %><br />
            <% =Html.DropDownList("Worktype.ID")%><span class="smallgrey">(required)</span><br />

.. you can see that either the initial selection (from DB) is persisted or the ViewData from post backs (like if the form fails validation) thru the use of the [null coalescing operator][1] (??).

Moreover, if i refactored this code, i'd prob like to use a ViewModel object like you are. The only thing is: (1) you'd never need to reference the ViewModel SelectList property in the view coz MVC auto binds this for us by the Html.DropDownList() overload.. and (2) i'd still need to ref the ViewData in the action anyway to get the selected value from a failed validation post back so what's the point really??