为什么我在mvc中的下拉列表中的项目没有被标记为选中状态？

Question

I'm using SelectLists to populate the following DropDownLists from the respective models.

<div class="editor-field">
    @Html.DropDownList("Companies", Model.Companies)
</div>
<div class="editor-label">
    @Html.LabelFor(model => model.Role, "User Role")
</div>
<div class="editor-field">
    @Html.DropDownList("Roles", Model.Roles)
</div>

Here's how it's populated:

public List<SelectListItem> GetCompaniesList(List<Company> Companies)
{
    var list = new List<SelectListItem>();

    foreach (Company comp in Companies)
    {
        var item = new SelectListItem { Text = comp.Companyname, Value = Convert.ToString(comp.PKey) };
        list.Add(item);
    }

    return list;
}

I'm simply adding this to the model by calling model.CompaniesList = GetCompaniesList(CompanyModel.GetCompanies()) .

These lists render markup that looks like this:

<select id="ddlRoles" name="ddlRoles" class="valid">
    <option selected="selected" value="0">Please select a Role</option>
    <option value="2">Admin</option>
    <option value="1">Master</option>
    <option value="3">User</option>
</select>

I manually wrote in the default value which is pre-selected.

My problem is that I can't seem to get the selected flag to move over to whichever item I'm selecting. As such, when it comes time to post to the controller, the selected values for these fields come up empty and throw an exception.

How can I get the dropdownlists to accept my selection and send the data along with the rest of the form in the ajax post?

EDIT

Here's my [HttpPost] Add ActionResult (it's a bit of a mess, but that's just some nasty data design on my part which I'll fix up later...

[HttpPost]
public ActionResult Add(UserModel model)
{
  // this check fails saying that Companies and Roles don't have values
  if (ModelState.IsValid)
  {
    try
    {
      // these fields are for auditing
      model.CreatedBy = String.Format("{0} {1}", Convert.ToString(Session["firstname"]), Convert.ToString(Session["lastname"]));
      model.CreatedOn = DateTime.Now;
      // get the selected values from the dropdownlists
      Int64 companyid = Convert.ToInt64(model.Companies.Single(x => x.Selected).Value);
      Int64 role = Convert.ToInt64(model.Roles.Single(x => x.Selected).Value);   
      // the UserModel accepts a User object to add to the database,
      // so copy the model's values over to the user object for adding.
      // this is a lot of the mess which will be cleaned up later
      User u = new User();
      WebCompany c = new WebCompany();
      WebRole r = new WebRole();
      u.Company = c.GetCompanyById(companyid);
      u.Role = r.GetRoleById(role);
      u.CreatedBy = model.CreatedBy;
      u.CreatedOn = model.CreatedOn;
      u.EmailAddress = model.EmailAddress;
      u.FirstName = model.FirstName;
      u.LastName = model.LastName;
      u.LastLogin = model.LastLogin;
      u.Password = model.Password;
      model.Add(u);
      return Json(new
      {
        Message = "Success",
        IsOK = bool.TrueString
      });
    }
    catch (Exception ex)
    {
      if (Request.IsAjaxRequest())
      {
        return ThrowError(ex, "Add");
      }
      else
      {
        return View("Index");
      }
    }
  }
  else
  {
    // return a Json that contains information about why the model is invalid
  }
}

EDIT 2

Here's the Model. A fair amount of inheritance is used here to provide common user values (email address, password) from a framework I'm writing.

NOTE: Until I tried to implement the dropdownlists, this all worked just fine.

UserModel : Provides MVC with access to the User object and includes CRUD functions

public class UserModel : User
{
    public List<SelectListItem> Companies { get; set; }
    public List<SelectListItem> Roles { get; set; }

    // CRUDs go here
}

User : Provide application-specific fields (companies, roles)

public class User : BaseUser
{
    public virtual Company Company { get; set; }
    public virtual Role Role { get; set; }
    public DateTime? LastLogin { get; set; }
}

BaseUser : Provides user-common fields (email address, password)

public abstract class BaseUser : BaseObject
{
    public string EmailAddress { get; set; }
    public string Password { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public BaseUser()
    {
        EmailAddress = String.Empty;
        Password = String.Empty;
        FirstName = String.Empty;
        LastName = String.Empty;
    }        
}

BaseObject : Provides globally common fields to the database tables (primary key, auditing)

public abstract class BaseObject
{
    [Key]
    public Int64 PKey { get; set; }
    public string CreatedBy { get; set; }
    public DateTime CreatedOn { get; set; }
    public string ModifiedBy { get; set; }
    public DateTime ModifiedOn { get; set; }

    public BaseObject()
    {
        CreatedBy = "System";
        CreatedOn = DateTime.Now;

        ModifiedBy = "System";
        ModifiedOn = DateTime.Now;
    }
}

Answer 1

I always use the "for" helper in this situation

@Html.DropDownListFor(x => x.selectedCompany, Model.Companies)

to pre set the selected you just need to set "selectedCompany" on the controller

model.selectedCompany = //the users previous selection

this will also tie the selection made by the user to your model so if the drop down is changed, the value of selectedCompany will be set to what it was changed to on the view.