视图模型未与DropDownListFor绑定

Question

I am trying to have a dropdown list populated by a view model and then, on HTTPPOST, try to retrieve the selected value but the model has no data bound in the controller in post. Any help would be appreciated.

View Model

public class TransferViewModel
{

    public IEnumerable<User> Users { get; set; }
    public string SortOrder { get; set; }
    public SelectList Departments { get; set; }
    public SelectList Sites { get; set; }

    public SelectListItem SelectedDepartment { get; set; }
    public SelectListItem SelectedSite { get; set; }
    public User SelectedUser { get; set; }
}

Controller

public ActionResult Sort(TransferViewModel model)
    {
        var users = from s in _db.Users select s;
        var departments = new SelectList(_db.Departments);
        var sites = new SelectList(_db.Sites);
        model.Departments = departments;
        model.Sites = sites;

        if (model.SelectedDepartment != null)
        {
            users = users.Where(s => s.Department.ID == Convert.ToInt32(model.SelectedDepartment));
        }
        else if (model.SelectedSite != null)
        {
            users = users.Where(s => s.Site.ID == Convert.ToInt32(model.SelectedSite));
        }

        switch (model.SortOrder)
        {
            case "name_desc":
                users = users.OrderByDescending(s => s.LName);
                break;
            case "dept":
                users = users.OrderBy(s => s.Department.Name);
                break;
            case "dept_desc":
                users = users.OrderByDescending(s => s.Department.Name);
                break;
            case "site":
                users = users.OrderBy(s => s.Site.Name);
                break;
            case "site_desc":
                users = users.OrderByDescending(s => s.Site.Name);
                break;
            default:
                users = users.OrderBy(s => s.LName);
                break;
        }
        model.Users = users;
        model.Departments = new SelectList(_db.Departments, "ID", "Name", model.SelectedDepartment);
        model.Sites = new SelectList(_db.Sites,"ID","Name", model.SelectedSite);
        return View(model);
    }

View

@using (Html.BeginForm("Sort", "Transfer", FormMethod.Post ))
{
<p>
    Display by @Html.DropDownListFor(model => model.SelectedDepartment, Model.Departments, htmlAttributes: new { @class = "form-control"})
    Department
</p>
<p>
    or by Location: @Html.DropDownListFor(model => model.SelectedSite, Model.Sites, htmlAttributes: new { @class = "form-control"})

</p>
<p>
    <input type="submit" value="Search"/>
</p>
}

Answer 1

All you need to have int properties in your model as in post the selected id for the dropdown will be posted back to the controller:

public class TransferViewModel
{
    ......................
    ......................
    public int SelectedDepartment { get; set; }
    public int SelectedSite { get; set; }
    .....................
    .....................
}

you are telling yourself in the action to use the ID column for the dropdownlists to be binded as DataValueMember :

// see that ID argument there that is binding the value which will be posted back
model.Departments = new SelectList(_db.Departments, "ID", "Name", model.SelectedDepartment);
model.Sites = new SelectList(_db.Sites,"ID","Name", model.SelectedSite);