ASP.NET MVC3, why does dropdownlist rely on viewbag even in a strongly typed view

Question

I'm new to MVC, so maybe this is a stupid question - I'm trying to get my head around strongly typed views in asp.net mvc. I'm working on version 3. If I have a project with 2 models - say Person and Department. A person must belong to a department. So I have my Department model (and I've generated my controller and CRUD interface):

public class Department
{
    public int Id { get; set;}
    public string DeparmentName { get; set;}
}

Then I have a Person model which references Department:

public class Person
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public int Department_Id { get; set; }
    [ForeignKey("Department_Id")
    public virtual Department Department { get; set;}
}

Now I generate my Controller and Views. Now, when I look into the PersonController, I have the following for the Create:

public ActionResult Create()
{
    ViewBag.Department_Id = new SelectList(db.Deparments, "Id", "DepartmentName");
    return View();
}

and in Person\Create.cshtml, the code to create the Department drop down is

@Html.DropDownList("Department_Id", String.Empty)

As I understand it, the DropDownList Html helper is using the ViewBag to create my drop down list. However, FirstName and LastName seem to have their input fields created without relying on the ViewBag - so in my View, I can have compile time checking on my FirstName and LastName fields because my view is strongly typed, but not on my DropDownList.

Is there a reason why the DropDownList for department not strongly typed or am I doing something wrong and there is a way to make it strongly typed?

Thanks :)

Totemism

Answer 1

The reason you need a viewbag is because your Person instance, to which your view binds, does not have the data for all possible departments, only the one department it belongs to. What happens in this line:

ViewBag.Department_Id = new SelectList(db.Deparments, "Id", "DepartmentName");

Is that all Departments from the DB are added to the Select List (for your DropDown to bind to) with Id as key asn DepartmentName as value. After your view is bound to the Person, the dropdown will show the appropriate Department.

Slavism

Hi Edwin - thank you - that sort of makes sense - one single Person has one Department_ID, but I thought that the view was binding to my Person model which defines the relationship to Departments through the "virtual" property? Maybe I'm just looking for too much "magic" in the MVC framework. – Totemism

@Rick: I guess you do. The view only binds to the single Department defined by your Person. You use the viewbag to transfer the full list of departments. By the way, the ViewBag is simply a wrapper around the ViewData (from the other answer) allowing you to create dynamic properties. – Slavism

Answer 2

If you are using strongly typed views, you should have a strongly typed model that you pass to the view:

public class ViewData {
   public IList<Departments> Departments {get; set;}
}

Then in you controller you have:

public ActionResult Create()
{
    ViewData model = new ViewData();
    model.Departments = db.Deparments;
    return View(model);
 }

The last step is to create the select list and drop down on the view:

@model ViewData
@SelectList list = new SelectList(Model.Deparments, "Id", "DepartmentName");
@Html.DropDownList("Department_Id", list);

Hope this makes sense.

Cyndie

Hi Steve - thanks - is the ViewData class you've created a "ViewModel"? – Totemism

Yes, I generally put all the view models in the models folder in the solution. Generally I name them {viewName}ViewData.cs – Cyndie