Model binding inner nested models with ASP.NET MVC

Question

I'm trying to create a basic registration form that takes in an address with a state from a drop down list. I can't seem to extract the data from the drop down into my model that contains a state object. I've tried creating a custom model binder as shown below. When I debug the ModelState is valid, but the state is always null.

Models:

public class AccountInfo 
{
    public virtual Guid accoundID { get; set; }
    public virtual string city { get; set; }
    public virtual string email_address { get; set; }
    public virtual string fax_number { get; set; }
    public virtual string first_name { get; set; }
    public virtual string last_name { get; set; }
    public virtual string mailing_address { get; set; }
    public virtual string phone_number { get; set; }
    public virtual State state { get; set; }
    public virtual string zip_code { get; set; }
}
public class State
{
    public virtual int ID { get; set; }
    public virtual string text { get; set; }
    public virtual string value { get; set; }
}

Model Binder:

 public class StateModelBinder : IModelBinder
{
    private RepositoryDB Database;
    public object BindModel(
        ControllerContext controllerContext,
        ModelBindingContext bindingContext
        )
    {
        var key = bindingContext.ModelName + ".state";
        var value = bindingContext.ValueProvider.GetValue(key);
        if (value == null)
        {
            return null;
        }
        var result = new State();
        Database = new RepositoryDB(ConfigurationManager.
            ConnectionStrings["ConnectionString"].ConnectionString);

        try
        {
            var query = from s in Database.States
                        where s.value == value.ToString()
                        select new State()
                        {
                            text = s.text,
                            ID = s.ID,
                            value = s.value
                        };
            result = (State)query.ToList().ElementAt(0);        
        }
        catch (Exception ex)
        {
            bindingContext.ModelState.AddModelError(bindingContext.ModelName, ex.Message);
            bindingContext.ModelState.SetModelValue(key, value);
        }


        return result;
    }

Controller:

[HttpPost]
    public ActionResult Index(AccountInfo accountModel)
    {
        try
        {
            if (ModelState.IsValid)
            {
                Database.Account.Add(accountModel);
                Database.SaveChanges();

                return Redirect(Url.Action("Success"));
            }
        }
        catch (Exception ex)
        {
            ModelState.AddModelError(String.Empty, ex);
        }
        // invalid info - return with error message         
        //repop select lists
        GetStates();
        return View(accountModel);
    }
public void GetStates()
    {
        IEnumerable<SelectListItem> states = from s in Database.States
                                                select new SelectListItem
                                                {
                                                    Text = s.text,
                                                    Value = s.value
                                                };

        ViewBag.state = states.ToList();
    }

View:

@Html.LabelFor(model => model.state, "*State")
@Html.DropDownListFor(model => model.state, (IEnumerable<SelectListItem>)ViewBag.state)

What are you doing? Why custom model binder? Why ViewBag? Why not view models? — Darin Dimitrov
– Darin Dimitrov, Commented Aug 1, 2012 at 16:30
I am trying to store the "State" (as in a mailing address) as an object inside the AccountInfo class. I'm populating the drop down with a linq query from a simple table on the database that has every possible state. Could you demonstrate how to use a viewmodel instead of my implementation? — Brian Wheat
– Brian Wheat, Commented Aug 1, 2012 at 17:25

John Culviner · Accepted Answer · 2012-08-01 16:59:54Z

Here is your problem:

@Html.DropDownListFor(model => model.state, (IEnumerable<SelectListItem>)ViewBag.state)

Binding to model.State cannot work because that is a complex type:

public class State
{
    public virtual int ID { get; set; }
    public virtual string text { get; set; }
    public virtual string value { get; set; }
}

If you think about it the only thing the form is going to post back is the value of the selected drop down item like "MN" (Minnesota) and not a full class. (You haven't serialized a "State" class to each drop down list item's value, just the primary key of each state hopefully) You need to make your state on the model be a string or int depending on what type that primary key is of your state like so:

public class AccountInfo 
{
    //...
    public virtual string/int State { get; set; }
    //...
}

From there you can re-hydrate your State object if you'd like from the unique identifier of a state (which should be the value that is posted back to the server as a string or int for example)

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