MVC 3 MultiSelectList Posting Values for Complex Types

If I have a complex type e.g.

public class Customer
{
   public int Id {get;set;}
   public List<ContactType> ContactTypes {get;set;}
}

public class ContactType
{
   public int Id {get;set;}
   public string Name {get;set;}
}

and my strongly typed view binds the ContactType like so ...

 @Html.ListBoxFor(m => m.ContactTypes , 
                  new MultiSelectList(ViewBag.ContactTypes, "Id", "Name"))

and my action method signature is ...

public ActionResult Create(Customer customer){}

why is the customer.ContactTypes null when I post the form? I c开发者_高级运维an see the posted data like ContactTypes=1&ContactTypes=2, I though this would bind to the ContactTypes?

Can anyone point me in the right direction?

---

Your model is wrong. You should have a collection property to bind the selected values to. You should not use the same property for the both arguments of your SelectList. So start by adding a property on your view model to hold the selected ids:

public class Customer
{
    public int Id { get; set; }

    public List<int> SelectedContactTypeIds { get; set; }
    public List<ContactType> ContactTypes { get; set; }
}

and then fix your view:

@Html.ListBoxFor(
    m => m.SelectedContactTypeIds, 
    new MultiSelectList(Model.ContactTypes, "Id", "Name")
)

Now inside your Create action you will get the SelectedContactTypeIds property containing the selected ids:

public ActionResult Create(Customer customer)
{
    // here the customer.SelectedContactTypeIds will contain the ids of the selected items
    ...
}

The reason why your code doesn't work is because you have bound the SelectList to the ContactTypes property which is a complex type and as you know the <select> element sends only the selected values when the form is submitted. That's how HTML works.