Modelbinding using Interfaces in ASP.NET MVC 2

Question

I have the following View Data:

public class ShoppingCartViewData
{
    public IList<IShoppingCartItem> Cart
    {
        get;
        set;
    }
}

I populate the viewdata in my controller:

viewData.Cart = CurrentSession.CartItems;

return View(viewData);

And send the data to the view and display it using:

<% for (int i = 0; i < Model.Cart.Count; i++ ) { %>
 <%= Html.TextBoxFor(m => m.Cart[i].Quantity)%>
    <%= Html.HiddenFor(m => m.Cart[i].Id) %>
<% } %>

I want to be able to catch the viewdata on the post. When I try:

[HttpPost]
public ActionResult UpdateCart(ShoppingCartViewData viewData)
{
    ...
}

When I run this I get a: System.MissingMethodException: Cannot create an instance of an interface.

Can anyone shed some light on this. What would I have to do to get this to work?

Many Thanks

Answer 1

You could try adding the formcollection as a parameter. And shouldn't viewdata be the viewmodel you're using?

[HttpPost]
public ActionResult UpdateCart(ShoppingCartViewModel viewModel, FormCollection collection)
{
    ...
}

Not sure if this is the exact solution, i'm also busy learning MVC2.0 and .NET4 ;-)

Answer 2

I'd create a model binder for your ViewModel, and then you can instantiate a concrete type that implements the appropriate interface when it binds to the method parameters.

You can insert logic into your model binder to read the form fields as appropriate and then instantiate the right IList or IShoppingCartItem data, so no need to worry about being pinned to a single implementation of the interface either.

Answer 3

Given my two comments this is how I would do it:

// you don't need this 
// viewData.Cart = CurrentSession.CartItems; 

// return View(viewData); 

// do it like this
return View(CurrentSession.CartItems);

Then have a strongly typed View either this:

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Administration.Master" Inherits="System.Web.Mvc.ViewPage<ShoppingCartViewData>" %>

or this:

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Administration.Master" Inherits="System.Web.Mvc.ViewPage<List<IShoppingCartItem>>" %>

Also this code won't work. This will generate you a bunch of textboxes with the same name and id. You need to generate textboxes with a count and for that you won't be able to use Html.TextBoxFor(). You will have to revert to Html.TextBox() or create a new extension TextBoxFor() method which would also accept a number (for you count).

<% for (int i = 0; i < Model.Cart.Count; i++ ) { %> 
 <%= Html.TextBoxFor(m => m.Cart[i].Quantity)%> // this won't work, if you want to post back all textboxes after they are edited
    <%= Html.HiddenFor(m => m.Cart[i].Id) %> 
<% } %> 

HTH