How to modify a value being roundtripped to an MVC action method?

Question

If I have a simple model like this:

Model:

  public class AnimalModel {
       public string Species {get;set;}
  }

View:

  <%= Html.TextBox("Species") %>

ActionMethod (POST):

  AnimalModel model = new AnimalModel();

  // update model 
  UpdateModel(model);   // updates ViewData.ModeState

  // correct spelling
  if (model.Species == "catt") {
      model.Species = "cat";
  }

  // return view
  return View(model);

So the action method on a POST is supposed to correct the spelling of 'catt' to 'cat'.

The problem is that the source code for 'TextBox' will take the value from ViewData.ModelState if it finds a 'Species' value in there. This means that my correct value is not updated on a postback because 'catt' takes priority.

So I thought - ah well I'll just make it explicit like this :

  <%= Html.TextBox("Species", Model.Species) %>

Surprisingly this doesnt work and STILL takes the value from ViewData.ModelState. I'm pretty sure this is a bug in MVC because looking at the source code for this overload i find this overload. It clearly indicates that the 'useViewData' parameter is supposed to be false if 'value' is provided (which I'm doing explicitly above) :

  public static string TextBox(this HtmlHelper htmlHelper, string name, object value, IDictionary<string, object> htmlAttributes) {
        return InputHelper(htmlHelper, InputType.Text, name, value, 
               (value == null) /* useViewData */, false /* isChecked */, true /* setId */, true /* isExplicitValue */, htmlAttributes);
    }

Unfortunatley though the 'InputHelper' method doesn't respect this property for a TextBox and still looks first in the ViewDataDictionary. It finds 'catt' in this dictionary because it came directly from the POST data.

I've found a solution when a UserControl is used, which passes in NULL for ViewDataDictionary.

 <% Html.RenderPartial("AnimalControl", Model, null); %>

What is the solution to this? I'm sure this is documented somewhere but i cant seem to find it. I don't want to have to clear out ViewDataDictionary manually but i see no other solution.

Answer 1

I'm glad I put aside model binding and built-in HTML helpers since the beginning.

You can either fix/override the helper you need or build your own. I don't see what else can be done in this situation.

Answer 2

It does appear to be a bug in MVC, from the way you described it, but...

If you have Species in the Model, why do you also have it in the ViewData Dictionary?

---

Consider checking out this posting in the NerdDinner Tutorial:

http://nerddinnerbook.s3.amazonaws.com/Part6.htm

Go down to where it says, "Using a ViewModel Pattern" and review that.

Answer 3

My best solution so far (since I'm not using ViewData or ModelState explicitly in my view) :

        ViewData.ModelState.Clear();
        return View(model);

Can someone PLEASE find a better solution. This stinks!

Answer 4

Try the code below to actually modify the "state" of the model. I know you'd expect it to work when you put <%= Html.TextBox("Species", Model.Species) %> there, but because of the reasons you explained, it doesn't work.

ModelState["Species"].Value =
            new ValueProviderResult("cat", "catt", CultureInfo.CurrentCulture);