What am I misunderstanding about how Html.TextBoxFor works

Question

I'm just trying to get back into .NET MVC with the new release and I can't get my head round the way may view is binding to the DataModel.

I have a model with a property "first_name" and within an HTML form I have the following

  <%= Html.TextBox("first_name", Model.first_name)%>
  <%= Html.TextBoxFor(model => model.first_name) %> 
  <input type="text" name="first_name" id="first_name" class="myLabel" value="<%=Model.first_name %>" />

In an action on a controller if I set the first_name property on my model and do

  mymodelObject.first_name = "Test";
  return View(mymodelObject);

What is the reason only the third textbox picks up on this first_name value and the other two don't?

Edit:

I probably haven't explained this well enough, sorry. Imagine I have 2 controller methods -

       public ActionResult Register()
    {
        Registration model = new Registration();
        model.first_name = "test";
        return View(model);
    }

With this one either binding works.

After this has been displayed I then click a button on the form and try and run this:

    [HttpPost]
    public ActionResult Register(Registration_ViewData model)
    {
        model.first_name = "Steve";
        return View(model);
    }

What I'm asking is why does the 3rd but not the first 2 bind to "Steve" as the new name.

Answer 1

This should work for the first two:

<%= Html.TextBox("first_name", x => x.first_name)%>
<%= Html.TextBoxFor(model => model.first_name) %> 

Answer 2

Because HTML helpers read the value from the ModelState and not from the model. In order the change that behavior you'll need to work with the ModelState as well.
(see: Changing model’s properties on postback)

Answer 3

You need to clear your model state so your code would look something like:

[HttpPost]
public ActionResult Register(Registration model)
{
    ModelState.Clear();
    model.first_name = "Steve";
    return View(model);
}