ASP.NET MVC : Changing model's properties on postback

Question

I've been playing with ASP.NET MVC and ran into something I can't figure out.

Suppose I have an object like this :

public class TestObject
{
    public string Name { get; set; }
    public int Age { get; set; }
}

And a view page (Create.aspx) like this :

<form action="/Create" method="post">
    <p>
        <%=Html.TextBox("Name") %>
    </p>
    <p>
        <%=Html.TextBox("Age")%>
    </p>
</form>

And on my controller I have these actions :

[AcceptVerbs(HttpVerbs.Get)]
public ActionResult Create()
{
    return View(new TestObject { Name = "DefaultName", Age = 10 } );
}

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create(TestObject o)
{
    o.Name = "ChangedNameToSomethingElse";
    o.Age = 15;
    return View(o);
}

The Html.TextBox() method always genereates the textboxes with the default values, even after the postback, where the object is passed back with different properties on its values. Now, granted, I can't think of a real world example why I'd want to do such a thing but I still don't understand why I always end up having textboxes populated with the model's values that were set on the Create action with the AcceptVerbs(HttpVerbs.Get) attribute.

Note : I've tried Html.TextBox("Name", Model.Name) but the result is still the same. And I verified that the Create action with AcceptVerbs(HttpVerbs.Post) actually runs, by passing a value via ViewData to the View. Also, the udated value is displayed when I output the value with <%=Model.Name %> but again, not on the textbox.

Is there something obvious I'm missing, or is there a reasoning behind this behaviour?

Answer 1

If you bind the result of a post request through the declaration of the method or by UpdateModel or TryUpdateModel to an object such as TestObject, a property called ModelState will get filled in with these values. The HTML helpers such as Textbox will always bind to modelstate over an explicitly passed model object.