MVC HttpPost modify Model and replace view - asp.net-mvc

Question

Can someone please help me understand this. I have the following code:
The controller
public ActionResult Stuff(PersonModel model)
{
model.Address = "Some Address";
return PartialView("_Registration", model);
}
The View
#Ajax.BeginForm("Stuff", new AjaxOptions(){ HttpMethod="POST", UpdateTargetId="the_form", InsertionMode=InsertionMode.Replace, OnSuccess="Stuff" })
{
<div id="the_form">
#{Html.RenderPartial("_Registration", new TypeScriptTest.Models.PersonModel());}
</div>
<input type="submit" value="Get Addreess" />
}
The Partial View
#model TypeScriptTest.Models.PersonModel
<table>
<tr>
<td>
Name:
</td>
<td>
#Html.TextBoxFor(c => c.Name)
</td>
</tr>
<tr>
<td>
Address:
</td>
<td>
#Html.TextBoxFor(c => c.Address)
</td>
</tr>
</table>
The data is posted with whatever I type in the two fields. But then I replace the Address property value with "Some Address" and return a PartialView I would expect to get a new view that replaces the old. This probably happens, but the view that replaces the old does not include the new "Some Address" value. It just looks exactly like the view I posted.
I have a workaround, but I want to understand what is happening here. My workaround is as follows:
public ActionResult Stuff(PersonModel model)
{
model.Address = "Some Address";
var v = PartialView("_Registration");
var view = v.ViewEngineCollection.OfType<System.Web.Mvc.RazorViewEngine>().Single().FindPartialView(new ControllerContext(HttpContext, RouteData, this), "_Registration", false);
var result = new PartialViewResult() { View = view.View };
result.ViewData.Model = model;
return result;
}
Here I can bypass cache using false as the last parameter in FindPartialView. Setting this parameter to true results in the same problem as the one in discussion.
Why is this happening and what is the correct way to get around this problem? My workaround works fine, but I would like to understand the problem.

Answer 1

In case you are still interested, this is because the values submitted from the form are stored in the ModelState and when rendering the HtmlHelpers, any value in the ModelState takes precedence over your current model object values.
If you want to change values submitted by the user then you could either clear the modelState or clear the entry for the single property that you want to change. (Take care when clearing the model state as that will also clear the errors for any submitted value, at least you may want to make sure the ModelState is valid before manually modifying it)
I guess that with your work around you will end up having a clean ModelState without the submitted values.
You might also find useful the answers to this similar question: Asp.net MVC ModelState.Clear