Why is my mvc3 viewmodel not updating correctly? - asp.net-mvc

Question

I am writing an image upload form using ASP.NET MVC 3.
In the view, please notice that I am displaying #Model.ImagePath as text and as a #Html.TextBoxFor(model => model.ImagePath).
View:
#using (Html.BeginForm(null, null, FormMethod.Post, new { enctype="multipart/form-data" }))
{
#Html.HiddenFor(model => model.BannerSlideId)
#Html.HiddenFor(model => model.Key)
<p>
Image Path:#(Model.ImagePath)<br />
Image Path in TextBoxFor: #Html.TextBoxFor(model => model.ImagePath)
</p>
<p>
<label class="styled">Upload Slide Image</label>
<input type="file" name="image" />
</p>
<p>
<button type="submit">Save</button> #Html.ActionLink("Back to List", "Index")
</p>
}
I then select an image using the file input, and I submit the form to the Controller.
Controller action:
[HttpPost]
public ActionResult Create(BannerSlide model, HttpPostedFileBase image)
{
if (image != null)
model.ImagePath = image.FileName;
return View("Edit", model);
}
When I debug with a breakpoint, the image.FileName string is assigned to model.ImagePath. However, when I get back to the View I get Two different values from ImagePath.
Results
Image Path:#(Model.ImagePath)<br />
Correctly returns the image filename that was assigned. But,
Image Path in TextBoxFor: #Html.TextBoxFor(model => model.ImagePath)
Incorrectly returns blank!
Any ideas for why this is happening?

Answer 1

You should remove the ImagePath property from ModelState if you intend to modify it in your POST controller action or HTML helpers such as TextBoxFor will first look for a value inside ModelState when binding and then in the model:
if (image != null)
{
ModelState.Remove("ImagePath");
model.ImagePath = image.FileName;
}

Answer 2

The TextBoxFor HtmlHelper method, and all input helper methods for that matter, set the value of the input control to the value held in ModelState rather than immediately binding to the property of the ViewModel. Since the ImagePath present in the POST to your Create method, it is displayed as a blank value on the subsequent Response.

Answer 3

I would suggest to use following attribute on such actions
public class ModelStateFixAttribute : ActionFilterAttribute
{
public ModelStateFixAttribute()
{
}
public override void OnActionExecuting(ActionExecutingContext filterContext)
{
base.OnActionExecuting(filterContext);
}
public override void OnActionExecuted(ActionExecutedContext filterContext)
{
ModelStateDictionary modelState = filterContext.Controller.ViewData.ModelState;
String[] arrKeys = new string[modelState.Keys.Count];
modelState.Keys.CopyTo(arrKeys, 0);
foreach (string key in arrKeys)
{
if (modelState.IsValidField(key))
modelState.Remove(key);
}
}
}