ViewBag message is changed even when a form has errors - asp.net-mvc

Question

I have a simple contact form that is using a model for it's fields, everything seems to work but the ViewBag message gets changed regardless if there are validation errors or not, user validation prevents this but I also need the HttpPost action to set the message based on if the the form was filled correctly.
I tried using if(ModelState.IsValid) but it doesn't seem to work. I realize I can probably manually check each variable in the home to see if it's empty, but that won't really tell me if it's valid or the post was returned with errors, is there a build in method for this?
ContactFormModel.cs
namespace TestApplication.Models
{
public class ContactFormModel
{
[Required]
[Display(Name = "Name")]
public string name { get; set; }
[Required]
[Display(Name = "Phone")]
public string phone { get; set; }
[Required]
[Display(Name = "Message")]
public string message { get; set; }
}
}
HomeController.cs
[HttpPost]
public ActionResult Contact(ContactFormModel contactForm)
{
ViewBag.Message = "Thank you. Your message has been sent.";
return View();
}
Contact.cshtml
#model TestApplication.Models.ContactFormModel
#{
ViewBag.Title = "Contact";
}
<h2>#ViewBag.Title.</h2>
<h3>#ViewBag.Message</h3>
<p>Use this area to provide additional information.</p>
#*#if (!IsPost)
{
#Html.EditorForModel(Model)
}*#
#using (Html.BeginForm())
{
#Html.AntiForgeryToken()
<div class="form-horizontal">
<hr />
#Html.ValidationSummary(true, "", new { #class = "text-danger" })
<div class="form-group">
#Html.LabelFor(model => model.name, htmlAttributes: new { #class = "control-label col-md-2" })
<div class="col-md-10">
#Html.EditorFor(model => model.name, "", new { htmlAttributes = new { #class = "form-control" } })
#Html.ValidationMessageFor(model => model.name, "", new { #class = "text-danger" })
</div>
</div>
<div class="form-group">
#Html.LabelFor(model => model.phone, htmlAttributes: new { #class = "control-label col-md-2" })
<div class="col-md-10">
#Html.EditorFor(model => model.phone, "", new { htmlAttributes = new { #class = "form-control" } })
#Html.ValidationMessageFor(model => model.phone, "", new { #class = "text-danger" })
</div>
</div>
<div class="form-group">
#Html.LabelFor(model => model.message, htmlAttributes: new { #class = "control-label col-md-2" })
<div class="col-md-10">
#Html.TextAreaFor(model => model.message, new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.message, "", new { #class = "text-danger" })
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<input type="submit" value="Create" class="btn btn-default" />
</div>
</div>
</div>
}

Answer 1

The pattern for doing what you want is to check ModelState.IsValid. If it's valid continue processing, if not return the view with the existing model contents to give the user a chance to correct their error(s).
[HttpPost]
public ActionResult Contact(ContactFormModel contactForm)
{
if (ModelState.IsValid)
{
ViewBag.Message = "Thank you. Your message has been sent.";
return View();
}
else
{
return View(contactForm);
}
}
Having said that, you should consider using a PRG (post-redirect-get) pattern. By returning the same view in the HttpPost version of the method you open yourself up to repeated posting of the data. If the user hits Refresh in their browser it will repost the data they just posted (after popping up a dialog that most non-technical users will never understand). You must have a HttpGet version that delivers the view in the first place, you should redirect to that on success. You'll have to switch to using TempData instead of ViewBag because the ViewBag won't survive the redirect.
[HttpPost]
public ActionResult Contact(ContactFormModel contactForm)
{
if (ModelState.IsValid)
{
TempData.Message = "Thank you. Your message has been sent.";
// Assumes there is a Get version of the Contact action method
return RedirectToAction("Contact");
}
else
{
return View(contactForm);
}
}