ModelState.Clear is not working

Question

I have a simple contact form built in MVC that uses the Html helper class to generate the textbox and drop down. I'd like to clear the values of the textboxes and drop down list to as it was when the page was rendered using a get (only after the inquiry was submitted properly).

I am using the method ModelState.Clear() to perform this clean up but my form values still remain, any idea on what I am doing wrong here? Upon success it does display the message in the code. Below you will find a copy of the code from my controller.

Thanks for taking the time!

[HttpPost]
public ActionResult Contact(ContactUsViewModel model)
{
    if (ModelState.IsValid)
    {
       bool isSuccess = _siteService.CreateInquiry(model.Inquiry);

       if (isSuccess)
       {
           model.SuccessMessage = "Thank you for contacting us.";
           ModelState.Clear();
       }
    }

    model.InquiryTypes = InquiryTypes;
    return View(model);
}

Answer 1

In case of success simply redirect to your GET action following the Post-Redirect-Get pattern:

public ActionResult Contact()
{
    var model = new ContactUsViewModel
    {
        SuccessMessage = TempData["SuccessMessage"] as string
    };
    return View(model);
}

[HttpPost]
public ActionResult Contact(ContactUsViewModel model)
{
    if (ModelState.IsValid)
    {
       bool isSuccess = _siteService.CreateInquiry(model.Inquiry);
       if (isSuccess)
       {
           TempData["SuccessMessage"] = "Thank you for contacting us.";
           return RedirectToAction("Contact");
       }
    }

    // since you are modifying the value of the InquiryTypes property
    // you need to remove it from the modelstate to avoid getting the 
    // old value rendered by the helpers
    ModelState.Remove("InquiryTypes");
    model.InquiryTypes = InquiryTypes;
    return View(model);
}

or since I am not a big fan of TempData (because it uses Session and personally I always disable it in my applications), you could simply pass a query string parameter to the Contact GET action like (success=1) and inside this action prepare the success message.