Post back Errors in MVC 4 - asp.net-mvc-4

Question

In regular Asp.Net, when you want to post an error, for ex, a duplicate ID, I use Panels for controls, error message labels, etc. I was wondering, how to achieve the same using MVC 4.
Currently, on my Index page, I have an Id, Name & Address column and a Submit button. My HTTPPOST ActionResult here:
[HttpPost]
public ActionResult Index(Person p)
{
if (ModelState.IsValid)
{
PersonInfo pi = new PersonInfo();
var duplicate = from d in db.PersonInfoes
where d.Id == p.Id
select d;
if (duplicate.Any())
{
return View("Duplicate");
}
else
{....}
When I enter a duplicate Id, upon submitting the page, I need to post to the same view ("Index") page to let user know that there is an Id already, but currently, I am redirecting to a different view to let user know as a duplicate, and I dont think this is the right way to do this. I am learning MVC, btw and hence this doubt. My duplicate cshtml here:
#{
ViewBag.Title = "Duplicate";
}
<h2>Duplicate</h2>
<h3>Duplicate ID found !!</h3>
<p>Please correct the ID and re-enter !</p>
<br/><br/>
#Html.ActionLink("Back to Data Entry","Index")
Any pointers would help.

Answer 1

You can return to the same view. You should consider adding an error to the model state dictionary so that you can show that in the UI.
[HttpPost]
public ActionResult Index(Person p)
{
if (ModelState.IsValid)
{
var duplicate= db.PersonInfoes.Where(s=>s.Id== p.Id);
if (duplicate.Any())
{
ModelState.AddModelError(string.Empty,"Duplicate found");
return View(p);
}
// to do : Your existing code
}
return View(p);
}
Just make sure you are calling the ValidationSummary method in your Index view to show this error message.
#model Person
#using (Html.BeginForm())
{
#Html.ValidationSummary(false)
<!-- Your form fields goes here-->
<input type="submit" />
}