How can I maintain ModelState with RedirectToAction?

Use Action Filters (PRG pattern) (as easy as using attributes)

Mentioned here and here.

Please note that tvanfosson's solution will not always work, though in most cases it should be just fine.

The problem with that particular solution is that if you already have any ViewData or ModelState you end up overwriting it all with the previous request's state. For example, the new request might have some model state errors related to invalid parameters being passed to the action, but those would end up being hidden because they are overwritten.

Another situation where it might not work as expected is if you had an Action Filter that initialized some ViewData or ModelState errors. Again, they would be overwritten by that code.

We're looking at some solutions for ASP.NET MVC that would allow you to more easily merge the state from the two requests, so stay tuned for that.

Thanks, Eilon

Store your view data in TempData and retrieve it from there in your Index action, if it exists.

   ...
   if (!ModelState.IsValid)
       TempData["ViewData"] = ViewData;

   RedirectToAction( "Index" );
}

 public ActionResult Index()
 {
     if (TempData["ViewData"] != null)
     {
         ViewData = (ViewDataDictionary)TempData["ViewData"];
     }

     ...
 }

[EDIT] I checked the on-line source for MVC and it appears that the ViewData in the Controller is settable, so it is probably easiest just to transfer all of the ViewData, including the ModelState, to the Index action.