How do I maintain ModelState errors when using RedirectToAction?

Question

The PRG pattern is ok, but I did this:

Base controller:

protected override void OnActionExecuted(ActionExecutedContext filterContext)
{
    if (TempData["ModelState"] != null && !ModelState.Equals(TempData["ModelState"]))
        ModelState.Merge((ModelStateDictionary)TempData["ModelState"]);

    base.OnActionExecuted(filterContext);
}

Action (I'm using xVal):

try
{
    user.Login();
    AuthenticationManager.SignIn(user);
}
catch (RulesException rex)
{
    // on bad login
    rex.AddModelStateErrors(ModelState, "user");
    TempData["ModelState"] = ModelState;
    return Redirect(Request.UrlReferrer.ToString());
}

The action throws an exception, adds the ModelState to TempData and redirects back to the referrer. Since the action is caught, OnActionExecuted is still executed, but the first time around the ModelState is the same as TempData["ModelState"], so you don't want to merge with yourself. When the redirect action is executed, OnActionExecuted fires again. This time, if there's anything in TempData["ModelState"], it merges with this action's ModelState.

You could expand it to multiple models by using TempData["ModelState.user"] = ModelState and then merging every TempData object that starts with ModelState..

I know this thread is old, but this blog about ASP.NET Best Practices has some excellent suggestions.
#13 on the page deals with using 2 Action filters to save and restore ModelState between redirects.

This is the pattern that my work uses, and I love it.

Here's the simplified example:

[ImportModelStateFromTempData]
public ActionResult Dashboard()
{
    return View();
}

[AcceptVerbs(HttpVerbs.Post), ExportModelStateToTempData]
public ActionResult Dashboard(string data)
{
    if (ValidateData(data))
    {
        try
        {
            _service.Submit(data);
        }
        catch (Exception e)
        {
            ModelState.AddModelError(ModelStateException, e);
        }
    }

    return RedirectToAction("Dashboard");
}

this blog post describes how you could implement the PRG-Pattern in MVC http://blog.simonlovely.com/archive/2008/11/26/post-redirect-get-pattern-in-mvc.aspx

hth

Use the TempData[] Collection

The tempdata is stored from one request to the next, then its gone.