How do I maintain ModelState errors when using RedirectToAction?

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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..

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