ASP.NET MVC - How to Preserve ModelState Errors Across RedirectToAction?

I have the following two action methods (simplified for question):

[HttpGet]
public ActionResult Create(string uniqueUri)
{
   // get some stuff based on uniqueuri, set in ViewData.  
   return View();
}

[HttpPost]
public ActionResult Create(Review review)
{
   // validate review
   if (validatedOk)
   {
      return RedirectToAction("Details", new { postId = review.PostId});
   }  
   else
   {
      ModelState.AddModelError("ReviewErrors", "some error occured");
      return RedirectToAction("Create", new { uniqueUri = Request.RequestContext.RouteData.Values["uniqueUri"]});
   }   
}

So, if the validation passes, i redirect to another page (confirmation).

If an error occurs, i need to display the same page with the error.

If i do return View(), the error is displayed, but if i do return RedirectToAction (as above), it loses the Model errors.

I'm not surprised by the issue, just wondering how you guys handle this?

I could of course just return the same View instead of the redirect, but i have logic in the "Create" method which populates the view data, which i'd have to duplicate.

Any suggestions?

I had to solve this problem today myself, and came across this question.

Some of the answers are useful (using TempData), but don't really answer the question at hand.

The best advice I found was on this blog post:

http://www.jefclaes.be/2012/06/persisting-model-state-when-using-prg.html

Basically, use TempData to save and restore the ModelState object. However, it's a lot cleaner if you abstract this away into attributes.

E.g.

public class SetTempDataModelStateAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        base.OnActionExecuted(filterContext);         
        filterContext.Controller.TempData["ModelState"] = 
           filterContext.Controller.ViewData.ModelState;
    }
}

public class RestoreModelStateFromTempDataAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        base.OnActionExecuting(filterContext);
        if (filterContext.Controller.TempData.ContainsKey("ModelState"))
        {
            filterContext.Controller.ViewData.ModelState.Merge(
                (ModelStateDictionary)filterContext.Controller.TempData["ModelState"]);
        }
    }
}

Then as per your example, you could save / restore the ModelState like so:

[HttpGet]
[RestoreModelStateFromTempData]
public ActionResult Create(string uniqueUri)
{
    // get some stuff based on uniqueuri, set in ViewData.  
    return View();
}

[HttpPost]
[SetTempDataModelState]
public ActionResult Create(Review review)
{
    // validate review
    if (validatedOk)
    {
        return RedirectToAction("Details", new { postId = review.PostId});
    }  
    else
    {
        ModelState.AddModelError("ReviewErrors", "some error occured");
        return RedirectToAction("Create", new { uniqueUri = Request.RequestContext.RouteData.Values["uniqueUri"]});
    }   
}

If you also want to pass the model along in TempData (as bigb suggested) then you can still do that too.

You need to have the same instance of Review on your HttpGet action. To do that you should save an object Review review in temp variable on your HttpPost action and then restore it on HttpGet action.

[HttpGet]
public ActionResult Create(string uniqueUri)
{
   //Restore
   Review review = TempData["Review"] as Review;            

   // get some stuff based on uniqueuri, set in ViewData.  
   return View(review);
}
[HttpPost]
public ActionResult Create(Review review)
{
   //Save your object
   TempData["Review"] = review;

   // validate review
   if (validatedOk)
   {
      return RedirectToAction("Details", new { postId = review.PostId});
   }  
   else
   {
      ModelState.AddModelError("ReviewErrors", "some error occured");
      return RedirectToAction("Create", new { uniqueUri = Request.RequestContext.RouteData.Values["uniqueUri"]});
   }   
}

If you want this to work even if the browser is refreshed after the first execution of the HttpGet action, you could do this:

  Review review = TempData["Review"] as Review;  
  TempData["Review"] = review;

Otherwise on refresh button object review will be empty because there wouldn't be any data in TempData["Review"].

Why not create a private function with the logic in the "Create" method and calling this method from both the Get and the Post method and just do return View().

I could use TempData["Errors"]

TempData are passed accross actions preserving data 1 time.

I suggest you return the view, and avoid duplication via an attribute on the action. Here is an example of populating to view data. You could do something similar with your create method logic.

public class GetStuffBasedOnUniqueUriAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        var filter = new GetStuffBasedOnUniqueUriFilter();

        filter.OnActionExecuting(filterContext);
    }
}


public class GetStuffBasedOnUniqueUriFilter : IActionFilter
{
    #region IActionFilter Members

    public void OnActionExecuted(ActionExecutedContext filterContext)
    {

    }

    public void OnActionExecuting(ActionExecutingContext filterContext)
    {
        filterContext.Controller.ViewData["somekey"] = filterContext.RouteData.Values["uniqueUri"];
    }

    #endregion
}

Here is an example:

[HttpGet, GetStuffBasedOnUniqueUri]
public ActionResult Create()
{
    return View();
}

[HttpPost, GetStuffBasedOnUniqueUri]
public ActionResult Create(Review review)
{
    // validate review
    if (validatedOk)
    {
        return RedirectToAction("Details", new { postId = review.PostId });
    }

    ModelState.AddModelError("ReviewErrors", "some error occured");
    return View(review);
}

I have a method that adds model state to temp data. I then have a method in my base controller that checks temp data for any errors. If it has them, it adds them back to ModelState.

Microsoft removed the ability to store complex data types in TempData, therefore the previous answers no longer work; you can only store simple types like strings. I have altered the answer by @asgeo1 to work as expected.

public class SetTempDataModelStateAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        base.OnActionExecuted(filterContext);

        var controller = filterContext.Controller as Controller;
        var modelState = controller?.ViewData.ModelState;
        if (modelState != null)
        {
            var listError = modelState.Where(x => x.Value.Errors.Any())
                .ToDictionary(m => m.Key, m => m.Value.Errors
                .Select(s => s.ErrorMessage)
                .FirstOrDefault(s => s != null));
            controller.TempData["KEY HERE"] = JsonConvert.SerializeObject(listError);
        }
    }
}


public class RestoreModelStateFromTempDataAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        base.OnActionExecuting(filterContext);

        var controller = filterContext.Controller as Controller;
        var tempData = controller?.TempData?.Keys;
        if (controller != null && tempData != null)
        {
            if (tempData.Contains("KEY HERE"))
            {
                var modelStateString = controller.TempData["KEY HERE"].ToString();
                var listError = JsonConvert.DeserializeObject<Dictionary<string, string>>(modelStateString);
                var modelState = new ModelStateDictionary();
                foreach (var item in listError)
                {
                    modelState.AddModelError(item.Key, item.Value ?? "");
                }

                controller.ViewData.ModelState.Merge(modelState);
            }
        }
    }
}

From here, you can simply add the required data annotation on a controller method as needed.

[RestoreModelStateFromTempDataAttribute]
[HttpGet]
public async Task<IActionResult> MethodName()
{
}


[SetTempDataModelStateAttribute]
[HttpPost]
public async Task<IActionResult> MethodName()
{
    ModelState.AddModelError("KEY HERE", "ERROR HERE");
}

My scenario is a little bit more complicated as I am using the PRG pattern so my ViewModel ("SummaryVM") is in TempData, and my Summary screen displays it. There is a small form on this page to POST some info to another Action. The complication has come from a requirement for the user to edit some fields in SummaryVM on this page.

Summary.cshtml has the validation summary which will catch ModelState errors that we'll create.

@Html.ValidationSummary()

My form now needs to POST to a HttpPost action for Summary(). I have another very small ViewModel to represent edited fields, and modelbinding will get these to me.

The new form:

@using (Html.BeginForm("Summary", "MyController", FormMethod.Post))
{
    @Html.Hidden("TelNo") @* // Javascript to update this *@

and the action...

[HttpPost]
public ActionResult Summary(EditedItemsVM vm)

In here I do some validation and I detect some bad input, so I need to return to the Summary page with the errors. For this I use TempData, which will survive a redirection. If there is no issue with the data, I replace the SummaryVM object with a copy (but with the edited fields changed of course) then do a RedirectToAction("NextAction");

// Telephone number wasn't in the right format
List<string> listOfErrors = new List<string>();
listOfErrors.Add("Telephone Number was not in the correct format. Value supplied was: " + vm.TelNo);
TempData["SummaryEditedErrors"] = listOfErrors;
return RedirectToAction("Summary");

The Summary controller action, where all this begins, looks for any errors in the tempdata and adds them to the modelstate.

[HttpGet]
[OutputCache(Duration = 0)]
public ActionResult Summary()
{
    // setup, including retrieval of the viewmodel from TempData...


    // And finally if we are coming back to this after a failed attempt to edit some of the fields on the page,
    // load the errors stored from TempData.
        List<string> editErrors = new List<string>();
        object errData = TempData["SummaryEditedErrors"];
        if (errData != null)
        {
            editErrors = (List<string>)errData;
            foreach(string err in editErrors)
            {
			    // ValidationSummary() will see these
                ModelState.AddModelError("", err);
            }
        }

    public class RestoreModelStateFromTempDataAttribute : ActionFilterAttribute
	{
		public override void OnActionExecuting(ActionExecutingContext filterContext)
		{
			var controller = filterContext.Controller as Controller;
			if (controller.TempData.ContainsKey("ModelState"))
			{
				var modelState = ModelStateHelpers.DeserialiseModelState(controller.TempData["ModelState"].ToString());
				controller.ViewData.ModelState.Merge(modelState);
			}
			base.OnActionExecuting(filterContext);
		}
	}
	public class SetTempDataModelStateAttribute : ActionFilterAttribute
	{
		public override void OnActionExecuted(ActionExecutedContext filterContext)
		{
			var controller = filterContext.Controller as Controller;
			controller.TempData["ModelState"] = ModelStateHelpers.SerialiseModelState(controller.ViewData.ModelState);
			base.OnActionExecuted(filterContext);
		}
	}

When I resolve some problem, I ran into a lot of non-obvious obstacles. I will indicate everything step by step. My comments will partially duplicate the answers from the current branch

1. Implement two attributes. You must explicitly specify the type for controller (filterContext.Controller as Controller) because default is object type.
2. Explicitly implement serialization ModelState from this article https://andrewlock.net/post-redirect-get-using-tempdata-in-asp-net-core/
3. If TempData is empty in destination action when check implementation cache in startup.cs. You need add memoryCache or SqlServerCache or another https://stackoverflow.com/a/41500275/11903993

I prefer to add a method to my ViewModel which populates the default values:

public class RegisterViewModel
{
    public string FirstName { get; set; }
    public IList<Gender> Genders { get; set; }
    //Some other properties here ....
    //...
    //...

    ViewModelType PopulateDefaultViewData()
    {
        this.FirstName = "No body";
        this.Genders = new List<Gender>()
        {
            Gender.Male,
            Gender.Female
        };

        //Maybe other assinments here for other properties...
    }
}

Then I call it when ever I need the original data like this:

    [HttpGet]
    public async Task<IActionResult> Register()
    {
        var vm = new RegisterViewModel().PopulateDefaultViewValues();
        return View(vm);
    }

    [HttpPost]
    public async Task<IActionResult> Register(RegisterViewModel vm)
    {
        if (!ModelState.IsValid)
        {
            return View(vm.PopulateDefaultViewValues());
        }

        var user = await userService.RegisterAsync(
            email: vm.Email,
            password: vm.Password,
            firstName: vm.FirstName,
            lastName: vm.LastName,
            gender: vm.Gender,
            birthdate: vm.Birthdate);

        return Json("Registered successfully!");
    }

I am giving just sample code here In your viewModel you can add one property of type "ModelStateDictionary" as

public ModelStateDictionary ModelStateErrors { get; set; }

and in your POST action menthod you can write code directly like

model.ModelStateErrors = ModelState; 

and then assign this model to Tempdata like below

TempData["Model"] = model;

and when you redirect to other controller's action method then in controller you have to read the Tempdata value

if (TempData["Model"] != null)
{
    viewModel = TempData["Model"] as ViewModel; //Your viewmodel class Type
    if(viewModel.ModelStateErrors != null && viewModel.ModelStateErrors.Count>0)
    {
        this.ViewData.ModelState.Merge(viewModel.ModelStateErrors);
    }
}

That's it. You don't have to write action filters for this. This is as simple as above code if you want to get Model state errors to another view of another controller.