Model State Validation for a JSON API isn't too hard because you only need to send back validation errors, especially with the ApiController attribute. However, for all those that are still using server-side rendering and Razor, it's still a bit challenging.

It's challenging because you need to return the correct View after posting a form. And the easiest method of getting the job done is writing this type of code in each Action .

if ( ModelState . IsValid ) { return RenderTheGetForm ( ) ; }

It does the job, but it's usually a lot of extra, tedious, code.

Instead we can keep the Controller s lean by shifting this code into an Action Filter attribute which can be used similarly to the ApiController attribute. So I decided to write one and publish it.

Here's how it works. You install the AutomaticModelStateValidation package and drop the AutoValidateModel attribute on the action that uses a View Model and validation.

[ Route ( "/[controller]" ) ] public class SubmissionController : Controller { [ HttpGet ] public ActionResult NewSubmission ( ) { return View ( new NewSubmissionViewModel ( . . . ) ) ; } [ HttpPost ( ) ] [ AutoValidateModel ( nameof ( NewSubmission ) ) ] public RedirectToActionResult SaveSubmission ( SaveSubmissionViewModel model ) { return RedirectToAction ( nameof ( ViewSubmission ) , new { Id = 1 } ) ; } [ HttpGet ] public ActionResult ViewSubmission ( int id ) { return View ( new ViewSubmissionViewModel ( . . . ) ) ; } }

The AutoValidateModel attribute on the SaveSubmission Action performs the ModelState.IsValid check. If the model is valid everything continues as normal, however, if the model is invalid then the specified fallback Action in is invoked and the ModelState from the previous Action is merged in.

This means you're able to render the invalid Form with validation messages and the appropriate HTTP status code with a single line of code!

If you want to get started, grab the Automatic ModelState Validation NuGet package. And keep reading if you're interested in some more of the details.

The deep dive

One of the highlights of this attribute is to invoke the fallback action programmatically without another round trip to the client. In the past, I've achieved similar functionality by temporarily storing the ModelState and redirecting to the previous action. However, this time around I have made use of the advancements in AspNetCore to bypass that step entirely.

The bulk of the code resides in the AutoValidateModelAttribute which implements the ActionFilterAttribute class. After first checking that the ModelState is invalid the next step is to determine check controller action to invoke.

var controllerName = SansController ( controller ?? context . Controller . GetType ( ) . Name ) ;

If the controller isn't explicitly specified then the fallback is the Type name of the controller. Here I also remove the Controller suffix.

Next, I have to locate the relevant ActionDescriptor by getting the IActionDescriptorCollectionProvider and finding the matching descriptor.

var controllerActionDescriptor = actionDescriptorCollectionProvider . ActionDescriptors . Items . OfType < ControllerActionDescriptor > ( ) . FirstOrDefault ( x => x . ControllerName == controllerName && x . ActionName == action ) ;

To invoke the ActionDescriptor the next thing I'll need is an ActionContext . Here is also where I pass along the previous ModelState so the validation errors are carried through to the new Action .

var actionContext = new ActionContext ( context . HttpContext , context . RouteData , controllerActionDescriptor , context . ModelState ) ;

The last major piece is getting an IActionInvokerFactory to create a ControllerActionInvoker and then invoking it.

var actionInvokerFactory = GetService < IActionInvokerFactory > ( ) ; var invoker = actionInvokerFactory . CreateInvoker ( actionContext ) ; await invoker . InvokeAsync ( ) ;

After that, there was just one more problem to fix. When no View is explicitly specified when returning a ViewResult, AspNet Mvc will fall back to using the action name from RouteData . Because I'm invoking a second Action inside a single Request the wrong view will be used unless I update the RouteData to match. So, I also set the action name to the name of the fallback action before calling the ControllerActionInvoker .

if ( context . RouteData . Values . ContainsKey ( ActionNameKey ) ) { context . RouteData . Values [ ActionNameKey ] = controllerActionDescriptor . ActionName ; }

If you're interested in the full source code you can check it on GitHub under Automatic ModelState Validation.