Question

I am implementing an error handling strategy in an ASP.Net MVC 3 application. I’ve written an attribute implementing IExceptionFilter etc. This functions correctly, handling exceptions throw in the action method and returns the exception information serialized as JSON.

I would like to use this attribute to also handle validation errors found by the model binder when passing data to an Action Method. For example, if I POST an object to an Action Method which is deserialized as the Action Method parameter I would like it to throw an exception if a validation error occurs like UpdateModel does. Right now the default model binder seems to behave like TryUpdateModel, simply flipping the ModelState.IsValid rather than throwing an exception.

[ActionExceptionJsonHandler]
public ActionResult CreateSomething(SomethingViewData account)
{
// If model binding fails validation an exception should be thrown and no code is executed here
// Do stuff here
}

If the default model binder threw an exception in the same way as UpdateModel does then the IExceptionFilter would catch it and handle returning the validation error to the client. Without that the developer must write code to check the ModelState etc.

So bottom line I have two related questions:

1. Is there a way to have the default model binder throw an exception when validation fails?
2. Any thoughts on using this approach versus manually checking ModelState in each action method?

Thanks.

Answer 1

My solution ended up being to implement an ActionFilterAttribute as shown below. In OnActionExecuting I check ModelState.IsValid, if it’s false I serialize the model state errors as JSON and set the Result object effectively canceling execution. This allows for me to return a custom JSON serialized object containing the model binding errors.

    public override void OnActionExecuting(ActionExecutingContext filterContext) {

        if (filterContext.Controller.ViewData.ModelState.IsValid) {
            base.OnActionExecuting(filterContext);
            return;
        }

        var returnDto = new ReturnDto
                            {
                                Success = false,
                                Errors = Tools.GetModelStateErrors(filterContext.Controller.ViewData.ModelState)
                            };

        // AllowGet is fine provided we are not returning a javascript array
        filterContext.Result = new JsonResult { Data = returnDto, JsonRequestBehavior = JsonRequestBehavior.AllowGet };
    }