Handling MVC5 errors with JQuery Ajax

Since you want to return the error content, I would suggest returning a JSON response (alternative is a partial view, but that would mean your JS uses delegated handlers, reset form validation, etc.). In this case, you will need to detect and return JSON if POST is AJAX and return normal view / redirect otherwise. If all checks have to be done client-side, and it's okay for no error text to appear, you could probably return the result of the exception and use an error handler to call.ajax()

to refresh the page. I found that browser support for getting error response text is inconsistent, so if you want actual errors, you need to return a 200 OK response with messages in JSON. My choice will probably depend on the specific use case - for example, if there were multiple errors that I could only detect on the server side, I would probably use an OK response with the content of the error. If only a few or all of the errors need to be handled on the client side, then I would post an exception route.

A custom error handler should not be used or needed to do so.

MVC with result status

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Edit([Bind(Include = "EventName")] EventViewModel model)
{
    //Get the specific record to be updated
    var eventRecord = (from e in db.Event
                       where e.EventID == model.EventID
                       select e).SingleOrDefault();

    if (eventRecord == null)
    {
        if (Request.IsAjaxRequest())
        {
            return new HttpStatusCodeResult(HttpStatusCode.NotFound, "Event not found.");
        }

        ModelState.AddModelError("EventID", "Event not found.");
    }

    //Update the data
    if (ModelState.IsValid)
    {
        eventRecord.EventName = model.EventName;
        db.SaveChanges();            

        if (Request.IsAjaxRequest())
        {
            return Json(new { Url = Url.Action("Index") });
        }

        return RedirectToAction("Index");
    }

    if (Request.IsAjaxRequest())
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest, /* ... collate error messages ... */ "" );
    }

    return View(model);
}

      

        
        
        
      

    

JS example with result status

$.ajax({
   type: "POST",
   url: "/EditEvent/Edit",
   data: $('#editEventForm').serialize(),
})
.done(function(result) {
     window.location = result.Url;
})
.fail(function(xhr) {
    switch (xhr.status) {  // examples, extend as needed
       case 400:
          alert('some data was invalid. please check for errors and resubmit.');
          break;
       case 404:
          alert('Could not find event to update.');
          break;
    }     
});

      

        
        
        
      

    

MVC with error content

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Edit([Bind(Include = "EventName")] EventViewModel model)
{
    //Get the specific record to be updated
    var eventRecord = (from e in db.Event
                       where e.EventID == model.EventID
                       select e).SingleOrDefault();

    if (eventRecord == null)
    {
        if (Request.IsAjaxRequest())
        {
            return Json(new { Status = false, Message = "Event not found." });
        }

        ModelState.AddModelError("EventID", "Event not found.");
    }

    //Update the data
    if (ModelState.IsValid)
    {
        eventRecord.EventName = model.EventName;
        db.SaveChanges();            

        if (Request.IsAjaxRequest())
        {
            return Json(new { Status = true, Url = Url.Action("Index") });
        }

        return RedirectToAction("Index");
    }

    if (Request.IsAjaxRequest())
    {
        return Json(new 
        {
            Status = false,
            Message = "Invalid data",
            Errors = ModelState.Where((k,v) => v.Errors.Any())
                               .Select((k,v) => new
                               {
                                   Property = k,
                                   Messages = v.Select(e => e.ErrorMessage)
                                               .ToList()
                               })
                               .ToList()
        });
    }

    return View(model);
}

      

        
        
        
      

    

JS example with error content

$.ajax({
   type: "POST",
   url: "/EditEvent/Edit",
   data: $('#editEventForm').serialize(),
})
.done(function(result) {
     if (result.Status)
     {
         window.Location = result.Url;
     }
     // otherwise loop through errors and show the corresponding validation messages
})
.fail(function(xhr) {
    alert('A server error occurred.  Please try again later.');     
});