How to handle errors in ASP.NET MVC

There are several blog posts regarding error handling for ASP.NET MVC. They use everything from Application_Error to exception handling in the base controller. With this post I’ll show how you can use the built-in features in MVC to handle errors.

MVC got an attribute called [HandleError] which you should set on your BaseController (or on each controller). There is no need to specify any of the options for the attribute.

The problem with [HandleError] is that it can’t handle 404 (not found), thus we need to create a custom error controller and tell ASP.NET to use it (by configuring web.config and creating and ErrorController):

<customErrors mode="Off" defaultRedirect="~/Error/">
  <error statusCode="404" redirect="~/Error/NotFound/" />
</customErrors>

The error controller

public class ErrorController : BaseController
{
	public ActionResult NotFound(string url)
	{
		var originalUri = url ?? Request.QueryString["aspxerrorpath"] ?? Request.Url.OriginalString;

		var controllerName = (string)RouteData.Values["controller"];
		var actionName = (string)RouteData.Values["action"];
		var model = new NotFoundModel(new HttpException(404, "Failed to find page"), controllerName, actionName)
		{
			RequestedUrl = originalUri,
			ReferrerUrl = Request.UrlReferrer == null ? "" : Request.UrlReferrer.OriginalString
		};

		Response.StatusCode = 404;
		return View("NotFound", model);
	}

	protected override void HandleUnknownAction(string actionName)
	{
		var name = GetViewName(ControllerContext, "~/Views/Error/{0}.cshtml".FormatWith(actionName),
													"~/Views/Error/Error.cshtml",
													"~/Views/Error/General.cshtml",
													"~/Views/Shared/Error.cshtml");

		var controllerName = (string)RouteData.Values["controller"];
		var model = new HandleErrorInfo(Server.GetLastError(), controllerName, actionName);
		var result = new ViewResult
		{
			ViewName = name,
			ViewData = new ViewDataDictionary<HandleErrorInfo>(model),
		};

		Response.StatusCode = 501;
		result.ExecuteResult(ControllerContext);
	}

	protected string GetViewName(ControllerContext context, params string[] names)
	{
		foreach (var name in names)
		{
			var result = ViewEngines.Engines.FindView(ControllerContext, name, null);
			if (result.View != null)
				return name;
		}
		return null;
	}
}

NotFound action

The notfound action builds information about the route and where the user came from. You might want to log that information. The model that it uses derives the built in error view model (HandleErrorInfo) and looks like this:

public class NotFoundModel : HandleErrorInfo
{
	public NotFoundModel(Exception exception, 
	       string controllerName, string actionName)
	: base(exception, controllerName, actionName)
	{
	}
	public string RequestedUrl { get; set; }
	public string ReferrerUrl { get; set; }
}

Which is used by the following view:

@using YourApplication.Models.Errors
@model NotFoundModel
@{
    ViewBag.Title = "Page could not be found";
}
<h2>@ViewBag.Title</h2>
<div>
You tried to visit '@Model.RequestedUrl' which cannot be found.
</div>

All other errors

The HandleUnknownAction method is used to handle all other error codes which [HandleError] can’t handle (it will be invoked since no other action methods exists). Look at the code for it and you’ll discover that it tries to find a customized view for each HTTP error.

Handling errors in your POST actions

Another important aspect is how you treat errors in your POST actions. Here is a sample method which is taking advantage of the built in model validation.

[HttpPost]
public virtual ActionResult Create(YourModel model)
{
	if (!ModelState.IsValid)
		return View(model);

	try
	{
		var dbEntity = _repository.Get(model.Id);
		Mapper.Map(model, dbEntity);
		_repository.Save(dbEntity);
		return RedirectToAction("Details", new { id = model.Id });
	}
	catch (Exception ex)
	{
		ModelState.AddModelError("", ex.Message);
		//log error here.
		return View(model);
	}
}

Keypoints:

• Always validate model first and display any errors)
• Fetch / Copy / Save – make sure that the view model only contains fields that can be changed)
• Include any error in the model state (to get it in the validation summary)
• Log all errors!