[EN] [ASP.NET MVC] Handle HTTP errors

This subject seems to be really basic when developing a web application but it seems that it’s very discussed on the web too. In a lot of forum threads or blog posts we can find very various methods to handle HTTP exception within an ASP.NET MVC application. This post exposes a way to do that. I’m not pretending that it’s the best way but I think that it’s an elegant one (so don’t hesitate to comment on this post if you’ve remarks or suggestions).

What’s the real need ?

I think that developers should manage HttpException in an other way than other exceptions for 3 main reasons :

- Respect of web-standards (a not found error returns a 404 http code, that’s it !) - SEO : search engines use http codes (because it’s standards) to build their indexes. - The user : a user-friendly page is already better than a stack trace…

I think that the most important is that the error code is well returned by the server : for example the URL http://www.mywebsite.com/Pages/some-page-that-does-not-exists is called the server should return a 404 status code and not a 302 (found) followed by a 200 (or 404) from a specific error page like http://www.mywebsite.com/Errors/Error404).

When HttpException are thrown in an ASP.NET MVC application ?

HttpException are thrown by the ASP.NET MVC engine or by the application code, directly in the asp.net controllers of your apps :

throw new HttpException(404, "Not found");

In some cases, they can also be thrown by the server (for internal errors, for example). But all HttpException can be handled in the application.

How to handle these exceptions ?

After a lot of researches and some tests I think that http exceptions should be handled in two places :

- In a base controller inherited by all the controllers of the application - In the Global.asax file

Handle http exception in the controller :

Exceptions that are thrown by the developer directly in the application code can be handle in a base controller. For example :

public ActionResult Product(int id)
{
var product = _unitOfWork.GetProduct(id);
if(product == null)
throw new HttpException(404, "Le produit est introuvable");

return View(product);
}

By inheriting the System.Web.Mvc.Controller class it’s possible to override an OnException method. This method is called when an exception occurs in a controller method (action) :

protected override void OnException(ExceptionContext filterContext) {
base.OnException(filterContext);

if (filterContext.Exception != null) {
filterContext.ExceptionHandled = true;

if (filterContext.Exception is HttpException) {
if (!ControllerContext.RouteData.Values.ContainsKey("error")) {
ControllerContext.RouteData.Values.Add("error", filterContext.Exception);
}

var httpException = (HttpException) filterContext.Exception;

switch (httpException.GetHttpCode()) {
case 404:
filterContext.HttpContext.Response.StatusCode = 404;
filterContext.HttpContext.Response.StatusDescription = httpException.Message;
View("Error404", null).ExecuteResult(ControllerContext);
break;
case 500:
filterContext.HttpContext.Response.StatusCode = 500;
filterContext.HttpContext.Response.StatusDescription = httpException.Message;
View("Error500", null).ExecuteResult(ControllerContext);
break;
default:
filterContext.HttpContext.Response.StatusDescription = httpException.Message;
View("GenericError", null).ExecuteResult(ControllerContext);
break;
}
}

//autre traitement si pas HttpException (log par exemple...)
}
}

As you can see in this code snippet the first step is to check that the exception is an HttpException. If yes, a redirection to an appropriate error page is done just after setting the status code and status description of the http response. The views are in the Shared folder of the application.

Now http exceptions that are thrown in a controller return the good status code and redirect to a user friendly page.

Handle HTTP exceptions in the Global.asax :

It’s in the Global.asax file that all other http exception are handled. To to that you should subscribe to the Error event of the mvc application (in the Init method override) :

public override void Init()
{
base.Init();
this.Error += new EventHandler(MvcApplication_Error);
}

In the Error event handler, initialize an errors-dedicated controller and create appropriate route data to work with the error :

var routeData = new RouteData();
routeData.Values.Add("controller", "Errors");

var lastException = Server.GetLastError();

if (lastException is HttpException) {
var httpException = (HttpException) lastException;
switch(httpException.GetHttpCode()) {
case 404:
routeData.Values.Add("action", "Error404");
break;
case 500:
routeData.Values.Add("action", "Error500");
break;
default:
routeData.Values.Add("action", "GenericError");
break;
}
}
else
{
routeData.Values.Add("action", "GenericError");
}

routeData.Values.Add("exception", lastException);

Server.ClearError();

IController errorController = _unityContainer.Resolve<ErrorsController>();
errorController.Execute(new RequestContext(
new HttpContextWrapper(Context), routeData));

The ErrorsController :

public class ErrorsController : Controller
{
public ActionResult Error404() {
Response.StatusCode = 404;
Exception exception = null;
if(RouteData.Values.ContainsKey("exception")) {
exception = (Exception) RouteData.Values["exception"];
}
return View(exception);
}

public ActionResult Error500() {
Response.StatusCode = 500;
Exception exception = null;
if (RouteData.Values.ContainsKey("exception"))
{
exception = (Exception)RouteData.Values["exception"];
}
return View(exception);
}

public ActionResult GenericError() {
Exception exception = null;
if (RouteData.Values.ContainsKey("exception"))
{
exception = (Exception)RouteData.Values["exception"];
}
return View(exception);
}
}

As you can see each action defines itself the http status code of the http response.

Now all http exceptions are handled in a user and SEO friendly way in your ASP.NET MVC applications !

Hope this helps