Requirements for 404
The following are my requirements for a 404 solution and below i show how i implement it:
- I want to handle matched routes with bad actions
- I want to handle matched routes with bad controllers
- I want to handle un-matched routes (arbitrary urls that my app can't understand) - i don't want these bubbling up to the Global.asax or IIS because then i can't redirect back into my MVC app properly
- I want a way to handle in the same manner as above, custom 404s - like when an ID is submitted for an object that does not exist (maybe deleted)
- I want all my 404s to return an MVC view (not a static page) to which i can pump more data later if necessary (good 404 designs) and they must return the HTTP 404 status code
Solution
I think you should save Application_Error in the Global.asax for higher things, like unhandled exceptions and logging (like Shay Jacoby's answer shows) but not 404 handling. This is why my suggestion keeps the 404 stuff out of the Global.asax file.
Step 1: Have a common place for 404-error logic
This is a good idea for maintainability. Use an ErrorController so that future improvements to your well designed 404 page can adapt easily. Also, make sure your response has the 404 code!
public class ErrorController : MyController
{
#region Http404
public ActionResult Http404(string url)
{
Response.StatusCode = (int)HttpStatusCode.NotFound;
var model = new NotFoundViewModel();
// If the url is relative ('NotFound' route) then replace with Requested path
model.RequestedUrl = Request.Url.OriginalString.Contains(url) & Request.Url.OriginalString != url ?
Request.Url.OriginalString : url;
// Dont get the user stuck in a 'retry loop' by
// allowing the Referrer to be the same as the Request
model.ReferrerUrl = Request.UrlReferrer != null &&
Request.UrlReferrer.OriginalString != model.RequestedUrl ?
Request.UrlReferrer.OriginalString : null;
// TODO: insert ILogger here
return View("NotFound", model);
}
public class NotFoundViewModel
{
public string RequestedUrl { get; set; }
public string ReferrerUrl { get; set; }
}
#endregion
}
Step 2: Use a base Controller class so you can easily invoke your custom 404 action and wire up HandleUnknownAction
404s in ASP.NET MVC need to be caught at a number of places. The first is HandleUnknownAction.
The InvokeHttp404 method creates a common place for re-routing to the ErrorController and our new Http404 action. Think DRY!
public abstract class MyController : Controller
{
#region Http404 handling
protected override void HandleUnknownAction(string actionName)
{
// If controller is ErrorController dont 'nest' exceptions
if (this.GetType() != typeof(ErrorController))
this.InvokeHttp404(HttpContext);
}
public ActionResult InvokeHttp404(HttpContextBase httpContext)
{
IController errorController = ObjectFactory.GetInstance<ErrorController>();
var errorRoute = new RouteData();
errorRoute.Values.Add("controller", "Error");
errorRoute.Values.Add("action", "Http404");
errorRoute.Values.Add("url", httpContext.Request.Url.OriginalString);
errorController.Execute(new RequestContext(
httpContext, errorRoute));
return new EmptyResult();
}
#endregion
}
Step 3: Use Dependency Injection in your Controller Factory and wire up 404 HttpExceptions
Like so (it doesn't have to be StructureMap):
public class StructureMapControllerFactory : DefaultControllerFactory
{
protected override IController GetControllerInstance(Type controllerType)
{
try
{
if (controllerType == null)
return base.GetControllerInstance(controllerType);
}
catch (HttpException ex)
{
if (ex.GetHttpCode() == (int)HttpStatusCode.NotFound)
{
IController errorController = ObjectFactory.GetInstance<ErrorController>();
((ErrorController)errorController).InvokeHttp404(RequestContext.HttpContext);
return errorController;
}
else
throw ex;
}
return ObjectFactory.GetInstance(controllerType) as Controller;
}
}
I think its better to catch errors closer to where they originate. This is why i prefer the above to the Application_Error handler.
This is the second place to catch 404s.
Step 4: Add a NotFound route to Global.asax for urls that fail to be parsed into your app
This route should point to our Http404 action. Notice the url param will be a relative url because the routing engine is stripping the domain part here? That is why we have all that conditional url logic in Step 1.
routes.MapRoute("NotFound", "{*url}",
new { controller = "Error", action = "Http404" });
This is the third and final place to catch 404s in an MVC app that you don't invoke yourself. If you don't catch unmatched routes here then MVC will pass the problem up to ASP.NET (Global.asax) and you don't really want that in this situation.
Step 5: Finally, invoke 404s when your app can't find something
Like when a bad ID is submitted to my Loans controller:
//
// GET: /Detail/ID
public ActionResult Detail(int ID)
{
Loan loan = this._svc.GetLoans().WithID(ID);
if (loan == null)
return this.InvokeHttp404(HttpContext);
else
return View(loan);
}
It would be nice if all this could be hooked up in fewer places with less code but i think this solution is more maintainable, more testable and fairly pragmatic.
Thanks for the feedback so far. I'd love to get more.
NOTE: This has been edited significantly from my original answer but the purpose/requirements are the same - this is why i have not added a new answer