Question

My ASP.NET MVC 3 web project was working fine, then I recompiled it and now I’m getting this error (some paths and namespaces changed to protect the innocent):

Object reference not set to an
instance of an object. Description: An
unhandled exception occurred during
the execution of the current web
request. Please review the stack trace
for more information about the error
and where it originated in the code.

Exception Details:
System.NullReferenceException: Object
reference not set to an instance of an
object.

Source Error:

Line 66:
RouteTable.Routes.MapRoute( Line 67:
“DefaultRoutes”, // Route name Line
68:
“{module}/{controller}/{action}/{id}”,
// URL with parameters

Source File: ….\Global.asax.cs
Line: 66

Stack Trace:

[NullReferenceException: Object
reference not set to an instance of an
object.]
Project.MvcApplication.RegisterRoutes(RouteCollection
routes) in D:…..\Global.asax.cs:66
Project.MvcApplication.Application_Start()
in D:…..\Global.asax.cs:139

[HttpException (0x80004005): Object
reference not set to an instance of an
object.]
System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext
context, HttpApplication app) +4051717
System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr
appContext, HttpContext context,
MethodInfo[] handlers) +191
System.Web.HttpApplication.InitSpecial(HttpApplicationState
state, MethodInfo[] handlers, IntPtr
appContext, HttpContext context) +352
System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr
appContext, HttpContext context) +407
System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr
appContext) +375

[HttpException (0x80004005): Object
reference not set to an instance of an
object.]
System.Web.HttpRuntime.FirstRequestInit(HttpContext
context) [phone]
System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext
context) +141
System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest
wr, HttpContext context) +4865877

I’m using IIS7.5 on Windows 7 x64, and the app pool is configured to run with .NET 4 and the ‘Integrated’ managed pipeline.

I’ve tried rebooting, and also cleaning out the bin and obj folders with no success.

Some extra information that turns out to be important – the line missing from the error page above:

RouteTable.Routes.MapRoute(
    "DefaultRoutes", // Route name
    "{module}/{controller}/{action}/{id}", // URL with parameters
    new { module = _loadedModules.FirstOrDefault().Name, controller = "Home", action = "Index", id = UrlParameter.Optional } // Parameter defaults
);

Answer 1

This project dynamically loads modules and what wasn’t clear from the original ASP.NET error page was that the NullReferenceException was being caused by the final line of the MapRoute() method call.

It turns out that _loadedModules was empty, so FirstOrDefault() was returning null – hence the exception.