Exceptions in System.Web.Routing.RouteCollection.GetRouteData

Question

I've been getting two exceptions at random times in my asp.net mvc code running on iis7:

Exception type: InvalidOperationException 
Exception message: Collection was modified; enumeration operation may not execute. 
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List'1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List'1.Enumerator.MoveNext()
   at System.Web.Routing.RouteCollection.GetRouteData(HttpContextBase httpContext)
   at System.Web.Routing.UrlRoutingModule.PostResolveRequestCache(HttpContextBase context)
   at System.Web.Routing.UrlRoutingModule.OnApplicationPostResolveRequestCache(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

and

Exception type: NullReferenceException 
Exception message: Object reference not set to an instance of an object. 
   at System.Web.Routing.RouteCollection.GetRouteData(HttpContextBase httpContext)
   at System.Web.Routing.UrlRoutingModule.PostResolveRequestCache(HttpContextBase context)
   at System.Web.Routing.UrlRoutingModule.OnApplicationPostResolveRequestCache(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

It's not consistently reproducible, but I assume it's something changing (or corrupting) RouteTable.Routes. The only place I access RouteTable.Routes in my project is in Global.asax.cs and I know that the code there is only being called once, so it's not the problem. Any idea on how to track it down?

Answer 1

In my case, it ended up being a HttpModule: Ext.Direct.Mvc (Ext.Direct for ASP.NET MVC). This module had a bug (Fixed in version 0.8.0) which registered routes again every time Init() was called for the IHttpModule. (which might be called multiple times). If the timing was right, it would corrupt the RouteTable.Routes collection, and cause one of the two exceptions above.