NHibernate: Collection was modified; enumeration operation may not execute - nhibernate

Question

I'm currently struggling with this "Collection was modified; enumeration operation may not execute" issue.
I have searched about this error message, and it's all related to the foreach statement. I do have the some foreach statements, but they are just simply representing the data. I did not using any remove or add inside the foreach statement.
NOTE:
The error randomly happens (about 4-5 times a day).
The application is the MVC website.
There are about 5 users operate this applications (about 150 orders a day). Could it be some another users modified the collection, and then occur this error?
I have log4net setup and the settings can be found here
Make sure that the controller has a parameterless public constructor I do have parameterless public constructor in AdminProductController
Does anyone know why this happen and how to resolve this issue?
A friend (Oskar) mentioned that
"Theory: Maybe the problem is that
your configuration and session factory
is initialized on the first request
after application restart. If a second
request comes in before the first
request is finished, maybe it will
also try to initialize and then
triggering this problem somehow."
Many thanks.
Daoming
Here is the error message:
System.InvalidOperationException
Collection was modified; enumeration operation may not execute.
System.InvalidOperationException: An error occurred when trying to create a controller of type 'WebController.Controllers.Admin.AdminProductController'. Make sure that the controller has a parameterless public constructor. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> NHibernate.MappingException: Could not configure datastore from input stream DomainModel.Entities.Mappings.OrderProductVariant.hbm.xml ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.Collections.ArrayList.ArrayListEnumeratorSimple.MoveNext()
at System.Xml.Schema.XmlSchemaSet.AddSchemaToSet(XmlSchema schema)
at System.Xml.Schema.XmlSchemaSet.Add(String targetNamespace, XmlSchema schema)
at System.Xml.Schema.XmlSchemaSet.Add(XmlSchema schema)
at NHibernate.Cfg.Configuration.LoadMappingDocument(XmlReader hbmReader, String name)
at NHibernate.Cfg.Configuration.AddInputStream(Stream xmlInputStream, String name)
--- End of inner exception stack trace ---
at NHibernate.Cfg.Configuration.LogAndThrow(Exception exception)
at NHibernate.Cfg.Configuration.AddInputStream(Stream xmlInputStream, String name)
at NHibernate.Cfg.Configuration.AddResource(String path, Assembly assembly)
at NHibernate.Cfg.Configuration.AddAssembly(Assembly assembly)
at DomainModel.RepositoryBase..ctor()
at WebController.Controllers._baseController..ctor()
at WebController.Controllers.Admin.AdminProductController..ctor()
at System.RuntimeType.CreateInstanceImpl(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean fillCache)
--- End of inner exception stack trace ---
at System.RuntimeType.CreateInstanceImpl(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean fillCache)
at System.Activator.CreateInstance(Type type, Boolean nonPublic)
at System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType)
--- End of inner exception stack trace ---
at System.Web.Mvc.DefaultControllerFactory.GetControllerInstance(RequestContext requestContext, Type controllerType)
at System.Web.Mvc.DefaultControllerFactory.CreateController(RequestContext requestContext, String controllerName)
at System.Web.Mvc.MvcHandler.ProcessRequestInit(HttpContextBase httpContext, IController& controller, IControllerFactory& factory)
at System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Answer 1

Oskar is right. Two separate threads are trying to initialize the session factory at the same time. Suggest you put some locking around the initialization code, perhaps just using the lock keyword and a suitable synchronization object. We've used a pattern like this, using one of the locks from the Wintellect PowerThreading library:
using (_lock.WaitToRead())
{
if (Factory != null) return Factory;
}
using (_lock.WaitToWrite())
{
if (Factory != null) return Factory;
Factory = ConfigureFactory();
return Factory;
}
You could more simply just use the lock keyword and a double-check locking pattern like so:
class NestedSessionManager
{
internal static SessionManager _sessionManager;
private static readonly object _syncRoot = new object();
internal static SessionManager sessionManager
{
get
{
if (_sessionManager != null) return _sessionManager;
lock (_syncRoot)
{
if (_sessionManager != null) return _sessionManager;
_sessionManager = new SessionManager();
return _sessionManager;
}
}
}
}