MySPList.Items[0] NullReferenceException

Question

Hi, here is a though one. I hope someone can help, as it is driving me crazy.
I have an assembly in GAC, which has a class, which is being executed by SPTimerScheduler . Everything related to the timer is fine, my code is being run, by SPConfigAcct (the user which is running the Windows SharePoint Timer). The user has all possible permissions, etc...
Now, the problem.
I create an instance of SPSite and SPWeb, which works fine. Using the SPWeb instance, I can access lists just fine. But when I do that:
web.Lists["my list"].Items[0]
I get a NullReferenceException.
I have tried all possible variations, like:
SPList list = web.Lists["my list"];
list.Items[0]; // Throws NullReference
and it just doesn't want to work. Now, the kicker. If I navigate with the browser to the list "my list" in sharepoint and View one of the items there, then run the timer, it can access the items just fine.
This leads me to believe that the problem is somehow related to the application pool. Anyway, I cross-referenced my logs with the logs in SharePoint and here is what I saw (in bold are items I have masked):
12/05/2009 03:00:59.15 NullReferenceException in SPRequest. stackTrace at release:
12/05/2009 03:00:59.15 ERROR:
request not found in the TrackedRequests. We might be creating and closing webs on different threads.
ThreadId = 24, Free call stack =
at Microsoft.SharePoint.SPRequestManager.Release(SPRequest request)
at Microsoft.SharePoint.SPWeb.Invalidate()
at Microsoft.SharePoint.SPWeb.InitWebPublic()
at Microsoft.SharePoint.SPWeb.get_ServerRelativeUrl()
at Microsoft.SharePoint.SPWeb.get_Url()
at Microsoft.SharePoint.SPListItemCollection.EnsureListItemsData()
at Microsoft.SharePoint.SPListItemCollection.get_Item(Int32 iIndex)
at [myAssembly].[myClass].SomeMethod(SPWeb web)
at [myAssembly].TimerJobs.[MyClass].MySynchronizer.Sync()
at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
at SharePointer.Utilities.InvokingMethod.Invoke()
at SharePointer.Utilities.MethodExecutor.PerformExecution()
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart() , Allocation call stack (if present) null
12/05/2009 03:00:59.13 An SPRequest object was not disposed before the end of this thread.
To avoid wasting system resources, dispose of this object or its parent (such as an SPSite or SPWeb) as soon as you are done using it.
This object will now be disposed. Allocation Id: {xxx-FA1C-474D-9DB7-zzzzz
} To determine where this object was allocated,
create a registry key at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\HeapSettings.
Then create a new DWORD named SPRequestStackTrace with the value 1 under this key.
If anyone has any idea, no matter how crazy, please let me know!Regards,
Hristo Yankov
My blog - http://blog.myitechnology.com

Answer 1

Debug the code by attaching the Timer process and see what is really happening.
Are you using the SPWeb in multiple threads??
Enrique Blanco PMP, Consultant http://www.eblanco.com
Hi all of you guys and thanks for your responses. I think I may have found out what the issue is and fixed it. I will test it again in an hour or so, because it didn't reproduce always.
Enrique, you are very close. Anyway, here is what I suspect:
This solution I am using - SPTimerScheduler ... it is run by SharePoint, as a timer job, every minute or so. Then, based on its configuration in a list, the SPTimerScheduler code instantiates my classes and runs them in separate threads. So basically, it works as a dispatcher.
Alright, so what happens is the following:
Main Thread (real timer job run by SharePoint) - running SPTimerScheduler feature class. It prepares to start an instance of my very own class (stored in another assembly). Here is the trick part - the constructor of my class is being run immediately, because that's how threading works. In the constructor of my class, I am getting an instance of SPWeb, based on a URL parameter. So far so good. Ok, so it has now prepared my class in a thread and starts it. Then this Main Thread finishes.
The Thread SPTimerScheduler started - the constructor is already done, right? Alright, Main Thread instructed this thread to execute a method of my class (in this thread). The method tries to access my SPWeb property. Which kind of works, but it is all messed up. At least SPListItem's properties are throwing exceptions.
So, when I moved the initialization of SPWeb from the constructor, to the main method (which the main thread starts in a separate thread), everything started working just fine.
I know it's complicated and I am not good in explanations, so let me illustrate with code. This is what my assembly looks like:
Members:
 SPWeb web;
Constructor: // Being called in Main Thread (the SharePoint timer job)
 web = new SPWeb(siteUrl);
Main Method: // Being called in a thread, separate from Main Thread
 web.Lists[xxx].Items[0]; // Throws NullReferenceException
Conclusion: If I let my SPWeb property be initialized in another thread, but the method of my class which accesses this property, being called in another thread, ugly things happen.
Well, I hope I am right. I will test again later. This is messed up.
Regards,
Hristo Yankov
My blog - http://blog.myitechnology.com