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Asked: 2026-06-01T11:57:06+00:00

I’ve been searching for answers for quite some time on this as it continues

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I’ve been searching for answers for quite some time on this as it continues to plague me. We store user login info and other data about the user’s current activities in Session State (InProc). Every so often I get a Null Reference exception trying to use one of the session variables. It happens on random pages, with random Session variables. I have modified the web.config httpRuntime and compliation tags to prevent appPool restarts:

<httpRuntime requestValidationMode="2.0" waitChangeNotification="86400" maxWaitChangeNotification="86400" />
<compilation debug="False" strict="false" explicit="true" targetFramework="4.0" numRecompilesBeforeAppRestart="1000" />

I have set IIS to restart the app pool at 3am to make sure it doesnt restart when people are busy using the server. And I’m logging app pool restarts in the event log to make sure I know when its happening. 

Dim runtime As HttpRuntime = GetType(System.Web.HttpRuntime).InvokeMember("_theRuntime", BindingFlags.NonPublic Or BindingFlags.Static Or BindingFlags.GetField, Nothing, Nothing, Nothing)
Dim shutDownMessage As String = runtime.GetType().InvokeMember("_shutDownMessage", BindingFlags.NonPublic Or BindingFlags.Instance Or BindingFlags.GetField, Nothing, runtime, Nothing)

Dim shutDownStack As String = runtime.GetType().InvokeMember("_shutDownStack", BindingFlags.NonPublic Or BindingFlags.Instance Or BindingFlags.GetField, Nothing, runtime, Nothing)
Dim evtSource As String = "ASP.NET"
Dim log As New EventLog
log.Source = evtSource
log.WriteEntry(String.Format("_shutDownMessage={0}{2}_shutDownStack={1}", shutDownMessage, shutDownStack, vbCrLf & vbCrLf), EventLogEntryType.Warning)

I get the event log entries when the app pool restarts.
The App Pool is NOT restarting when these errors happen.

When particular Session variables are lost, most of the other Session variables for the same user are still in place. Also, there are typically another 10-20 users logged into the site that are unaffected when it happens.
The user that gets the error will back up, go through the same pages again, and it will work fine.

I was having this problem on a Windows Server 2003 (32bit) running IIS6 with .NET 3.5 32bit and 4GB of memory.. As part of our server upgrades about a year ago we got a new webserver – Windows Server 2008 (64bit) running IIS 7 with 16GB memory. I upgraded the website to .NET 4.0 64bit. Still having the same problems on the new machine (usually 1-3 times per day – at random times through the day).

I cant make it happen in my debugging due to its random nature, but I do believe it happens randomly on our dev environment as well. The dev server has virtually the same specs as the production one.
Both environments are isolated and running as a single web server, not a part of a web farm.

I’m thinking that I may try to implement a State Server to get out of the InProc mode, but that’s just another stab in the dark..
Other than trying the State Server, is there anything else I can do to identify when this happens or prevent it?

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1 Answer

  1. Editorial Team

    For anyone that is interested, or dealing with similar issues, I wanted to follow up with the cause of my problem here.

    I implemented NCache out-of-process state server for Application Cache and Session State about 7 or 8 months ago. Unfortunately, moving the session out-of-process has not had any impact of my problem of losing random session variables during report selection on my site. And, as I had been unable to replicate this problem, I had not put more effort into trying to fix it until recently when another problem made the light go off in my head.

    To get to the point – I was not overwriting the session variables somewhere that I didnt realize, but the problem was the user was opening up a second (or third) tab to compare report selection options side by side. We have several custom reports where the user can select multiple options to generate custom reports (think of it like a wizard control where there are several steps to create a custom report). If a user is on step 3 of 5, and then opens a new tab and starts going through the report selection process again, the new selections are overwriting the old selections b/c the 2 tabs shared the same session. I verified this was the case by opening multiple tabs and stepping through the selection process.

    I am in the process of trying to distinguish between multiple report runs so that the selections for one report are stored using a unique session key from other report selections. That is proving difficult as well, but is not really related to the problem I thought I was having with missing session data.

    If anyone finds this post and thinks they are losing session data randomly and cant replicate it, try debugging your site and opening multiple tabs. Stepping through both tabs at the same time illuminated the problem for me.

    HTH

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