NullReferenceException on IIS/ASP.net - out of nowhere?

Question

I'm currently working on a website for a software company. I have a mechanism that runs as soon as the page loads (in Page_OnLoad, Masterpage code file), creates a new instance of an own class and runs a function from there. Everything was running fine for weeks, and I worked on the website at completely different pages/areas that do not affect my current cause in any way.

So the following happend: I tried the website on the asp.net dev server: Everything was fine and working correctly. So I uploaded it to my IIS server. Accessed the website: Everything still running without errors. 20 minutes or so later, I refreshed the page: NullReferenceException, out of nowhere. And now I can't get rid of it, whatever I do.

This is the message from the stack:

   [NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.]
   EITS.WWW.Helper.cUserLog.TrackUserLog(HttpRequest request) in     C:\EIT\Projekte\eits.ch\www.eits.ch\EITSWeb\www.eits.ch\Includes\cUserLog.cs:72
   EITS.WWW.IndexMaster.Page_Load(Object sender, EventArgs e) in C:\EIT\Projekte\eits.ch\www.eits.ch\EITSWeb\www.eits.ch\Index.Master.cs:22
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +24
   System.Web.UI.Control.LoadRecursive() +70
   System.Web.UI.Control.LoadRecursive() +189
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3063

The Masterpage code file:

http://codetidy.com/3244/

The class that is invoked:

http://codetidy.com/3245/

Answer 1

Here's the section of code that's throwing the exception:

String vSQL = "SELECT * FROM eits_visitor WHERE ip = '" + oUser.usrIP + "'";
DataTable dtCurrent = SQLCon.GetDataTableFromSQL(vSQL, oHelper.cfgConnection);

if (dtCurrent != null && dtCurrent.Rows.Count > 0) {
    // Check if IP is registered today
    vSQL = "SELECT TOP(1)* FROM eits_log WHERE visitor_ip = '" + oUser.usrIP + "' ORDER BY date DESC;";
    dtCurrent = SQLCon.GetDataTableFromSQL(vSQL, oHelper.cfgConnection);

    // Next line is where the exception is thrown:
    if (SQLCon.DateFromString(dtCurrent.Rows[0]["date"].ToString()).DayOfYear != DateTime.Now.DayOfYear) {
        // do stuff
    }
}

The problem is with this second query, where you're not checking that dtCurrent.Rows[0] even exists.

In the first query, you're doing the right thing - checking that dtCurrent.Rows.Count > 0 before doing anything. You seem to then just assume that because the first query returned a result, the second query will too, but since this is where the exception is being thrown you may need to rethink that assumption.

Your comment says the second query is aimed at "check[ing] if IP is registered today", but you're not actually checking whether a record exists - you're just assuming the record is there and using it, which makes the code fail when there is no such record.

If the IP wasn't registered today, is there still a record? If not, that's where your problem lies.

My first response to a problem such as this would be to add a check to make absolutely sure my assumption is correct:

if (dtCurrent.Rows.Count == 0)
    throw new Exception("The unthinkable happened - our assumption was wrong!");

// Next line is where the exception was being thrown before.
// Do we still get this far now?
if (SQLCon.DateFromString(dtCurrent.Rows[0]["date"].ToString()).DayOfYear != DateTime.Now.DayOfYear) {
    // do stuff
}