SessionID抛出空异常

Question

I have an object that encapsulates a System.Web.SessionState.HttpSessionState object as one of its data members. Rarely (to the point where we are unable to reproduce exactly when or how) we will get a NullReferenceException error with this object when accessing customSession.Session.SessionID

I've used the debugger to trace the object, and customSession.Session is not null. It is a valid HttpSessionState object.

Mode: InProc

StaticObjects: {System.Web.HttpStaticObjectsCollection}

SyncRoot: {System.Web.SessionState.HttpSessionStateContainer}

Keys: {System.Collection.SpecializedNameObjectCollectionBase.KeysCollection}

IsNewSession: true

IsReadOnly: false

Keys has 4 records in it.

But even in the debugger, if I explore the object and try to get the SessionID, it says

"cs.Session.SessionID threw an exception of type System.NullReferenceException.

Does anyone know if this is just a bug in Visual Studio's development server? I just don't want to have this issue occur on a production release and the only way to fix it is to restart IIS.

-- Edit --

Additional information:

I'm using the "break when common language runtime exception is thrown" option, so what's happening is the system is trying to access the CustomSession object, as cs.Session.SessionID and getting a null-ref exception, and the debugger kicks in. Then I use the watch window and put in "cs.Session" in the watch, and it shows all the properties of the Session object, and the line for SessionID says it throws a null reference exception. This null ref exception causes the application to break if not manually handled.

I was able to reproduce the error by modifying code in the App_Code directory repeatedly (took about 5 or 6 changes I think). I don't know for certain if this is the only way of this issue happening as others here have reported it happening after what they suspected was a session timeout. In any case, it seems to be happening within the "Session_End" event coming out of the global.asax. But the error occurs before the "Session.Abandon()" statement is hit.

I have confirmed that I can access values stored in the session. The only thing giving an error is accessing the SessionID.

--- Additional (oct30) ----

The line where the error occurs is the if-statement:

foreach (CustomSession cs in allSessions) {
    if (cs.Session.SessionID == curSession.Session.SessionID) {
        // do stuff
    }
}

The "cs" and "curSession" objects are both not null, the "Session" property of each is also not null at this point.

Answer 1

You are watching an exception which would be handled ordinarily by the framework.

Essentially it is implementing a per-connection singleton pattern using the exception to detect when the session does not exist. So it just uses the session object, and if that throws an exception, it creates one on the spot and carries on.

Since you have selected "break when exception is thrown" and presumably not selected "Just my code", it is breaking. Because you asked it to.