Session becomes null in different thread - asp.net-mvc

Question

I have a task that runs in a different thread and requires the session. I've done:
public GenerateDocList(LLStatistics.DocLists.DocList docs)
{
this.docs = docs;
context = HttpContext.Current;
}
and
public void StartTask()
{
//this code runs in a separate thread
HttpContext.Current = context;
/* rest of the code */
}
Now the thread has knowledge of the session and it works for a while but at some point in my loop HttpContext.Current.Session becomes null. Any ideas what can I do about this?
public static LLDAC.DAL.DBCTX LLDB
{
get
{
LLDAC.DAL.DBCTX currentUserDBContext = HttpContext.Current.Session["LLDBContext"] as LLDAC.DAL.DBCTX;
if (currentUserDBContext == null)
{
currentUserDBContext = new LLDAC.DAL.DBCTX();
HttpContext.Current.Session.Add("LLDBContext", currentUserDBContext);//this works only for a few loop iterations
}
return currentUserDBContext;
}
}

Answer 1

In general, this is a very fragile pattern for a multi-threaded operation. Long-running tasks (which I assume this is) are best suited to instance methods in a class rather than static methods such that the class can maintain any dependent objects. Also, since the session state is not thread safe and can span multiple requests you are getting into some very risky business by cashing your DB context in the session at all.
If you are convinced this is best done with static methods and stored in the session, you may be able to do something like this:
public static HttpSessionState MySession { get; set; }
public GenerateDocList(LLStatistics.DocLists.DocList docs)
{
this.docs = docs;
MySession = HttpContext.Current.Session;
}
Then:
public static LLDAC.DAL.DBCTX LLDB
{
get
{
LLDAC.DAL.DBCTX currentUserDBContext = MySession["LLDBContext"] as LLDAC.DAL.DBCTX;
if (currentUserDBContext == null)
{
currentUserDBContext = new LLDAC.DAL.DBCTX();
if (MySession == null)
{
thow new InvalidOperaionException("MySession is null");
}
MySession.Add("LLDBContext", currentUserDBContext);
}
return currentUserDBContext;
}
}
Note that you could still run into issues with the session since other threads could still modify the session.
A better solution would probably look something like this:
public class DocListGenerator : IDisposable
{
public LLDAC.DAL.DBCTX LLDB { get; private set; }
public DocListGenerator()
{
LLDB = new LLDAC.DAL.DBCTX();
}
public void GenerateList()
{
// Put loop here.
}
public void Dispose()
{
if (LLDB != null)
{
LLDB.Dispose();
}
}
}
Then your calling code looks like this:
public void StartTask()
{
using (DocListGenerator generator = new DocListGenerator()
{
generator.GenerateList();
}
}
If you really want to cache something, you could cache your instance like this:
HttpContext.Current.Sesssion.Add("ListGenerator", generator);
However, I still don't think that is a particularly good idea since your context could still be disposed or otherwise altered by a different thread.

Answer 2

Using anything related to the HttpContext.Current on anything besides the main Request thread is generally going to get you into trouble in ASP.net.
The HttpContext is actually backed on a thread belonging to a Thread Pool and the thread may very well get reused on another request.
This is actually a common issue with using the new Async/Await keywords in ASP.net as well.
In order to help you, it would help to know why you're attempting this in the first place?
Is this a single server or a web farm with multiple load balanced servers?
Are you hosting it yourself, or is it the site hosted by a provider?
What is the SessionState implementation (SQL Server, State Server, In-Process, or something custom like MemCached, Redis, etc...)
What version of ASP .net?
Why are you starting a new thread instead of just doing the processing on the request thread?
If you really can't (or shouldn't) use session. Then you could use something like a correlation ID.
Guid correlationID = Guid.NewGuid();
HttpContext.Current.Session["DocListID"] = correlationID;
DocList.GoOffAndGenerateSomeStuffOnANewThread(correlationID);
... when process is done, store the results somewhere using the specified ID
// Serialize the result to SQL server, the file system, cache...
DocList.StoreResultsSomewhereUnderID();
... later on
DocList.CheckForResultsUnderID(HttpContext.Current.Session["DocListID"]);