Question

I have a scenario where i have a page which opens a dialog on click of a button, in the opened dialog form on button click i can read a list of data from a selected .txt file and build a query and add the data to some database tables. Since there could be large amount of data this process can take large time because of this the user would not be able to work on the application until the upload completes. Hence to make the upload process Asynk i am using the PageAsyncTask. Below is the code sample, but in the method called in the PageAsyncTask the HttpContext.Current is null hence i am not able to use session handling. Please any guidance on this why would this be null and how can i use the session in this case

   protected void BtnUpload_click(object sender, EventArgs e)
    {
        PageAsyncTask asyncTask1 = new PageAsyncTask(OnBegin, OnEnd, OnTimeout, SessionManager.UserData, true);

        Page.RegisterAsyncTask(asyncTask1);
        Page.ExecuteRegisteredAsyncTasks();
    }

public IAsyncResult OnBegin(object sender, EventArgs e,
            AsyncCallback cb, object extraData)
    {
        _taskprogress = "AsyncTask started at: " + DateTime.Now + ". ";
        uData = extraData as UserData;

        _dlgt = new AsyncTaskDelegate(BeginInvokeUpload);
        IAsyncResult result = _dlgt.BeginInvoke(cb, extraData);

        return result;
    }

private void BeginInvokeUpload()
    {
        string selectedFileName = string.Empty;
        string returnValuePage = string.Empty;
        User teller = new User();
        SessionManager.UserData = uData;
    }



  private void BeginInvokeUpload()
    {
        string selectedFileName = string.Empty;
        string returnValuePage = string.Empty;
        User teller = new User();
        SessionManager.UserData = uData;
    }

public class SessionManager
    {
public static UserData UserData
        {
            get 
            {
                UserData userData = null;
                if (HttpContext.Current.Session["UserData"] != null)
                {
                    userData = HttpContext.Current.Session["UserData"] as UserData;                    
                }
                return userData;
            }
            set 
            {
                 HttpContext.Current.Session["UserData"]=value;                  
            }
        }
}

Answer 1

The answer is simple : you can not use the session if the HttpContext.Current is null

So if you need to modify the session you simple can not and the only alternative is to make your totally custom session module/solution.

If you only need to read some values, then you can pass them when you create your thread.

And finally the only solution is to not use the thread if you won to manipulate the session variables.

why this design?

why MS session did not allow you to handle it out side of a page and inside a thread ? the answer is because is need to lock the session data on page processing – with this lock even if you start a thread and been able to get the session data, will not been able to use it parallel.

Also if you been able to use the session your self in a thread, then this thread may lock the entire page view process, because I say it again, session is lock the entire page view, and each page that use the same session are not work in parallel

This lock of session on the entire page is necessary the way the MS session works, and the only way to avoid that is to make a totally custom session solution, and handle special cases with different code.

The good about that design is that you avoid to make a lot of locking and synchronization by your self on every page call – for example if you disable the session on a page, and use that page for data inserting, if a use make multiple double clicks on the insert, and you do not handle it with synchronization on the insert, you end up with multiple same insertions.

More about session lock:
Replacing ASP.Net's session entirely
Web app blocked while processing another web app on sharing same session
jQuery Ajax calls to web service seem to be synchronous
ASP.NET Server does not process pages asynchronously

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How to get Session Data with out having HttpContext.Current, by SessionID