Why does session object throw a null reference exception?

asp.net, c#, session

Solution

You first need to check whether that session variable exists

if(Session["YourAssessment"] != null)
    // Do something with it
else
    // trying to call Session["YourAssessment"].ToString() here will FAIL.

That happens since your session has a lifecycle, which means - it expires (the cookie that defines it expires) - thus your objects vanish. you could increase `sessionState timeout` in web.config for your sessions to last longer.

For example, in web.config

  <system.web>
      <sessionState timeout="40" />
  </system.web>

Will make your sessions last for 40 minutes, as long as the client doesn't clear it, and the web server is up&running.

Problem

on some of my aspx page I am checking session like this ``` if (bool.Parse(Session["YourAssessment"].ToString()) == false && bool.Parse(Session["MyAssessment"].ToString()) == true) { Response.Redirect("~/myAssessment.aspx"); } ``` It works fine if I keep playing with the pages frequently, but if I don't do anything with the page at least even for 5 min, running the page throws the error ``` Object reference not set to an instance of an object. ``` Following is the stack for this ``` [NullReferenceException: Object reference not set to an instance of an object.] yourAssessment.Page_Load(Object sender, EventArgs e) in d:\Projects\NexLev\yourAssessment.aspx.cs:27 System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35 System.Web.UI.Control.OnLoad(EventArgs e) +91 System.Web.UI.Control.LoadRecursive() +74 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2207 ``` Could some body explain me this weird behaviour? And as we know by default the session last for is 20 min. EDITED See I have a page default aspx, it has got a button which fixes on the some basis where to redirect On default page it check like this ``` protected void Page_Load(object sender, EventArgs e) { if (!HttpContext.Current.Request.IsAuthenticated) { Response.Redirect("~/login.aspx"); } else { Session["YourAssessment"] = false; Session["MyAssessment"] = false; } } ``` on button click it has ``` protected void imgClientFreeEval_Click(object sender, System.Web.UI.ImageClickEventArgs e) { if (HttpContext.Current.Request.IsAuthenticated) { string sqlQuery = "SELECT count(*) FROM SurveyClient WHERE UserID='" + cWebUtil.GetCurrentUserID().ToString() + "'"; SqlParameter[] arrParams = new SqlParameter[0]; int countSurvey = int.Parse( Data.GetSQLScalerVarQueryResults(sqlQuery).ToString()); if (countSurvey > 0) { Session["YourAssessment"] = true; Session["MyAssessment"] = false; } Response.Redirect((countSurvey > 0) ? "~/yourAssessment.aspx" : "~/myAssessment.aspx"); } else { Response.Redirect("~/login.aspx"); } ``` and on myAssessment page it check like this ``` protected void Page_Load(object sender, EventArgs e) { if (!HttpContext.Current.Request.IsAuthenticated) { Response.Redirect("~/login.aspx"); } else { if (Session["YourAssessment"] != null && Session["MyAssessment"] != null && bool.Parse(Session["YourAssessment"].ToString()) && !bool.Parse(Session["myAssessment"].ToString())) { Response.Redirect("~/yourAssessment.aspx"); } } } ``` and on yourAssessmtn it check like this ``` protected void Page_Load(object sender, EventArgs e) { if (!HttpContext.Current.Request.IsAuthenticated) { Response.Redirect("~/login.aspx"); } else { if (Session["YourAssessment"] != null && Session["MyAssessment"] != null && !bool.Parse(Session["YourAssessment"].ToString()) && bool.Parse(Session["MyAssessment"].ToString())) { Response.Redirect("~/myAssessment.aspx"); } PopulateAllSurveyByUser(); if (ViewState["surveyClientID"] != null) { grdSurveyDetail.Visible = true; PopulateSurveyDetails( int.Parse(ViewState["surveyClientID"].ToString())); } else { grdSurveyDetail.Visible = false; } } } ``` what's wrong please explain?

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