c# - Problems with Session for Sorting my GridView -

i have problems session["tasktable"] datasource gridview. when open .aspx site first time session["tasktable"] null, if reload page (f5) session["tasktable"] datatable tasktable. how be? i'm able sort if first reload page. ideas? thanks

protected void page_load(object sender, eventargs e)     {          if (!page.ispostback)         {             datatable tasktable = new datatable("tasklist");             tasktable = dtcloned;                             session["tasktable"] = tasktable;             gv_projekte.datasource = session["tasktable"];             gv_projekte.databind();           }      } 

and sorting of gridview

protected void gv_sorting(object sender, gridviewsorteventargs e)     {         datatable dt = session["tasktable"] datatable;          if (dt != null)         {             //sort data.             dt.defaultview.sort = e.sortexpression + " " + getsortdirection(e.sortexpression);             gv_projekte.datasource = session["tasktable"];             gv_projekte.databind();         }     }      private string getsortdirection(string column)     {         // default, set sort direction ascending.         string sortdirection = "asc";          // retrieve last column sorted.         string sortexpression = viewstate["sortexpression"] string;          if (sortexpression != null)         {             // check if same column being sorted.             // otherwise, default value can returned.             if (sortexpression == column)             {                 string lastdirection = viewstate["sortdirection"] string;                 if ((lastdirection != null) && (lastdirection == "asc"))                 {                     sortdirection = "desc";                 }             }         }          // save new values in viewstate.         viewstate["sortdirection"] = sortdirection;         viewstate["sortexpression"] = column;          return sortdirection;     } 

that because of page.ispostback condition when page not postback assigned session variable value there untill session expire whatever session time out there.

take following exercise.

 protected void page_load(object sender, eventargs e)     {         if (session["a"]==null)         {             response.write("session empty");         }         else         {             response.write("session not empty");             response.write(session["a"].tostring());         }         if (!page.ispostback)         {             session["a"] = "jalpesh";         }     } 

here in blank page i'm doing same thing doing first time page load @ time print 'session empty' after session not empty till session expires print 'session not empty'.

so if need session value null have assign session null somewhere.