Getting unhandled exception error from code

when try run code error on "in"

on "foreach (var item2 in listbox1.items) line.

htmlelementcollection elc = this.webbrowser1.document.getelementsbytagname("a");                                              int q;                                              foreach (htmlelement el in elc)                                              {                                                  foreach (var item2 in listbox1.items)                                                  {                                                                                                            if (item2.tostring() == label2.text)                                                      {                                                          listbox1.selecteditem = item2.tostring();                                                          q = listbox1.selectedindex;                                                          listbox2.selectedindex = q;                                                                                                                    if (el.innertext == listbox2.selecteditem)                                                          {                                                                el.invokemember("click");                                                              break;                                                                                                                            messagebox.show("done");                                                          }                                                                                                                                                                          }                                                  }                                                                                                  }

i got work way searching items in listbox.

Visual Studio Languages  ,  .NET Framework  >  Visual C#