c# - Selenium WaitFor Ajax -

i want use solution :

public static void waitforajax(this iwebdriver driver) {     while (true) // handle timeout somewhere     {         var ajaxiscomplete = (bool)(drivermanager.driver ijavascriptexecutor).executescript("return jquery.active == 0");         if (ajaxiscomplete)             break;         thread.sleep(100);     } } 

but error:

an exception of type 'system.invalidoperationexception' occurred in webdriver.dll not handled in user code additional information: unknown error: jquery not defined

thanks in advanced