'System.Web.HttpApplicationState' does not contain a definition for 'DoEvents'

While convert .aspx pages to Image format, i am getting this Error.

'System.Web.HttpApplicationState' does not contain a definition for 'DoEvents' and no extension method 'DoEvents' accepting a first argument of type 'System.Web.HttpApplicationState' could be found (are you missing a using directive or an assembly reference?) 

Got code from these links:

http://pietschsoft.com/post/2008/07/23/C-Generate-WebPage-Thumbmail-Screenshot-Image.aspx#comment

http://stackoverflow.com/questions/3012728/how-to-convert-aspx-html-page-to-image-format/15987582#15987582