httpwebrequest异步参考

http://www.cnblogs.com/SanMaoSpace/archive/2011/07/27/2118133.html

http://www.cnblogs.com/qianlifeng/archive/2010/04/23/1718343.html

http://www.cnblogs.com/SanMaoSpace/archive/2011/07/27/2118128.html

http://www.csharpwin.com/csharpspace/11503r9846.shtml

http://bbs.csdn.net/topics/340040257