删除目录后，session会被清空

 System.IO.Directory.Delete(fullfile.Substring(0, fullfile.Length - 4), true);

删除目录后，session会被清空

 删除文件没有影响。

解决方法：

http://www.dotnetindia.com/2003/10/why_aspnet_rest.html 

http://connect.microsoft.com/VisualStudio/feedback/details/102228/system-io-directory-delete-clears-page-session

 http://topic.csdn.net/u/20070203/12/db43d6e2-8bde-4740-8275-ed4b4494bdd0.html

http://topic.csdn.net/u/20080215/00/629d47b4-0148-486b-9696-05d4317c8c7b.html