如果您需要保存文件，用它来获取应用程序的路径。

If you need to save a file use this to get the application path

Server.MapPath(@"filename.txt");

试试这个，因为当你只在您尝试访问Microsoft.Net文件夹中的Web服务器写入的文件名。

Try this because when you only write the filename at an web server you are trying to access to a Microsoft.Net folder.