刚刚用 BackgroundTransferRequest 做了一个后台下载文件的demo[WP7]，代码写完了运行时报了一个ArgumentException（MSDN给出的说明 The path must end with a file name.），但是我写的代码没有问题的啊、确实一个合法的文件名。
一丝不解中，再抬头一个Remarks:
To help isolate the files to which the background transfer service has access, all transfers must be downloaded to a file within the \shared\transfers directory in the application’s isolated storage. You can add additional directories under this directory, but attempting to set a URI that is outside of the transfer directory will throw an exception.
很郁闷的独立存储，安全性确实高了，不过有时确让程序员很郁闷，幸好的是微软的文档做的很棒，很多信息还是可以在msdn上找到的。
BackgroundTransferRequest Class 链接：
http://msdn.microsoft.com/en-us/library/hh203110(v=vs.92).aspx