这是更新面板的一个令人沮丧的特点。给这篇文章读肚里在这个问题的原因以及分辨率：

This is a frustrating feature of the update panel. Give this post a read which goes over the cause of this issue as well as a resolution:

的http://geekswithblogs.net/mmintoff/archive/2009/04/01/fileupload-within-updatepanel.aspx