当您尝试访问属性时，似乎您的异步下载操作尚未完成.在访问文件之前，您确定下载已完成吗?

Looks like your asynch download operation is yet to complete when you try to access the properties. Have you made sure that the download is completed before accessing the file?

您可以在DownloadFileCompleted事件中访问文件.

You can access the file in the DownloadFileCompleted event.

http://msdn.microsoft.com/zh-CN/library/system.net.webclient.downloadfilecompleted.aspx