Cannot attach the file as database这个异常是在EF的code frist里经常出现的，解决方法很简单，只要重新启动一下V11实例即可。

　　 CMD> sqllocaldb.exe stop v11.0
    LocalDB instance "v11.0" stopped.

    CMD> 
    CMD> sqllocaldb.exe delete v11.0
    LocalDB instance "v11.0" deleted.

    CMD> sqllocaldb.exe start v11.0
    LocalDB instance "v11.0" started.

再次运行应用程序，搞定！

本文转自博客园张占岭（仓储大叔）的博客，原文链接：EF架构~Cannot attach the file as database，如需转载请自行联系原博主。