升级到VS2012后无法识别的标记前缀或设备过滤器'asp'

I upgraded a Visual Studio 2010 Class Library project to 2012. This project contains several web user controls. After the upgrade, all of my ascx files show the error "Unrecognized tag prefix or device filter 'asp'" for every asp.net control in use.I upgraded a Visual Studio 2010 Class Library p