Unrecognized tag prefix or device filter 'asp'
July 01, 2013
I had this problem recently in Visual Studio that drove me mad! It was a VB ASP.NET web project and Intellisense would not work on ASPX pages, complaining about the asp tag prefix.
There is a lot of documented bugs/fixes for this online (google it), including deleting the solution .suo file, clearing out bin/obj files, deleting the visual studio reflected schemas files (under the app data folder), clearing your VS settings.
I tried them all but to no avail.. My problem though, was quite different in 2 ways.. Firstly my project was building and running absolutely fine (not sure about this for the other people's problems, but maybe worth noting). Also, in my opinion, the cause of my problem was the way I had set up the project to re-use an existing web projects files and code (I was re-using quite a lot of code files from another project by adding them as linked files for the code-infront and a DLL reference for the code behind.)
I have had some problems with VB project in the past due to this kind of thing, because VB does not define the root namespace in the code files it has caused some issues, which led me to believe the problem was being caused by the vbproj settings.
I remembered that by default I have option strict set to "on" for VB projects, since the project was building but the designer was just being "whingy" I decided to turn option strict off... It worked! :)