User281315223 posted
This is occurring in any Web Project?
If that is the case, then something something sounds wrong with Visual Studio. You might consider closing Visual Studio and restarting it (running it with administrative rights via Right-click > Run as Administrator). This generally resolves most Visual
Studio issues, if that in fact is one. (Restarting your machine can help as well).
With regards to your actual error message, it sounds like you are missing an assembly reference. Try checking the References area of your application to ensure the proper references are there and that none of them have any caution icons over them (indicating
issues).