Visual Studio 2012 IDE Complaining about namespaces after upgrade

Question

I've recently upgraded my VS 2010 Solution to a VS 2012 solution. It is a solution with various class libraries and an MVC web application. I've upgraded all projects to .NET 4.5 but have this strange error.

The solution builds 100%, but whenever I'm in a controller (any controller), any references to classes within my MVC project are unrecognized, i.e. I get the 'The type of namespace name 'xxx' could not be found etc.' and I have no intellisense on any of my own classes etc. but the project builds and works perfectly.

Any ideas??? And yes, it's very sporadic, it will work from one minute and won't the next.

Answer 1

Run Build -> Clean Solution.

Or..

Right-click on the project that Visual Studio cannot find the reference for and select Clean.