Yesterday I ran into a very interesting problem. I have a solution with three projects. When I compile, I get an error saying namespace not found or might be missing assembly. The error was pointing one of the project in the same solution. So I ended up building each project by itself and two projects compiled properly but the one kept failing with the same error. Initially I thought, something might have messed up in the project dependencies and it turned out, that is not the case.
After looking at the properties of each project I found the real culprit. It is the target system in properties. So if you run into a problem, make sure Target Frame of all projects pointing to same type. In my case the failing one was pointing to .Net Framework 4 Client for some reason.
Hope this saves someone’s valuable development time.