Check your build types of each project under project properties - I bet one or the other will be set to build against .NET XX - Client Profile
.
With inconsistent versions, specifically with one being Client Profile
and the other not, then it works at design time but fails at compile time. A real gotcha.
There is something funny going on in Visual Studio 2010 for me, which keeps setting projects seemingly randomly to Client Profile
, sometimes when I create a project, and sometimes a few days later. Probably some keyboard shortcut I'm accidentally hitting...