I have two ASP.NET Web projects (ProjectA and ProjectB). When class in ProjectA is instantiating a class of ProjectB which uses a resource file Blah.resx, I get this error:
Just because you are referencing Project B's DLL doesn't mean that the Resource Manager of Project A is aware of Project B's App_GlobalResources directory.
Are you using web site projects or web application projects? In the latter, Visual Studio should allow you to link source code files (not sure about the former, I've never used them). This is a little-know but useful feature, which is described here. That way, you can link the Project B resource files into Project A.