ASP.Net build error - assembly not referenced

Question

I am getting a build error when I attempt to build my asp.net application. The error is:

The type 'MediCare.Framework.Authentication.IUserAuthenticate' is defined in an assembly that is not referenced. You must add a reference to assembly 'MediCare.Framework, Version=1.0.1.95, Culture=neutral, PublicKeyToken=1999fa3c42b9'.

I can see the class in the library which is in the references folder. How do I debug this issue?
My intention is to point the references folder to the library source code's bin directory (since the pdb file is lcoated there and any updates to the library will reflect in the application). However, when I point the references folder to the bin directory of the source code, I get the error message above.

Update The application was originally developed in VS 2008 and I am modifying it in 2010. I think I am confused about where the GAC assemblies would be, if I am using VS 2010 (2.0 framework location or 4.0 framework location) ? .NET 4.0 has a new GAC, why?

Update (06-26-2012)
The file in the GAC ("C:\\Windows\\assembly\\") has been deleted. But this error is still occuring.

Answer 1

This could possibly be caused by version inconsistencies. Meaning, in order to fix this, I would do a complete clean build of your assemblies (and also an assemblies that might be nested inside an assembly). Just go out cold and do a new build on all assemblies. That should work.

Answer 2

Go into windows explorer, right click on the DLL file that your project is using that contains this class, and look at the version tab. Make sure version number matches what you see in the error message. My guess is that it won't.

If it does match, then check the GAC and make sure there isn't a different version of the DLL there that it could be picking up. To do that, go to windows explorer, and navigate to c:\\windows\\assembly. See if that assembly is in there with a different version number. If it is, you may need to delete it from the GAC (use caution though because other apps may depend on it being there).