Why can't I reference my class library?

Also, check that the new solution's projects run against a compatible framework to the project you're trying to include. I was trying to include a reference to a 4.0 project in a 3.5 project.

I found how to fix this issue (for me at least). Why it worked, I'm not sure, but it did. (I just tried against a second website that was having the same problem and the following solution worked for that as well).

I tried the normal cleaning of the projects and rebuilding, shutting down all my Visual Studio instances and restarting them, even tried restarting my computer.

What actually worked was opening up the project in Visual Studio, closing all the open tabs, and then shutting it down.

Before I had left the tabs open because I didn't think it mattered (and I hardly ever close the tabs I'm using).

One possibility is that the target .NET Framework version of the class library is higher than that of the project.