Just to add to @JeremyThompson's response:
My issue was due to referencing a COM object, by going "Add Reference" and browsing to the the .exe for the application (i.e. C:\Program Files (x86)\ProFile\profile.exe). This added an "Interop.Profile.dll" (obviously this will be different depending on the SDK you're trying to access) reference which was located in the obj/Debug folder as described by @JeremyThompson.
In my specific case, I had this Interop.Profile.dll referenced in two projects, one of which was not getting the error above and built successfully. I went to the obj/Debug folder for the successfully built project and copied the Interop.Profile.dll to my Solution's DLL folder (this is just a folder I created to keep all of my assemblies in one place, but you could technically put your COM DLL anywhere outside of the obj/Debug folder) and then changed the references of both my projects to point at the Interop.Profile.dll which is located in my DLLs folder, rather than the one in the obj/Debug folder.
Once I did this, my application built successful (using Rebuild, or Clean and then Build).
Obviously, my case is pretty specific, but hopefully it helps someone out there.