I have a custom WSP with (among other things) a user control and a DLL for GAC deployment. It's been built for SharePoint 2010 and .NET 3.5 and is fully working on my SharePoint 2010 Farm.
I also have a new SP2013 farm and installed the same WSP using:
Add-SPSolution -LiteralPath C:\WSPs\mypackage.wsp Install-SPSolution -Identity "mypackage.wsp" -WebApplication http://mywebapp -GACDeployment
This gives no errors. I have attached my 2010 content database, ran test-spcontentdatabase with no errors and have mounted it to the 2013 web app. No worries so far.
When opening the page where I expect to see the user control, I receive the error "Could not load type 'MY.DLLname'. This is the DLL contained in the WSP and referenced by my user control.
The DLL IS present in the "old" gac: c:\windows\assembly, but NOT present in the .net 4.5 gac, which is expected (I guess).
Why isn't this working? Are SharePoint 2013 sites running in "2010 mode" only looking for dll's in the "new" GAC? If so, what is the correct deployment procedure for legacy 2010 WSPs containing assemblies?