I've done an add-in in Visual Studio 2008 (targeting .NET Framework 2.0) in a Windows 8.1 64 bit machine.

With the option "Register for COM Interop" checked, it compiles and registers correctly in "Release - x64" configuration.

But whenever I try to compile in "Debug - x64", I get the error:

     C:\Windows\Microsoft.NET\Framework\v3.5\Microsoft.Common.targets(3019,9): error MSB3097: File "..." is not a valid assembly.

I've tried to register manually (C:\Windows\Microsoft.NET\Framework64\v2.0.50727\regasm) and it works fine.

Thanks in advance.

Best Regards,

Ignasi