Plugins using C# DLL Interface

I'm having a problem switching from Mac, PC platform to iOS platform. I have written my C# interface to the plugins (.bundle, .a (iOS)) as DLLs. We do not wish to expose our code as C# scripts.

I set up my folders as such...

Assets/Plugins -> interfaceToPlugin.dll (for Mac using DLLImport("pluginName"))

plugin.bundle

Assets/Plugins/iOS -> interfaceToPlugin.dll (for iOS using DLLImport("_Internal"))

plugin.a

The "interfaceToPlugin" dlls were compiled separately for Mac and iOS.

I can build and run the Mac version. However, when I switch platforms to build for iOS. The Mac version of interfaceToPlugin.dll is still being referenced and I get pinvoke errors on the plugin methods.

All the Unity documentation explains how to build plugins with the interface to the plugins being C# scripts and the compiler directives will take care of DLLImporting method setups.

I know I cannot be the only one writing the C# interface to the plugins as a dll. The Unity documentation mentions that some developers might do this for the very reason that I am, not to expose our proprietary code.

How can I set up the project environment so that the appropriate interface dlls for the different platforms is used?