You might be asking yourself at this point, how do I set up my solution in Xamarin Studio to handle shared code and also have platform-specific projects? Xamarin.iOS applications can only reference Xamarin.iOS class libraries; so, setting up a solution can be problematic. There are several strategies for setting up a cross-platform solution, each with its own advantages and disadvantages.
Options for cross-platform solutions are as follows:
File Linking: For this option, you would start with either a plain .NET 4.0 or .NET 4.5 class library containing all the shared code. You would then have a new project for each platform you want your app to run on. Each platform-specific project would have a subdirectory with all of the files linked in from the first class library. To set this up, add the existing files to the project, and select the Add a link to the file option. Any unit tests can run against the original class library. The advantages and disadvantages...