I think you can use 'Portable Class Library', it is a project type in Visual Studio helps you build cross-platform apps and libraries for Microsoft platforms quickly and easily, Portable class libraries can help you reduce the time and costs of developing and testing code. Use this project type to write and build portable .NET Framework assemblies, and then reference those assemblies from apps that target multiple platforms such as Windows and Windows Phone.
look out this link for
sample[
^]
Hope it helps