.NET Standard is a formal specification of the .NET APIs that allow you to share even more code across the platforms you target. Xamarin developers are able to use .NET Standard libraries for sharing business logic, and .NET Standard support came to Xamarin.Forms recently, allowing you to easily share both your business and UI logic across multiple platforms. You’ve been asking us when you can File -> New -> Mobile App with Xamarin.Forms and .NET Standard, and we’re excited to share that this is now available in the Visual Studio 2017 version 15.5 Preview 3.
In this blog post, you’ll learn how to use new project templates in Visual Studio 2017 that enable you to utilize .NET Standard as a code sharing strategy when creating new Xamarin.Forms apps. We’ll also take a look at some of the other improvements that landed in our Xamarin.Forms templates in Visual Studio 2017 version 15.5, such as migration to PackageReference and checkboxes for selecting which platforms to target.
https://blog.xamarin.com/net-standard-comes-xamarin-forms-project-templates/