.NET Core 2.0 + .NET Standard 2.0 comes with 20000 APIs,
but apparently about a half of them are cross-platform (Linux, Mac, not only Windows).
To help identify what APIs are portable there is a new tool:
Windows Compatibility Pack for .NET Core | Connect(); 2017 | Channel 9
Porting to .NET Core - Using the Windows Compatibility Pack | Microsoft Docs
Introducing API Analyzer | .NET Blog
Trying out new .NET Core Alpine Docker Images - Scott Hanselman