How do you make a choice on which framework to use? Where the right version of the .NET Framework or Mono has to be installed machine-wide to support your app, you can have applications using different versions of .NET Core on one server. You can also update one application to use a newer version of .NET Core without affecting the others. With the .NET Framework or Mono, you have the best chance to use the existing code. It offers a huge selection of classes and other types. CoreFX is a different class library, and you will probably have to refactor when using the existing code. Also, CoreFX has a much smaller choice of usable types, and not everything you're used to is available using extra packages. It is an ecosystem that still has to evolve where the FCL is very complete, and it is proven technology. However, it contains a lot you probably don't need. Using Mono or .NET Core are your choices if your app has to run cross-platform. The .NET Framework runs on Windows only. Targeting multiple frameworks makes sense if you're developing component libraries for use in your own business or if you want to distribute them commercially. Your libraries can be used by applications using all the chosen frameworks.

Alternatively, maybe you have to write an app now and decide the framework it has to run on later. In the next section, we'll see the role that DNX plays in all this: