Two major implementations of .NET Standard 2.0 are the traditional .NET Framework and .NET Core, so libraries targeting .NET Standard 2.0 can be used on both platforms without modification.
If you're a typical .NET customer, then you're used to the .NET Framework, which also means you're probably only used to doing development on Windows. That was no longer a viable strategy for .NET, so we're now pursing a cross-platform strategy. A good chunk of the complexity you see today is a result of the changed dynamics of software engineering: the PC is no longer the only relevant form factor, server applications have to be rethought as scalable cloud services, and the UI paradigm is no longer a sequence of dialogs but has to be tailored to multiple clients, and on top of that you now also have to deal with multiple different operating systems.
.NET has a long standing history of embracing the underlying platform (hello COM, hello P/Invokes) while also providing a ton of conveniences on top that make it approachable (hello WinForms). We can't really shield you from all the complexities that result from the changed dynamics. But what we can do is making it more consistent and productive.
Over the last years, various different .NET stacks were created, several of them outside of the Microsoft bubble (Mono, Xamarin, Unity). So in order to build modern experiences, you often have to use various different stacks to get the job done. We understand that this isn't free of challenges, which is why we try very hard to reconcile the differences. For instance, we acquired Xamarin to fully embrace the mobile support they offer and make it a fully integrated part of the .NET development platform. We changed the license on Mono to enable Unity to use the latest version and pick up innovation instantaneously. And we've created the
.NET Standard is a way to achieve API consistency between different .NET stacks. This makes it much easier for application and library authors to share code between different .NET implementations.
Our goal is to empower .NET developers to build any kind of app, for any kind of operating system. That's the world we live in now and we're fully committed to make this experience as productive as possible.
The whole thing reminds me a little of the UI situation. Start with Winforms, don't improve it but start WPF (and keep Winforms), then start Silverlight, then start UWP. All in parallel instead of building on top of each other.
MS doesn't need more more frameworks, but less.
.Net Core has API's that aren't applicable to the Windows full-fat Framework, Mono or Xamarin - you don't need or want them there.
For newer .NET implementations we push folks to start with .NET Core and adding their specific technologies on top. That is, for instance, what Samsung has been doing with Tizen. So if the open source community innovates in .NET Core, Samsung can just move to a later build of .NET Core in order to benefit from it. They are a pure superset of .NET Core, by construction. However, for all other cases someone needs to port the changes from .NET Core to Mono which makes it consumable by Xamarin and Unity. .NET Framework is in a similar boat, although porting is somewhat easier as the .NET Core code base originated from .NET Framework.
Since we can't (easily) move Mono/Xamarin/Unity/.NET Framework on top of .NET Core, we need a way to standardize the API set so that it's not all chaos. And that's where .NET Standard comes in.
They are. NET Core fully implements NET Standard.
Is there a list somewhere that specifies what is not implemented per target, though it's in the standard?