There are other open source community-driven alternative to MS Build - you can use Cake, Fake or Nuke. They are even simpler to use and yet powerful.
Regarding the dotnet core 3.0 support for WPF and WinForms, it has nothing to do with cross-platform. It just that those desktop technology will take advantage of some of the strengths of dotnet core such as performance, speed, signle deployment model and side-by-side deploy. I think Microsoft specifically gave an indepth explanation about this. So asp.net core and dotnet core are cross-platform.
I think this cuts across all the stack, to be hoest. In some countries - even in some county or states - some people love Java more .NET, while in some other countries some people love .NET more than Java. I think the reason Microsoft open sourced .NET was because many companies would love to use .NET but it was not open source. Now it is open sourced by Microsoft and many people are contributing, many people have started using it also. But your statement little "little effort", could you elaborate on that? Note Samsung is adopting .NET, Google etc. By the way, C# is still one of the most popular and loved programming languages.
it has nothing to do with cross-platform
That's the problem. In previous versions Microsoft constantly patted themselves on the back about how cross platform dotnet core was. Now they may or may not be doing an about face on that and you have to start to wonder how committed they really are to being platform agnostic.
Microsoft doesn't need to give an in-depth explanation. All they need is some basic product differentiation. Call it something like "The Modern Development Framework For Windows" and only mention dotnet core in passing, as opposed to the "Windows development is coming to dotnet core" messaging that they've been doing. They could even go one better and blow the dust off some of Mono's Linux-y bits (like GTK support) and port that to dotnet core as the "Modern Development Framework for Linux". OK, maybe not call it "Modern" because of the UI connotations and the Windows and Linux specific products should have fairly different names to avoid confusion, but hopefully you'll see my point that the platform specific code should be highlighted as not being the same thing as the platform agnostic core product.
In some ways the mixed messaging over Windows development in dotnet core is also part of the low effort to attract outsiders. It might make perfect sense to people who are already on the platform, but if you were evaluating the platform cold it looks like there's no clear direction of where it's headed and when you combine that with Microsoft's mercurial track record it really doesn't look good.
C# might still be popular and loved, but from the outside looking in it looks really, really insular.
You have already said you are a Java developer. So I rest my case.
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