You would need to target multiple frameworks in the csproj file. In the original launch of Visual Studio 2017 there's no UI for this, but you can do it manually. I believe that there will be UI support for this in an update.
It's just a matter of changing the <TargetFramework>
element to <TargetFrameworks>
and using a semi-colon-separated list of targets. For example, in Noda Time I have:
<TargetFrameworks>net45;netstandard1.3</TargetFrameworks>
You could have:
<TargetFrameworks>netcoreapp1.0;netstandard1.3</TargetFrameworks>
However, you'd only want to do this if you wanted to take advantage (conditionally) of some features that are only available in .NET Core, and not in .NET Standard.
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