support .net 4.0

[seayxu]

support .net 4.0

[Numpsy]

I think the CI failure is because the build scripts also need updating to use a newer dotnet sdk version, e.g.

• change appveyor.yml to use the Visual Studio 2019 image instead of Visual Studio 2015 and remove the step that installs the dotnet core 1.0 SDK
• either remove global.json or update it to use newer SDK version

?

[seayxu]

@Numpsy I fixed.

[nblumhardt]

Thanks for your PR. Unfortunately, it's not practical for us to add further support for obsolete framework versions - though it should be straightforward to maintain a private (or public) fork of the package that adds it. Happy to help if you need a hand to get it running.

[Numpsy]

I think the PR changes to use up to date build tools and to add a netstandard2.0 target are a useful change themselves, .net 4.0 aside?

[nblumhardt]

@Numpsy definitely! If anyone is keen to bundle up the modernization that would be most welcome 👍

[seayxu]

ok,just move .net 4.0 framework if you think it not need.