Add netstandard2.0 Compile Target once RTM

[caleblloyd]

Explore new APIs in NetStandard.Library: 2.0-*

If all of our conditional compile APIs are now supported, this will allow us to target only NetStandard: 1.3 in the library code and drop specific targeting for the full framework. We should still test against both the full framework and .NET Core so that we can detect platform-specific anomalies.

Code changes should happen in a branch that gets merged to master once .NET Core/Standard 2.0 RTM is released.

[caleblloyd]

I think that my previous understanding of NetStandard.Library was incorrect. NetStandard.Library: 2.0 does not polyfill older versions of netstandard1.x to support new APIs.

NetStandard.Library is really just a metapackage that links to other packages, as explained in this blog post

That means that in order to make advantage of the new APIs in netstandard2.0, we will have to build a package for netstandard2.0. netstandard2.0 targets:

• .NET Core >= 2.0
• .NET >= 4.6.1

Therefore in order to maintain backwards compatibility we would still need to build:

• net451 for .NET [4.5.1 - 4.6.1) support
• netstandard1.3 for .NET Core [1.0.0, 2.0.0) support

All we should need to do when .NET Standard 2.0 RTM lands is add netstandard2.0 as a target. All of our conditional compile flags for netstandard1.3 will stay as-is.

Since .NET Core 2.0 is supported on all of the same platforms as .NET Core 1.0, it should be reasonable to expect developers to upgrade rather quickly. Support for netstandard1.3 in future development could be phased out maybe a year or so after 2.0 RTM.

Once Windows 8.1 and Windows Server 2012 R2 are no longer supported we could drop support for net451 in future development releases, but that likely won't be for a couple years or more.

There's really not all that many conditional compilation flags in the codebase now, and most are for netstandard1.3. The difficulty will be keeping up with testing against different versions of the framework.

[caleblloyd]

Regarding end of life dates for Microsoft products:

• Windows Server 2012 End of Mainstream Support is currently 10/9/2018
• .NET Core 1.0 and 1.1 End of Support will be 1 year after .NET Core 2.0 is released

Since .NET Core 2.0 will be released Q3 2017, these dates will coincidentally be very close. Pending the needs of one of this project's stakeholders, we could likely switch to only targeting netstandard2.0 in October 2018.

We would need to publish a major version (1.0) by then, and once we made the change would need to increment another major version (2.0).

[caleblloyd]

NET Standard 2.0 is now finalized and the announcement of .NET Core 2.0 will be on September 18-19.

[yukozh]

@caleblloyd The 2.0.0 rtm libraries are ready in nuget