Add System.Json

[stephentoub]

System.Json.dll is part of Silverlight and doesn't ship in the .NET Framework, nor do we encourage its usage. But, it's available and used with Mono, and in order to enable compatibility between Mono and .NET Core, we're adding it as an available assembly.

This PR ports the Mono implementation over to corefx. This includes porting the tests, adding packaging, cleaning up the source, etc. I've not done any performance work, increased code coverage of the tests, etc. This is mainly about making the APIs available. We can tweak is subsequently as is necessary.

cc: @danmosemsft, @KrzysztofCwalina, @ericstj, @ianhays

[ericstj]

What actually uses handles?

[stephentoub]

Nothing. I copied this from another assembly and never trimmed it. I can probably get rid of most of these.

[ericstj]

Targeting netstandard1.0 with the following dependencies worked for me:

    "netstandard1.0": {
      "dependencies": {
        "System.Collections": "4.0.11",
        "System.IO": "4.1.0",
        "System.Linq": "4.1.0",
        "System.Runtime": "4.1.0",
        "System.Runtime.Extensions": "4.1.0",
        "System.Resources.ResourceManager": "4.0.1"
      }
    }

[ericstj]

See ericstj@9a1cf68 & ericstj@2400019. If you make me a collaborator I'll push to your branch.

[ericstj]

@mhutch, would like your opinion on ericstj@2400019. This has us deferring to the inbox implementation of System.Json.dll on xamarin profiles. Otherwise we could give Xamarin profiles the implementation from the package itself.

[ericstj]

Should this expose more interfaces like IReadOnly*, same q for all types.

[stephentoub]

We could consider it, but my understanding is we're only really adding this assembly for compatibility with Mono (it doesn't exist on desktop), and that we'd suggest developers write new JSON-related code using Json.NET and/or https://github.com/dotnet/corefxlab/tree/master/src/System.Text.Json depending on how that experiment turns out, rather than writing new code using this library. So I'm not sure how much effort we want to expend on adding APIs, interface implementations, etc. We could certainly look into it, though, but I'd like that to be a separate effort it if happens.

[danmoseley]

yay for a new assembly! thank you

[danmoseley]

[ericstj]

Yeah, LGTM too. For the question about adding more interfaces I think that could be addressed separately.

[stephentoub]

Thanks, guys, and Eric, thank you very much for helping to fix the packaging.

[mhutch]

I would definitely prefer to defer to our implementation on Mono-based frameworks in order to fuse the types (e.g. what happens if someone gets a System.Json object from a netstandard library and passes it to an Xamarin.iOS library) and to shrink our deployed app size by ensuring we don't deploy multiple implementations.

[ericstj]

I think this version needs to match the version xamarin has inbox (2.0.5.0). We should test that.