Build our own packages using FrameworkReference

[natemcmaster]

Depends on NuGet/Home#7342.
Follow up to #4178

The packages we ship which reference the shared framework should be built using FrameworkReference. Once NuGet/Home#7342 is resolved, the metadata for this item will be preserved in the nuspec, which should ensure consumers get all the necessary runtime settings and compiler references.

Workaround
Consumers using our AspNetCore packages, such as Microsoft.AspNetCore.NodeServices, need to make sure they also have <FrameworkReference Include="Microsoft.AspNetCore.App" />. This will be implicitly added with Microsoft.NET.Sdk.Web, but not other MSBuild SDKs.

Changes required

1. We need to change our own packages to build with FrameworkReference. (Related work Convert projects in this repo to use ProjectReference #4246)
2. Update our SDK in BuildTools to support using FrameworkReference

[natemcmaster]

Prototype in progress: https://github.com/aspnet/AspNetCore/compare/natemcmaster:local-fx

Current challenges:

• I get compiler errors due to duplicate APIs in the same namespace. I think this is the result of having InternalsVisibleTo and duplicate copies of API due to shared source.
• Visual Studio go-to-definition is broken when navigating from something outside the shared framework into a shared framework assembly.
• Working around https://github.com/dotnet/core-setup/issues/4809 - the only way to make tests run against the shared framework is to install the shared framework into DOTNET_ROOT. My workaround is to make DOTNET_ROOT = "$repoRoot/.dotnet/$(arch)/".
• Requires VS 2019 and a Preview 2 SDK (blocked on Make Visual Studio 2019 a prerequisite to building this repo #7005).

cc @pakrym @ryanbrandenburg

[natemcmaster]

@Pilchie I think this is still worth addressing, but this is a challenging issue to address. The challenges I described in the previous comment are still unresolved. We would probably need help from the teams which own VS, the runtime host, and the SDK to completely resolve this. It's extremely unlikely this will land in 3.0. 3.1 or 5.0 is a more appropriate timeframe for this.

What we have now in the code currently is a workaround for this. There is a post-build step which manually alters the metadata in .nuspec file in NuGet packages to remove incorrect <dependency> items and add a <frameworkReference>. IMO it would be ideal if dotnet pack produced the right nuspec in the first place so this post-build hack wasn't necessary.

The other major workaround we have is that test projects don't actually run against the shared framework bits that ship to customers. The host doesn't provide a way to easily swap in a new shared framework, so test projects run on ASP.NET Core binaries as if they are simple class libraries projects. This means test code isn't actually executing the same crossgened binaries that ship to customers.

[JunTaoLuo]

@dotnet/aspnet-build Let's chat about this once everyone is back in the office and see if there is any work we need to do here. I suspect some of the details mentioned here is out of date.

[dougbu]

Seems minor and won't affect anyone using our packages. The cleanup task run after every build runs reliably and does the needful.

About the only external improvement coming from building against framework references would be cleanup of the very few .deps.json files we ship. But, this comes with the major downside of basically requiring two builds to get the tests using the just-built ASP.NET Core assemblies.

[Pilchie]

I think I agree that it's probably ok to let this go.

cleanup of the very few .deps.json files we ship

@dougbu could you give me an example of the before and after here?