[mtouch] Support netstandard library nuget package with reference assembly

[VincentDondain]

See test case: https://www.dropbox.com/s/btb6rzb636pnutf/Issue3199Test2.zip?dl=0

In this test case the iOS project references a netstandard class library, which references a Nuget Package (System.Runtime.CompilerServices.Unsafe) but the app does NOT reference that package directly.

In this specific case mtouch will fail with MTOUCH : error MT2101: Can't resolve the reference '!!1& System.Runtime.CompilerServices.Unsafe::As(!!0&)', referenced from the method 'System.Void LibWithPackage.Class1::Test()' in 'System.Runtime.CompilerServices.Unsafe, Version=4.0.3.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. as it doesn't have a reference to System.Runtime.CompilerServices.Unsafe.dll.

Full build log: https://gist.github.com/VincentDondain/3a067c97e7f034a2a5c542dc6d3d7072

Env: https://gist.github.com/VincentDondain/20ff66a750277c383d75d518a925ed82

[spouliot]

note: logs are truncated when clicking on the link (GitHub ui size restriction). Click on RAW to see the complete version, including the invocation of mtouch

mtouch is not given a reference to System.Runtime.CompilerServices.Unsafe.dll

[spouliot]

    Dependency "System.Runtime.CompilerServices.Unsafe, Version=4.0.3.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
        Could not resolve this reference. Could not locate the assembly "System.Runtime.CompilerServices.Unsafe, Version=4.0.3.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.

is not looking into the nuget directories so it fails to find

/Users/vidondai/.nuget/packages/system.runtime.compilerservices.unsafe/4.4.0/ref/netstandard2.0/System.Runtime.CompilerServices.Unsafe.dll

[pjcollins]

I believe that a related discussion has been happening in the following PRs, and may be worth consulting:

• [Xamarin.Android.Build.Tasks] Better support for netstandard libraries. android#1356
• [Xamarin.Android.Build.Tasks] Rework how we resolve netstandard Nugets android#1459

[VincentDondain]

Yup, thanks @pjcollins.

I'm well aware of these android changes but I'm investigating to see if a simpler solution can be found.

For instance #3791 is much simpler than what XA is doing (scanning all the dlls to check if they are reference assemblies).

[pjcollins]

@VincentDondain perfect, happy to hear that you're aware of the work going on in XA. I think I should also cc @dellis1972 on here to continue to promote cross team collaboration on this problem 🙂. Ideally we could land on a similar approach across products, assuming other limitations would not prevent that.

[VincentDondain]

I slacked with Dean quite a bit too :P
Cross team collaboration at max power!

[VincentDondain]

Msbuild on windows seems to be handling packages with reference assemblies properly. See #4009 where we're investigating the differences between msbuild on mac and windows.

I think we should wait before introduce further "fixes/hacks" before we have a better understand of msbuild on windows and why it works properly.