Xamarin.Mac.Sdk bundling the wrong assemblies

This solution contains two projects:

• WitnessMe, a traditional Xamarin.Mac project that is only notable in that it explicitly references .NET 4.6.1.
• SurpriseWitness, a Xamarin.Mac.Sdk project that is otherwise meant to be identical to WitnessMe.

Building and Running

Test the sample like so:

msbuild /r /bl &&  find . -name System.Memory.dll | xargs md5

Notice that SurpriseWitness bundles a different version of System.Memory.dll than WitnessMe. Your output will look something like this:

MD5 (./WitnessMe/bin/Debug/WitnessMe.app/Contents/MonoBundle/System.Memory.dll) = 89ec6e101de3a70ed140c62c2980f24e
MD5 (./WitnessMe/bin/Debug/System.Memory.dll) = 89ec6e101de3a70ed140c62c2980f24e
MD5 (./SurpriseWitness/bin/Debug/net472/SurpriseWitness.app/Contents/MonoBundle/System.Memory.dll) = e1ce1dc0f9f08765f3ea59a24d0d905e
MD5 (./SurpriseWitness/bin/Debug/net472/System.Memory.dll) = 89ec6e101de3a70ed140c62c2980f24e

SurpriseWitness is bundling the ref assembly of System.Memory.dll. You can verify like so:

find ~/.nuget/packages/system.memory/4.5.1 -name System.Memory.dll | xargs md5

MD5 (/Users/sandy/.nuget/packages/system.memory/4.5.1/ref/netstandard1.1/System.Memory.dll) = 8e35651c648c424af199e6dc3ec71676
MD5 (/Users/sandy/.nuget/packages/system.memory/4.5.1/ref/netstandard2.0/System.Memory.dll) = e1ce1dc0f9f08765f3ea59a24d0d905e
MD5 (/Users/sandy/.nuget/packages/system.memory/4.5.1/lib/netstandard1.1/System.Memory.dll) = a8771302c97226729d2b3c0b849c2969
MD5 (/Users/sandy/.nuget/packages/system.memory/4.5.1/lib/netstandard2.0/System.Memory.dll) = 89ec6e101de3a70ed140c62c2980f24e

What's Going On?

Read https://github.com/xamarin/xamarin-macios/blob/d7c2a45ca9351106f83fcf7f08c1b09a8a5fb563/msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.targets#L504-L508, then look at your log.

It seems that the _CompileToNative target depends on values (like _ReferencesFromNuGetPackages) being set from earlier NuGet targets (like ResolveNuGetPackageAssetes). Those targets run for WitnessMe but not SurpriseWitness, presumably because Xamarin.Mac.Sdk inherits behavior from Microsoft.NET.Sdk that differs from the old NuGet targets.

_ReferencesFromNuGetPackages contains ref assemblies, apparently. _CompileToNative removes those from the list of references sent to MMP...but in our case that does not happen, so the ref version of System.Memory.dll remains in the list, and gets chosen instead of the lib version (which is also in the list).

The result is an app bundle with the wrong assemblies, and runtime issues, as seen in this sample (launch SurpriseWitness.app and it will crash).

This same problem exists for other packages, such as System.Threading.Tasks.Extensions.