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While testing https://bugzilla.xamarin.com/show_bug.cgi?id=58504 I attempted to reproduce _without_ .NET core installed.
I did not know I needed it, nor did the Xamarin stable installers install it.
I received for my trouble a bunch of errors like this:
/Users/donblas/Library/Caches/VisualStudio/7.0/MSBuild/78994_1/Sdks/Microsoft.NET.Sdk/build/Microsoft.NET.Sdk.Common.targets(5,5): Error: Project '/Users/donblas/Projects/MySpecialConsoleProject/NetStandardLib/NetStandardLib.csproj' targets '.NETStandard,Version=v2.0'. It cannot be referenced by a project that targets '.NETFramework,Version=v4.6.1'. (MySpecialConsoleProject)
which do not help me understand that I need to install a secondary item.
Once installing that, the things that should work started working.
Probably what should happen here is the same as what happens if you open a .NET Core 2.0 project when .NET Core 2.0 SDK is not installed. That will show a dialog saying you need to install the .NET Core 2.0 SDK and it will show an error for the project with a tooltip saying the same thing. So we just need to extend that logic to check for .NET Standard 2.0 and show the same dialog.
.NET Standard 1.x projects are supported by MSBuild that ships with Mono so there is no prompt for .NET Standard projects originally. I added support for the dialog for .NET Core 2.0 projects but not for .NET Standard 2.0 projects.
I will see if I can get this included in 15.4 otherwise it will have to be 15.5.
So what is discussed in Comment 1 is already implemented in VS Mac 220.127.116.117 but not in VS Mac 18.104.22.1684 (stable channel). You see a dialog saying that .NET Core 2.0 is not installed and an error icon next to the project in the Solution window.
VS Mac 7.1 has been closed to new features/bug fixes for a while so the only change made to VS Mac 7.1 recently was to allow support for .NET Core 2.0 final.