Hello everybody,
I'm facing a strange problem regarding Xamarin.iOS (in Xamarin.Forms projects).
Since 2 days, I'm unable to build Xamarin.iOS project with a strange error regarding "netstandard" :
MTOUCH : warning MT0136: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' referenced from '/Users//.nuget/packages/xamarin.forms/4.0.0.425677/lib/Xamarin.iOS10/Xamarin.Forms.Core.dll'.
MTOUCH : warning MT0137: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51', referenced by a Xamarin.Forms.Xaml.TypeConversionAttribute attribute in 'Xamarin.Forms.Core.dll'.
MTOUCH : warning MT0137: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51', referenced by a Xamarin.Forms.Xaml.TypeConversionAttribute attribute in 'Xamarin.Forms.Core.dll'.
MTOUCH : warning MT0137: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51', referenced by a Xamarin.Forms.Xaml.TypeConversionAttribute attribute in 'Xamarin.Forms.Core.dll'.
MTOUCH : warning MT0137: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51', referenced by a Xamarin.Forms.Xaml.TypeConversionAttribute attribute in 'Xamarin.Forms.Core.dll'.
MTOUCH : warning MT0136: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' referenced from '/Users/Shared/Dev/Xamarin/Tests/EmptyXamarinFormsProject/EmptyXamarinFormsProject/bin/Debug/netstandard2.0/EmptyXamarinFormsProject.dll'.
MTOUCH : warning MT0136: Cannot find the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' referenced from '/Users//.nuget/packages/xamarin.forms/4.0.0.425677/lib/Xamarin.iOS10/Xamarin.Forms.Xaml.dll'.
MTOUCH : warning MT5215: References to 'gdi32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'kernel32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'comdlg32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'winspool' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'user32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'shell32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'oleaut32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'gdiplus' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'gdi32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'user32' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'libX11' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'libcups' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'gdiplus' might require additional -framework=XXX or -lXXX instructions to the native linker
MTOUCH : warning MT5215: References to 'kernel32' might require additional -framework=XXX or -lXXX instructions to the native linker
clang : error : linker command failed with exit code 1 (use -v to see invocation)
error MT5209 : Native linking error : framework not found Carbon
I faced this issue in an existing project so I tried to downgrade Xamarin.Forms (I worked with first version of Xamarin 4, and 3.6 also) but none of those downgrade helped me 
I created a fresh new project that immediately having this issue.
I also downloaded "dotnet-sdk-2.2.300-osx-x64.pkg" but still cannot build.
Does someone faced this issue recently ?
I cannot build any of my project and some of them are currently in production with bug fixes in the pipe.
Any solution ?
Best regards,
Cyril