.NET Core projects can reference Portable Class Libraries if one of the .NET Standard compliant profiles is used (e.g. the profile 259).
However, after installing a PCL NuGet package in a .NET Core projects, you may end up with the following compiler error:
The type '???' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=188.8.131.52, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
To fix this, just install the following PCL compatibility NuGet in your project:
The package provides bindings for the missing “mscorlib” so that the project can be compiled without problems.