Rico Suter's blog.

.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=, 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.