There was a conflict between "mscorlib, Version = 4.0.0.0" and "mscorlib, Version = 2.0.5.0",

While playing with ASP.Net Core, I tried to create my own custom code library and got the following messages:

Unable to resolve the conflict between "mscorlib, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = b77a5c561934e089" and "mscorlib, Version = 2.0.5.0, Culture = neutral, PublicKeyToken = 7cec85d7bea7798e, Retargetable = Yes". Choosing "mscorlib, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = b77a5c561934e089" is arbitrary.

Unable to resolve conflict between "System, Version = 4.0.0.0", Culture = neutral, PublicKeyToken = b77a5c561934e089 "and" System, Version = 2.0.5.0, Culture = neutral, PublicKeyToken = 7cec85d7bea7798e, Retargetable = Yes ". Selecting" System, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = b77a5c561934e089 "is arbitrary.

Q: Is it possible for the two versions to "play well" together?
Q: Can I even try?

NOTES:
running has update-package -reinstall

removed all references to Entity Framework, Unity, etc ... so this is not a "fix".