What is System.Private.CoreLib?

[colejohnson66]

What is the relationship of System.Private.CoreLib and mscorlib? Is it the replacement? If so, why the name change?

[jkotas]

The implementation assemblies were refactored as part of the .NET Framework to .NET Core transition. For example: In .NET Framework, the implementations of cryptography related types are spread over multiple assemblies. X509Certificate lives in mscorlib.dll and X509Certificate2 lives in System.dll that does not make sense and introduces implementation difficulties. In .NET Core, the implementation of both types lives in System.Security.Cryptography.

At the same time, .NET Core wanted to allow running .NET Framework binaries using type forwarders. .NET Core includes mscorlib.dll, System.dll, System.Core.dll, etc. These compatibility shims do not contain any code. They just forward the type from where it lives in .NET Framework to where it lives in .NET Core.

It explains why we could not just use mscorlib as the core assembly name in .NET Core and needed an assembly with a different name. We choose System.Private.CoreLib for the implementation assembly and System.Runtime for the reference assembly. The different names for implementation assembly and reference assembly were not strictly required in the hindsight, it is more an artifact of how the system evolved.