When searching for the implementation of a specific .NET API, the apisof.net has proven to be an invaluable resource.

Want to know where IEnumerator is defined?

Here is an excerpt the reply:

• .NET Core 1.0 → System.Runtime, Version=4.1.0.0, PublicKeyToken=b03f5f7f11d50a3a
• .NET Framework 4.5 → mscorlib, Version=4.0.0.0, PublicKeyToken=b77a5c561934e089
• .NET Standard 1.3 → System.Runtime, Version=4.0.20.0, PublicKeyToken=b03f5f7f11d50a3a

This is really useful information when trying to make various projects based on different technologies work together with the help of app.config binding redirection.