C# (.NET) library. GitHub
A comprehensive, easy to use and powerful .NET configuration library, fully covered with unit tests and tested in the wild on thousands of servers and applications.
This library eliminates the problem of having configuration in different places, having to convert types between different providers, hardcoding configuration keys accross the solution, depending on specific configuration source implementation. It’s doing that by exposing an abstract configuration interface and providing most common implementation for configuration sources like app.config, environment variables etc.
The library is still going strong today (year 2020).