dotnet/corefx

CoreFX was the library layer of .NET Core, Microsoft's open-source, cross-platform version of the .NET framework. This repository contained the standard library implementations that developers use when building .NET Core applications, including foundational components like System.Collections (data structures), System.IO (file and stream handling), and System.Xml (XML parsing), among many others. .NET Core itself is a runtime environment: when you write a C# or F# program and run it.NET Core is what executes it. CoreFX was the companion repo containing the class libraries, the pre-built tools and utilities that make up the "standard library" every .NET developer can call on. The actual runtime engine lived in a separate companion repository called CoreCLR. This repository is now archived. According to the description, it is only used for servicing maintenance updates for older versions (.NET Core 2.1 and 3.1). All active development has moved to the unified github.com/dotnet/runtime repository, which merged CoreFX and CoreCLR together. If you are looking to work with or contribute to .NET, that is the current home. The code is licensed under the MIT license and was maintained by Microsoft and open-source contributors as part of the .NET Foundation.