referencesource

The Reference Source is a publicly available collection of source code for software components developed by Microsoft. It is primarily associated with the .NET Framework and .NET Core, providing developers with access to the original implementation of classes, libraries, and runtime components. By exposing the full source code, the Reference Source enables developers to understand internal behavior, examine bug fixes, and learn best practices implemented by Microsoft.

The collection is hosted on GitHub under the Microsoft/referencesource repository. It contains a subset of the entire source tree, focusing on stable, documented APIs rather than experimental or unsupported portions. While the code is not intended for building or distributing applications, it can be referenced for educational and debugging purposes. The repository includes multiple subdirectories, each correlating to a specific .NET assembly such as System, Microsoft.CSharp, System.Data, and platform-specific extensions. The source files are annotated with XML comments and are accompanied by a license that permits viewing and reading but not redistribution of compiled binaries.

Microsoft introduced the Reference Source in 2006 to increase transparency of the .NET Framework. Since then, it has expanded to cover newer releases, including .NET Core and .NET 5+ through the "dotnet/runtime" and "dotnet/standard" projects. The Reference Source is built into Visual Studio’s “Go to Definition” and “Navigate” features, allowing developers to jump directly to the referenced source when inspecting third‑party libraries without having to acquire separate source packages.

The reference source is not a full replacement for official documentation; it complements API references and guidebooks by providing concrete implementation details. It has proven valuable for forensic analysis of library behavior, compliance audits, and as a learning tool for experienced developers wanting insight into how Microsoft implements complex runtime features.