Where can I find the actual code for the classes that are built into .NET framework?

The source code for the .NET Framework is available on GitHub, and you can browse and download it from the official GitHub repository. Microsoft has open-sourced a significant portion of the .NET Framework code, and it is available under the MIT License.

The official GitHub repository for the .NET runtime and class libraries is located at:

https://github.com/dotnet/runtime

Within this repository, you can find the source code for various components, including the runtime itself and the base class libraries. The source code is organized into different directories, each corresponding to a specific area or component of the .NET Framework.

If you are interested in exploring the source code for a particular class or namespace, you can navigate through the repository's directory structure to find the relevant files. The src directory contains the source code for the runtime, and the src/libraries directory contains the source code for the class libraries.

Keep in mind that not all parts of the .NET Framework are open-sourced. Some components, such as Windows-specific libraries and certain APIs, are not included in the open-source release.

Here's a step-by-step guide to exploring the .NET runtime and class libraries on GitHub:

1. Visit the official .NET runtime GitHub repository: https://github.com/dotnet/runtime.

2. Navigate through the repository's directories to find the source code for the specific area or component you are interested in.

3. Use the search functionality on GitHub to find specific classes, namespaces, or keywords within the repository.

4. Browse the source code, view changes, and explore the commit history.

Please note that exploring large codebases like the .NET Framework source code can be complex, so having a good understanding of the structure of the framework and using tools like Visual Studio Code or Visual Studio with appropriate extensions can enhance the browsing experience.