The Base Class Library (BCL), part of the Framework Class Library (FCL), is a library of functionality available to all languages using the .NET Framework. The BCL provides classes which encapsulate a number of common functions, including file reading and writing, graphic rendering, database interaction and XML document manipulation.

Some of the Base class libraries are as follows:

1. System
2. System.Collections
3. System.Diagnostics
4. System.Globalization
5. System.IO
6. System.Net
7. System.Reflection
8. System.Security
9. System.Text
10. System.Threading
11. System.Xml