Base Class Library

The Standard Libraries are the standard libraries of the Common Language Infrastructure (CLI). The CLI includes the Standard Libraries in order to encapsulate a large number of common functions, such as file reading and writing, XML document manipulation, exception handling, application globalization, network communication, threading and reflection, which makes the programmer's job easier. It is much larger in scope than standard libraries for most other languages, including C++, and is comparable in scope and coverage to the standard libraries of Java. The Framework Class Library (FCL) is the origin of the Standard Libraries as the .NET Framework, which includes the FCL, is the first implementation of the CLI.

The main standard libraries are organized into two Standard Profiles, the Kernel Profile and the Compact Profile. The following standard libraries do not belong to any profile: the Extended Array Library, the Extended Numerics Library, the Parallel Library and the Vararg Library.

The Kernel Profile is a subset of the Compact Profile. The Kernel Profile contains the Base Class Library (BCL) and Runtime Infrastructure Library.

The Compact Profile is a superset of the Kernel Profile. The Compact Profile contains the Network Library, the Reflection Library and the XML Library.

The Base Class Library is a simple runtime library for modern programming languages. It serves as the Standard for the runtime library for the language C# as well as one of the CLI Standard Libraries. It provides types to represent the built-in data types of the CLI, simple file access, custom attributes, security attributes, string manipulation, formatting, streams, collections, among other things. It defines types in the following namespaces:

The Runtime Infrastructure Library provides the services needed by a compiler to target the CLI and the facilities needed to dynamically load types from a stream in a specified file format. It defines types in the following namespaces:

The Network Library provides simple networking services including direct access to network ports as well as HTTP support. It defines types in the following namespaces:

...
Wikipedia