.NET FCL is the foundation of classes, interfaces, value types, services and providers that are used to construct .NET Framework desktop and Web-based (i.e., ASP.NET) applications. The fundamental elements of the FCL are defined as classes located in the System namespace. All of the most primitive aspects of .NET are stored in System, including built-in value types, the Object type, and support for exception handling and garbage collection. Thousands of more classes are located in second- and third-level namespaces that include support for network and file I/O, graphics, security, configuration management, and Web services. All CLS-compliant compilers can use the FCL.

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