TheFramework Class Library (FCL) is a component of Microsoft's.NET Framework, the first implementation of theCommon Language Infrastructure (CLI). In much the same way asCommon Language Runtime (CLR) implements the CLIVirtual Execution System (VES), the FCL implements the CLI foundationalStandard Libraries. As a CLI foundational class libraries implementation, it is a collection of reusableclasses,interfaces, andvalue types, and includes an implementation of the CLIBase Class Library (BCL).^[1]

With Microsoft's move to.NET Core, the CLI foundational class libraries implementation is known asCoreFX instead of Framework Class Library.^[2]

The Framework Class Library is organized into a hierarchy of namespaces, with the most commonly used functionality found in theSystem.* andMicrosoft.* namespaces. It provides a wide range of reusable types, including collections, input/output, base data types, networking, andXML processing, which support the development of desktop, web, and server applications across the.NET platform.^[3]

• Standard Libraries (CLI)
• Base Class Library (BCL)

Difference Between .NET Core and .NET Framework: Head to Head Comparison

• .NET Framework Developer Center. Microsoft Developer Network (MSDN).
• .NET Framework 3.5 namespaces

• .NET Framework terminology
• Microsoft application programming interfaces

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