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The Framework class library (FCL) is a comprehensive collection of reusable types including classes, interfaces and data types included in the .NET Framework to provide access to system functionality.
The .NET FCL forms the base on which applications, controls and components are built in .NET. It can be used for developing applications such as console applications, Windows GUI applications, ASP.NET applications, Windows and Web services, workflow-enabled applications, service oriented applications using Windows Communication, XML Web services, etc.
The reusable types of FCL provide a simple interface to developers due to:
FCL acts as a standard library, which can be used in a consistent manner by all the .NET languages and common language compliant (CLC-compliant) compilers.
The .NET FCL is the key component of .NET framework. It provides core functionalities of .NET architecture, which include:
FCL is designed to provide services similar to the Windows application programming interface (API), which was used before .NET was created. FCL has its code base as managed, object-oriented and easy to use, while Windows API is unmanaged, modular and cumbersome to use.
The .NET FCL is integrated with the Common Language Runtime (CLR) of the Framework, which manages the code execution. Its classes follow the object model as used by the Intermediate Language (IL) and are based on single inheritance. The classes and interfaces are grouped into namespaces so that they can be accessed easily.
Namespaces represent a hierarchy of the defined types formed by a logical group of related classes and interfaces, which can be used by any language targeting the .NET framework. They reside in assemblies, which are deployable units containing details about classes, interfaces and structures. The first part up to the last dot of the full name of a type indicates the namespace, while the last part specifies the type name. This way of using namespaces avoids a naming conflict, which can arise if two class names are same. While "System" is the root namespace for fundamental types in .NET framework, "Object" forms the root for all objects.
The classes and interfaces provide an option to use the functionality through implementation (in a concrete class considering it as a base) or only the signatures of methods defined in interface or abstract classes. When using Visual Studio for development of an application, the most common base classes are already referenced in the project, while the types not defined, such as user-defined types in a separate dynamic link library, have to be added explicitly so they can be used. The class servicing the needed functionality can be used in code by including an import directive for the namespace containing the class.
Microsoft has also provided guidelines necessary to be adopted for library development, which extend and interact with the .NET Framework. These guidelines cover naming types and members in class libraries, using static and abstract classes, interfaces, members of type, exceptions, etc. Improper usage of the FCL library can adversely affect developer productivity and discourage its usage.
FCL is similar to Java Foundation Classes. The main challenge in using FCL is to know the specific class that can provide the required functionality.