Tài liệu Programming the .NET Framework

Most modern programming languages include some form of runtime that
provides common services and access to the underlying operating systems
and hardware. Examples of this range from a simple functional library, such
as the ANSI C Runtime used by C and C++, to the rich object-oriented class
libraries provided by the Java Runtime Environment.
Similar to the way that Java programs depend on the Java class libraries and
virtual machine, C# programs depend on the services in the .NET Framework
such as the framework class library (FCL) and the Common Language
Runtime (CLR).
For a high-level overview of the FCL, see Chapter 4.
This chapter addresses the most common tasks you need to perform when
building C# programs. These topics generally fall into one of two categories:
leveraging functionality included in the FCL and interacting with elements
of the CLR.
Common Types
Certain types in the FCL are ubiquitous, in that they are fundamental to the
way the FCL and CLR work and provide common functionality used
throughout the entire FCL.
This section identifies some of the most common of these types and provides
guidelines to their usage. The types mentioned in this section all exist
in the System namespace.