The following are the common namespaces in CSharp. System Contains the definition of fundamental types,
conversion between types, mathematics, program
invocation, and environment management. System.Collections Includes types for working with collections of
objects. Collections can generally follow either
list or dictionary type storage mechanisms. System.Collections.Generics This C# 2.0 added namespace works with
strongly typed collections that depend on
generics (type parameters). 
System.Data Contains types used for working with data that
is stored within a database. 
System.Drawing Contains types for drawing to the display
device and working with images. System.IO Contains types for working with files and directories
and provides capabilities for manipulating,
loading, and saving files. 
System.Linq Provides classes and interfaces for querying
data in collections using a C# 3.0 added API,
Language-Integrated Query. System.Text Includes types for working with strings and various
text encodings, and for converting between
those encodings. This namespace includes a
subnamespace called System.Text.Regular-
Expressions, which provides access to regular-
expression-related APIs. 
System.Threading Handles thread manipulation and multithreaded
programming. System.Windows.Forms Includes types for creating rich user interfaces
and the components within them.
System.Xml Contains standards-based support for XML
processing