System
The System namespace contains the fundamental types of the runtime, including the Object, Int32, and String types.
System.Collections
The System.Collections namespace contains classes for storing collections of objects, including lists, hastables, queues, and stacks.
System.Diagnostics
This namespace contains classes for interacting with system resources like event logs, processes, and performance counters.
System.DirectoryServices
System.IO
The System.IO namespace contains classes for reading and writing data from streams and interacting with the filesystem.
System.Reflection
Reflection is the ability for a program at run-time to get information about loaded type and assemblies. It can be used to access and invoke members and fields by their name, for instance.
This namespace contains classes for creating new assemblies and types at run-time by emitting IL instructions.
System.Text
The System.Text namespace contains types for converting text between encodings, such as ASCII and UTF-8. The namespace also contains the StringBuilder class which can be used to create and modify string values more efficiently than by concatenating String objects.
System.Threading
This namespace provides the core classes for multi-threaded programming, including the Thread class to launch threads as well as several classes for synchronization and multi-threaded resource management.
System.Xml
The System.Xml namespace contains classes for reading, writing, and manipulating Xml documents.
This namespace contains an implementation of Xml Stylesheet Transformations (XSLT).
This namespace contains classes for high-performance tree navigation and evaluation of XPath expressions on Xml documents.