The CSHTML5 namespace contains classes that are specific to C#/XAML for HTML5, such as the 'Interop' class for executing JavaScript code from within C#.

The System namespace contains fundamental classes and base classes that define commonly-used value and reference data types, events and event handlers, interfaces, attributes, and processing exceptions.

The System.Collections namespaces contain types that define various standard, specialized, and generic collection objects.

The System.Collections.Generic namespace contains interfaces and classes that define generic collections, which allow users to create strongly typed collections that provide better type safety and performance than non-generic strongly typed collections.

The System.Collections.ObjectModel namespace contains classes that can be used as collections in the object model of a reusable library. Use these classes when properties or methods return collections.

The System.Collections.Specialized namespace contains specialized and strongly-typed collections; for example, a linked list dictionary, a bit vector, and collections that contain only strings.

The System.ComponentModel namespaces contain types that implement the run-time and design-time behavior of components and controls. Child namespaces support the Managed Extensibility Framework (MEF), provide attribute classes that define metadata for ASP.NET Dynamic Data controls, and contain types that let you define the design-time behavior of components and their user interfaces.

The System.Diagnostics namespaces contain types that enable you to interact with system processes, event logs, and performance counters. Child namespaces contain types to interact with code analysis tools, to support contracts, to extend design-time support for application monitoring and instrumentation, to log event data using the Event Tracing for Windows (ETW) tracing subsystem, to read to and write from event logs and collect performance data, and to read and write debug symbol information.

The System.Globalization namespace contains classes that define culture-related information, including language, country/region, calendars in use, format patterns for dates, currency, and numbers, and sort order for strings. These classes are useful for writing globalized (internationalized) applications. Classes such as StringInfo and TextInfo provide advanced globalization functionalities, including surrogate support and text element processing.

The System.IO namespaces contain types that support input and output, including the ability to read and write data to streams either synchronously or asynchronously, to compress data in streams, to create and use isolated stores, to map files to an application's logical address space, to store multiple data objects in a single container, to communicate using anonymous or named pipes, to implement custom logging, and to handle the flow of data to and from serial ports.

The System.Net namespaces contain classes that provide a simple programming interface for a number of network protocols, programmatically access and update configuration settings for the System.Net namespaces, define cache policies for web resources, compose and send e-mail, represent Multipurpose Internet Mail Exchange (MIME) headers, access network traffic data and network address information, and access peer-to-peer networking functionality. Additional child namespaces provide a managed implementation of the Windows Sockets (Winsock) interface and provide access to network streams for secure communications between hosts.

The System.Net.Security namespace provides network streams for secure communications between hosts.

The System.Reflection namespaces contain types that provide a managed view of loaded types, methods, and fields, and that can dynamically create and invoke types. A child namespace contains types that enable a compiler or other tool to emit metadata and Microsoft intermediate language (MSIL).

The System.Resources namespaces contain types that enable developers to create, store, and manage an application's culture-specific resources.

The System.Runtime.CompilerServices namespace provides functionality for compiler writers who use managed code to specify attributes in metadata that affect the run-time behavior of the common language runtime.

The System.Runtime.InteropServices namespace provides a wide variety of members that support COM interop and platform invoke services. If you are unfamiliar with these services, see Interoperating with Unmanaged Code.

The System.Runtime.Serialization namespace contains classes that can be used for serializing and deserializing objects. Serialization is the process of converting an object or a graph of objects into a linear sequence of bytes for either storage or transmission to another location. Deserialization is the process of taking in stored information and recreating objects from it.

The System.ServiceModel namespaces contain the types necessary to build Windows Communication Foundation (WCF) service and client applications.

The System.ServiceModel.Channels provides classes related to service model channels.

The System.Text namespaces contain types for character encoding and string manipulation. A child namespace enables you to process text using regular expressions.

The System.Threading namespaces contain types that enable multithreaded programming. A child namespace provides types that simplify the work of writing concurrent and asynchronous code.

The System.Windows namespaces contain types used in Windows Presentation Foundation (WPF) applications, including animation clients, user interface controls, data binding, and type conversion. System.Windows.Forms and its child namespaces are used for developing Windows Forms applications.

The System.Windows.Markup namespace provides types to support XAML. Some of these types are located in WPF assemblies and are specific to WPF scenarios that involve XAML. Other types in this namespace provide support for .NET Framework XAML Services in general, and do not require referencing WPF assemblies.

The System.Xml namespaces contain types for processing XML. Child namespaces support serialization of XML documents or streams, XSD schemas, XQuery 1.0 and XPath 2.0, and LINQ to XML, which is an in-memory XML programming interface that enables easy modification of XML documents.

The System.Xml.Schema namespace contains the XML classes that provide standards-based support for XML schema definition language (XSD) schemas.

The System.Xml.Serialization namespace contains classes that are used to serialize objects into XML format documents or streams.

The Windows.ApplicationModel.Activation namespace enables an app to handle the activation parameters for the activation contracts, appointments, and extensions supported by Windows.

The Windows.Devices.Input namespace provides support for identifying the input devices available (pointer, touch, mouse, and keyboard) and retrieving information about those devices.

The Windows.Foundation namespace enables fundamental Windows Runtime functionality, including managing asynchronous operations and accessing property stores. This namespace also defines common value types that represent Uniform Resource Identifiers (URIs), dates and times, 2-D measurements, and other basic values.

The Windows.UI namespace provides an app with access to core system functionality and run-time information about its UI.

The Windows.UI.Core namespace provides the basic user interface window components, including core input events and event dispatch.

The Windows.UI.Input namespace provides support for the Windows input system.

The Windows.UI.Text namespace provides support for working with textual content.

The Windows.UI.Xaml namespace provides general framework APIs and application model APIs, and a variety of support classes that are commonly used by many different feature areas.

The Windows.UI.Xaml.Controls namespace provides UI controls and classes that support existing and custom controls.

The Windows.UI.Xaml.Controls.Primitives namespace defines classes that represent the component parts of UI controls, or otherwise support the control composition model. Also defines interfaces for control patterns such as snapping and selection.

The Windows.UI.Xaml.Data namespace defines the data binding infrastructure for apps.

The Windows.UI.Xaml.Input namespace defines the input and input event infrastructure for apps and UI elements.

The Windows.UI.Xaml.Markup namespace provides XAML language support API for Windows Store apps and the Windows Runtime.

The Windows.UI.Xaml.Media namespace provides basic media support, graphics primitives, and brush-drawing APIs.

The Windows.UI.Xaml.Media.Imaging namespace provides types related to imaging and obtaining resources and source files for bitmap images.

The Windows.UI.Xaml.Navigation namespace provides types that support navigation events as initiated by the Page and Frame classes.

The Windows.UI.Xaml.Shapes namespace defines basic shapes that are intended for decorative rendering or for compositing non-interactive parts of controls.