.NET contains a lot of namespaces and many more, if you include the third-party libraries. However, there are a few that you will use over and over. Here are the twenty that will get you through 80% of the common, recurring programming problems.
Contains the most fundamental types. These include commonly used classes, structures, enums, events, interfaces, etc.
Contains classes that represent ASCII and Unicode character encodings. Classes for converting blocks of characters to and from blocks of bytes.
Provides regular expression functionality.
Provides classes and interfaces that support queries that use Language-Integrated Query (LINQ).
Contains the classes for LINQ to XML. LINQ to XML is an in-memory XML programming interface that enables you to modify XML documents efficiently and easily.
Provides support for processing XML.
Contains classes that are used to serialize objects into XML format documents or streams.
Provides classes that allow you to interact with system processes, event logs, and performance counters.
Provides classes and interfaces that enable multithreaded programming. In addition to classes for synchronizing thread activities and access to data (Mutex, Monitor, Interlocked, AutoResetEvent, and so on), this namespace includes a ThreadPool class that allows you to use a pool of system-supplied threads, and a Timer class that executes callback methods on thread pool threads.
Provides types that simplify the work of writing concurrent and asynchronous code. The main types are Task which represents an asynchronous operation that can be waited on and cancelled, and Task, which is a task that can return a value. The TaskFactory class provides static methods for creating and starting tasks, and the TaskScheduler class provides the default thread scheduling infrastructure.
Contains types that allow reading and writing to files and data streams, and types that provide basic file and directory support.
Provides a simple programming interface for many of the protocols used on networks today.
Provides a programming interface for modern HTTP applications.
Contains classes used to send electronic mail to a Simple Mail Transfer Protocol (SMTP) server for delivery.
Provides a managed implementation of the Windows Sockets (Winsock) interface for developers who need to tightly control access to the network.
Contains types that retrieve information about assemblies, modules, members, parameters, and other entities in managed code by examining their metadata.
Provides the underlying structure of the common language runtime security system, including base classes for permissions.
Provides cryptographic services, including secure encoding and decoding of data, as well as many other operations, such as hashing, random number generation, and message authentication.
Provides the support for dynamic programming.