A namespace is designed to provide a way to separate a set of names from others . The name of a class declared in one namespace does not conflict with the same class name declared in another .
Defining namespaces : The definition of a namespace is based on keywords namespace start , Followed by the name of the namespace .Using keyword : Indicates that the program is using a name from a given namespace .
Nested namespace : A namespace can be nested , That is, you can define another namespace within one .

Namespace : The default is the name of the current project .

Common namespace :
Using System: Contains the definition data type , event , Event handler and other basic classes
Using System.Data: Contains the namespace and classes that provide data access .
Using System.IO: Contains data flow and read-write related functions of the class .
Using System.Windows.Forms: Contains Windows Form development tools needed form class .
Using System.Web: Contains all the site development related namespace and class .

<> In the same project , Reference namespace :

<> In different projects , Reference namespace :

Click quote , Select Add Reference . Select project , Then add it .

<> Reference the namespace of the system itself :

Click quote , Select Add Reference . Select assembly , Then add it .