The Namespace Statement The Imports Statement
2.6 Namespaces
Thousands of types are defined in the .NET Framework. In addition, programmers can define new types for use in their programs. With so many types, name clashes are inevitable. To prevent name clashes, types are considered to reside inside of namespaces. Often, this fact can be ignored. For example, in Visual Basic .NET a class may be defined like this: Public Class SomeClass ... End Class This class definition might be in a class library used by third-party customers, or it might be in the same file or the same project as the client code. The client code that uses this class might look something like this: Dim x As New SomeClass x.DoSomething Now consider what happens if the third-party customer also purchases another vendors class library, which also exposes a SomeClass class. The Visual Basic .NET compiler cant know which definition of SomeClass will be used. The client must therefore use the full name of the type, also known as its fully qualified name . Code that needs to use both types might look something like this: The namespace is FooBarCorp.SuperFoo2100. Dim x As New FooBarCorp.SuperFoo2100.SomeClass x.DoSomething ... The namespace is MegaBiz.ProductivityTools.WizardMaster. Dim y As New MegaBiz.ProductivityTools.WizardMaster.SomeClass y.DoSomethingElse Note that a namespace name can itself contain periods . . When looking at a fully qualified type name, everything prior to the final period is the namespace name. The name after the final period is the type name. Microsoft recommends that namespaces be named according to the format CompanyName.TechnologyName . For example, Microsoft.VisualBasic.2.6.1 The Namespace Statement
So how does a component developer specify a types namespace? In Visual Basic .NET, this can be done several ways. One is to use the Namespace keyword, like this: Namespace MegaBiz.ProductivityTools.WizardMaster Public Class SomeClass ... End Class End Namespace Note that it is permissible for different types in the same source file to have different namespaces. 41 A second way to provide a namespace is to use the rootnamespace switch on the Visual Basic .NET command-line compiler. For example: vbc src.vb t:library rootnamespace:MegaBiz.ProductivityTools.WizardMaster All types defined within the compiled files then have the given namespace. If youre compiling in the Visual Studio .NET IDE, the root namespace is specified in the Project Property Pages dialog box, which can be reached by right-clicking the project name in the Solution Explorer window of the IDE, then choosing Properties see Figur e 2- 1 for the resulting WizardMaster Property Pages dialog. By default, Visual Studio .NET sets the root namespace equal to the name of the project. Figure 2-1. Setting the root namespace in the Visual Studio .NET IDE Note that regardless of which compiler is used command line or Visual Studio .NET, if a root namespace is specified and the Namespace keyword is used, the resulting namespace will be the concatenation of the root namespace name and the name specified using the Namespace keyword.2.6.2 The Imports Statement
So far, the discussion has implied that its not necessary for the user of a type to specify the types full name unless there is a name clash. This isnt exactly true. The CLR deals with types only in terms of their full names. However, because humans dont like to deal with long names, Visual Basic .NET offers a shortcut. As an example, the .NET Framework provides a drawing library, in which a type called Point is defined. This types namespace is called System.Drawing, so the types fully qualified name is System.Drawing.Point. Code that uses this type might look like this: Dim pt As System.Drawing.Point pt.X = 10 pt.Y = 20 ... Typing the full name of every type whenever it is used would be too cumbersome, though, so Visual Basic .NET offers the Imports statement. This statement indicates to the compiler that the types from a given namespace will appear without qualification in the code. For example: 42 At the top of the source code file: Imports System.Drawing ... Somewhere within the source code file: Dim pt As Point pt.X = 10 pt.Y = 20 ... To import multiple namespaces, list each one in its own Imports statement. Its okay if multiple imported namespaces have some name clashes. For the types whose names clash, the full name must be specified wherever the type is used. The Imports statement is just a convenience for the developer. It does not set a reference to the assembly in which the types are defined. See the discussion of assemblies in Chapt er 3 to learn how to reference assemblies that contain the types you need. Finally, note that namespaces, too, are just a convenience for the developer writing source code. To the runtime, a type is not in a namespace—a namespace is just another part of a type name. It is perfectly acceptable for any given assembly to have types in different namespaces, and more than one assembly can define types in a single namespace.2.7 Symbolic Constants
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» VB.NET - (O'Reilly) Programming Visual Basic NET
» What Is the Microsoft .NET Framework?
» hello, world An Example Visual Basic .NET Program
» Hello, Windows An Example Visual Basic .NET Program
» Hello, Browser An Example Visual Basic .NET Program
» Source Files Identifiers The Visual Basic .NET Language
» Numeric Literals String Literals Character Literals
» Date Literals Boolean Literals Nothing Summary of Literal Formats
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» The Namespace Statement The Imports Statement
» Symbolic Constants Scope The Visual Basic .NET Language
» Access Modifiers Assignment The Visual Basic .NET Language
» Unary Operators Arithmetic Operators
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» String-Concatenation Operators Bitwise Operators
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» Object Instantiation and New Constructors
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» Interfaces The Visual Basic .NET Language
» Enumerations The Visual Basic .NET Language
» Exceptions The Visual Basic .NET Language
» Delegates The Visual Basic .NET Language
» Using Events and Delegates Together
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» Standard Modules Conditional Compilation
» Summary The Visual Basic .NET Language
» Common Language Infrastructure CLI and Common Language Runtime CLR
» Global Assembly Cache GAC Comparison of Assemblies, Modules, and Namespaces
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» A Brief Tour of the .NET Framework Namespaces
» Configuration File Format Configuration
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» Adding event handlers Creating a Form in Code
» Handling Form Events Windows Forms I: Developing Desktop Applications
» Relationships Between Forms Windows Forms I: Developing Desktop Applications
» Merging Menus MDI Applications
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» Component Attributes Windows Forms I: Developing Desktop Applications
» The Graphics Class 2-D Graphics Programming with GDI+
» The Pen Class 2-D Graphics Programming with GDI+
» The Brush Class 2-D Graphics Programming with GDI+
» System colors The Color Structure
» Alpha Blending 2-D Graphics Programming with GDI+
» Antialiasing 2-D Graphics Programming with GDI+
» The PrintPageEventArgs Class Printing
» The OnBeginPrint and OnEndPrint Methods Choosing a Printer
» The PageSettings Class Printing
» The PrinterSettings Class Printing
» Page Setup Dialog Box Print Preview
» Summary Windows Forms I: Developing Desktop Applications
» The Button Class The CheckBox Class The ComboBox Class
» The DateTimePicker Class The GroupBox Class The ImageList Class
» The Label Class The LinkLabel Class
» The ListBox Class Common Controls and Components
» The ListBox.ObjectCollection Class
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» Control Events Windows Forms II: Controls, Common Dialog Boxes, and Menus
» The Anchor Property Form and Control Layout
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» ColorDialog FontDialog OpenFileDialog Common Dialog Boxes
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» Adding Menus in the Visual Studio .NET Windows Forms Designer
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» Summary Windows Forms II: Controls, Common Dialog Boxes, and Menus
» Setting control properties using attributes Adding event handlers
» AutoEventWireup Handling Page Events
» The Button control Web Controls
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» The ListBox control Web Controls
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» The Table control Web Controls
» The TextBox control Web Controls
» Other web controls Web Controls
» HTML Controls Handling Control Events
» Programmatically Instantiating Controls More About Server Controls
» More About Validation-Control Tag Attributes
» Using Validation-Control Properties Providing a Summary View of Validation Failures
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» Using Directives to Modify Web Page Compilation
» The Server Object ASP.NET Objects: Interacting with the Framework
» The Application Object The Session Object The Cache Object The Request Object
» The Response Object ASP.NET Objects: Interacting with the Framework
» Discovering Browser Capabilities ASP.NET and Web Forms: Developing Browser-Based Applications
» The Session Object Maintaining State
» The Application Object Maintaining State
» Session and Application Startup and Shutdown global.asax Compiles to a Class
» Adding Global Objects Application-Level Code and global.asax
» ASP.NET authorization Authorization
» Windows NTFS authorization Code-access authorization
» IUSR_ComputerName Impersonation Accessing Network Resources
» User Controls Designing Custom Controls
» Creating a custom server control using Visual Studio .NET
» Creating a custom server control in code Using a custom server control in Visual Studio .NET
» Summary ASP.NET and Web Forms: Developing Browser-Based Applications
» The WebService Attribute The WebMethod Attribute
» Testing a Web Service with a Browser
» Consuming a Web Service in Visual Studio .NET
» Consuming a Web Service in Notepad
» Synchronous Versus Asynchronous Calls
» Web-Service Descriptions Web-Service Discovery
» Limitations of Web Services Summary
» A Brief History of Universal Data Access Managed Providers
» Connecting to a SQL Server Database
» Connecting to an OLE DB Data Source
» The DataSet Class Finding Tables
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» Writing Updates Back to the Data Source
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» The DataSets XML Capabilities
» Binding a DataSet to a Windows Forms DataGrid
» Binding a DataSet to a Web Forms DataGrid
» Typed DataSets ADO.NET: Developing Database Applications
» Reading Data Using a DataReader
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