Recipe 3.8. Using Namespaces

Problem

You want to place your classes within a specific .NET namespace.

Solution

Use the Namespace statement together with the default namespace identified in a project's properties.

Discussion

Every Visual Basic application resides within a default namespace, what we'll call the "absolute namespace position" for your application. Visual Studio automatically sets this to a top-level namespace with the same name as your project. For instance, if you just accept the default "WindowsApplication1" name for a new Windows Forms application, the namespace is also named WindowsApplication1 . Since it's a top-level namespace, it resides at the same hierarchy position as the System namespace.

To alter the namespace for your project, open the Project Properties window, and change the "Root namespace" field on the Application tab. You can change it to use an existing namespace, such as System.Windows.Forms , but then you must take care to avoid naming conflicts with your classes.

When generating a full .NET application (EXE), your choice of namespace is not too problematic because that namespace exists only within the view of your program and its lifetime. Two applications using the WindowsApplication1 namespace will not conflict with each other. However, if you generate a .NET library (DLL) for general distribution to others outside your organization, you should select a namespace that will avoid conflicts with others. Microsoft recommends that you use a combination of your company name and the product name, as they did with the Microsoft.VisualBasic namespace.

Beyond the absolute namespace position, you can place your classes and other types in a "relative namespace position" within the larger default absolute namespace. When you add a class (or other type) to your project, it appears in the absolute namespace position:

Class Class1

	End Class

If your project uses WindowsApplication1 as its absolute namespace, this class appears as WindowsApplication1.Class1 . In relative positioning, you can insert a new namespace between the absolute position and the class:

Namespace CoolClasses
	   Class Class1

	   End Class
	End Namespace

Now, Class1 is fully referenced as WindowsApplication1.CoolClasses.Class1 .

The Namespace keyword may include multiple namespace components (separated by periods), and you can nest them as well:

Namespace CoolClasses
	   Namespace SomewhatCool.BarelyCool
	      Class Class1

	      End Class
	   End Namespace
	End Namespace

This Class1 lives at WindowsApplication1.CoolClasses.SomewhatCool.BarelyCool .

Recipe 3.9. Splitting a Class Across Multiple Files

Problem

You have a class that is simply too much to manage reasonably in a single sourcecode file, and you would like to split it up.

Solution

Use the Partial keyword on a class to enable splitting the implementation of that class across multiple physical source files:

Partial Class Class1

	End Class

Discussion

Visual Basic now includes a partial class feature that Visual Studio uses to separate automatically generated code from nongenerated code. This feature is available to use in your own classes. Before Visual Basic 2005, if you tried to split a class by using the Class statement multiple times on the same class name , the program would not compile. But now you can break up your class into separate sections:

Class Class1
	   ' ----- Some class members are defined here.
	End Class
	Partial Class Class1
	   ' ----- More class members are defined here.
	End Class

The key is the word Partial . Adding the keyword Partial to at least one of the class components tells the Visual Basic compiler to collect all the parts and put them together before it builds the compiled version of your program, even if those parts exist in different files.

You do not need to include Partial on every part of the class, just on one of the parts. Also, if your class inherits from another class or implements an interface, you need to include only the Inherits or Implements keyword in one of the class portions.

All class parts must exist in the context of the same namespace. If you create different class definitions with the same name but in different namespaces, they will be distinct and unrelated classes.