CHAPTER 10 NAMESPACES AND ASSEMBLIES

The using Directives

Fully qualified names can be quite long, and using them throughout your code can become quite cumbersome. There are two compiler directives, however, that allow you to avoid having to use fully qualified names—the using namespace directive and the using alias directive.

Two important points about the using directives are the following:

•They must be placed at the top of the source file, before any type declarations.

•They apply for all the namespaces in the current source file.

The using Namespace Directive

You saw in the MyWidgets example several sections back that you can specify a class by using the fully qualified name. You can avoid having to use the long name by placing using namespace directives at the top of the source file.

The using namespace directive instructs the compiler that you will be using types from certain specific namespaces. You can then go ahead and use the simple class names without having to fully qualify them.

When the compiler encounters a name that is not in the current namespace, it checks the list of namespaces given in the using namespace directives and appends the unknown name to the first namespace in the list. If the resulting fully qualified name matches a class in this assembly or a referenced assembly, the compiler uses that class. If it does not match, it tries the next namespace in the list.

The using namespace directive consists of the keyword using, followed by a namespace identifier.

Keyword

↓

using System ;

↑

Name of namespace

One method I have been using throughout the text is the WriteLine method, which is a member of class Console, in the System namespace. Rather than use its fully qualified name throughout the code, I simplified our work just a bit, by the use of the using namespace directive at the top of the code.

For example, the following code uses the using namespace directive in the first line to state that the code uses classes or other types from the System namespace.

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CHAPTER 10 NAMESPACES AND ASSEMBLIES

The using Alias Directive

The using alias directive allows you to assign an alias for either of the following:

•A namespace

•A type in a namespace

For example, the following code shows the use of two using alias directives. The first directive instructs the compiler that identifier Syst is an alias for namespace System. The second directive says that identifier SC is an alias for class System.Console.

The following code uses these aliases. All three lines of code in Main call the

System.Console.WriteLine method.

•The first statement in Main uses the alias for a namespace—System.

•The second statement uses the fully qualified name of the method.

•The third statement uses the alias for a class—Console.

namespace MyNamespace

{

class SomeClass

{

static void Main() { Alias for namespace

↓

Syst.Console.WriteLine ("Using the namespace alias.");

System.Console.WriteLine("Using fully qualified name.");

SC.WriteLine ("Using the type alias");

↑

} Alias for class

}

}

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