Information Technology Reference
In-Depth Information
The using Directives
Fully qualified names can be quite long, and using them throughout the code could become
quite tedious. 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 classes from
certain specific namespaces. You can then 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 direc-
tive 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.
using System; // using namespace directive
...
System.Console.WriteLine("This is text 1"); // Use fully qualified name.
Console.WriteLine("This is text 2"); // Use directive.
