Information Technology Reference
In-Depth Information
Standard Event Usage
GUI programming is event driven, which means that while the program is running, it can be
interrupted at any time by events such as button clicks, key presses, or system timers. When
this happens, the program needs to handle the event and then continue on its course.
Clearly, this asynchronous handling of program events is the perfect situation to use C#
events. Windows GUI programming uses events so extensively that there is a standard .NET
Framework pattern for using them, which you are strongly encouraged to follow.
The foundation of the standard pattern for event usage is the
EventHandler
delegate type,
which is declared in the
System
namespace. The declaration of the
EventHandler
delegate type
is shown in the following code.
The first parameter is meant to hold a reference to the object that raised the event. It is
of type
object
and can, therefore, match any instance of any type.
The second parameter is meant to hold state information of whatever type is appropri-
ate for the application.
The return type is
void
.
public delegate void EventHandler(object sender, EventArgs e);
Using the EventArgs Class
The second parameter in the
EventHandler
delegate type is an object of class
EventArgs
, which
is declared in the
System
namespace. You might be tempted to think that, since the second
parameter is meant for passing data, an
EventArgs
class object would be able to store data of
some sort. You would be wrong.
The
EventArgs
class is designed to carry no data. It is used for event handlers that do
not
need to pass data—and is generally ignored by them.
If you want to pass data, you must declare a class
derived
from
EventArgs
, with the
appropriate fields to hold the data you want to pass.
Even though the
EventArgs
class does not actually pass data, it is an important part of the
pattern of using the
EventHandler
delegate. Class
object
and class
EventArgs
are the base
classes of whatever actual types are used as the parameters. This allows
EventHandler
to pro-
vide a signature that is the lowest common denominator for all events and event handlers,
allowing them to have exactly two parameters, rather than having different signatures for
each case.
Passing Data by Extending EventArgs
To pass data in the second parameter of your event handler, and adhere to the standard con-
ventions, you need to declare a custom class derived from
EventArgs
that can store the data
you need passed. The name of the class should end in
EventArgs
. For example, the following
code declares a custom class that can store a string in a field called
Message
.
