14.3. Event Handling

Normally, a user interacts with an application’s GUI to indicate the tasks that the application should perform. For example, when you write an e-mail in an e-mail application, clicking the Send button tells the application to send the e-mail to the specified e-mail addresses. GUIs are event driven. When the user interacts with a GUI component, the interaction—known as an event—drives the program to perform a task. Common events (user interactions) that might cause an application to perform a task include clicking a Button, typing in a TextBox, selecting an item from a menu, closing a window and moving the mouse. All GUI controls have events associated with them. Objects of other types can also have associated events as well. A method that performs a task in response to an event is called an event handler, and the overall process of responding to events is known as event handling.

14.3.1. A Simple Event-Driven GUI

The Form in the application of Fig. 14.5 contains a Button that a user can click to display a MessageBox. In line 6, notice the namespace declaration, which is inserted for every class you create. We’ve been removing these from earlier simple examples because they were unnecessary. Namespaces organize groups of related classes. Each class’s name is actually a combination of its namespace name, a dot (.) and the class name. This is known as the class’s fully qualified class name. You can use the class’s simple name (the unqualified class name—SimpleEventExample) in the application. If you were to reuse this class in another application, you’d use the fully qualified name or write a using directive so that you could refer to the class by its simple name. We’ll use namespaces like this in Chapters 15 and Chapters 21. If another namespace also contains a class with the same name, the fully qualified class names can be used to distinguish between the classes in the application and prevent a name conflict (also called a name collision).