Concepts of Programming Languages

14.7 Event Handling in C# 661

14.7 Event Handling in C

Event handling in C# (and in the other .NET languages) is similar to that

of Java. .NET provides two approaches to creating GUIs in applications, the

original Windows Forms and the more recent Windows Presentation Founda-

tion. The latter is the more sophisticated and complex of the two. Because our

interest is just in event handling, we will use the simpler Windows Forms to

discuss our subject.

Using Windows Forms, a C# application that constructs a GUI is created by

subclassing the Form predefined class, which is defined in the System.Windows

.Forms namespace. This class implicitly provides a window to contain our

components. There is no need to build frames or panels explicitly.

Text can be placed in a Label object and radio buttons are objects of the

RadioButton class. The size of a Label object is not explicitly specified

in the constructor; rather it can be specified by setting the AutoSize data

member of the Label object to true, which sets the size according to what

is placed in it.

Components can be placed at a particular location in the window by assign-

ing a new Point object to the Location property of the component. The

Point class is defined in the System.Drawing namespace. The Point con-

structor takes two parameters, which are the coordinates of the object in pixels.

For example, Point(100, 200) is a position that is 100 pixels from the left

edge of the window and 200 pixels from the top. The label of a component is

set by assigning a string literal to the Text property of the component. After

creating a component, it is added to the form window by sending it to the Add

method of the Controls subclass of the form. Therefore, the following code

creates a radio button with the label Plain at the (100, 300) position in the

output window:

private RadioButton plain = new RadioButton();

plain.Location = new Point(100, 300);

plain.Text = "Plain";

Controls.Add(plain);

All C# event handlers have the same protocol: the return type is void

and the two parameters are of types object and EventArgs. Neither of the

parameters needs to be used for a simple situation. An event handler method

can have any name. A radio button is tested to determine whether it is clicked

with the Boolean Checked property of the button. Consider the following

skeletal example of an event handler:

private void rb_CheckedChanged (object o, EventArgs e){

if (plain.Checked)...

...
}