C# 0 The Complete Reference

ButtonForm

, which is derived from 

Form

. It contains 

a

Button

 field called 

MyButton

. Inside the constructor, the button is created, initialized, and 

added to the form. When run, the program displays the window shown in Figure 26-2. You 

can click the button, but nothing will happen. To make the button do something, you must 

add a message handler, as described in the next section.

Handling Messages

In order for a program to respond to a button press (or any other type of control interaction), 

it must handle the message that the button generates. In general, when a user interacts with 

a control, those interactions are passed to your program as messages. In a form-based C# 

program, these messages are processed by event handlers. Therefore, to receive messages, 

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your program adds its own event handler onto the list of handlers called when a message is 

generated. For button-press messages, this means adding your handler to the 

Click

 event.

The

Click

 event is specified by 

Control

 and defined by 

Button

. It has this general form:

public Event EventHandler Click;

The

EventHandler

 delegate is defined as shown here:

public delegate void EventHandler(object 

who

, EventArgs 

args

)

The object that generated the event is passed in 

who.

 Any information associated with that 

event is passed in 

args.

 For many events, 

args

 will be an object of a class derived from 

EventArgs

. Since a button click does not require any additional information, we don’t need 

to worry about event arguments when handling the button.

The following program adds button-response code to the preceding program. Each time 

the button is clicked, the location of the button is changed.

// Handle button messages.

using System;

using System.Windows.Forms;

using System.Drawing;

class ButtonForm : Form {

  Button MyButton;

  public ButtonForm() {

    Text = "Respond to a Button";

    MyButton = new Button();

    MyButton.Text = "Press Here";

    MyButton.Location = new Point(100, 200);

    // Add button event handler to list.

    MyButton.Click += MyButtonClick;

    Controls.Add(MyButton);

  }

F

IGURE

 26-2 

Adding a button

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