Keyboard-Event Handling

Key events occur when keyboard keys are pressed and released. Such events can be handled for any control that inherits from System.Windows.Forms.Control. There are three key eventsKeyPress, KeyUp and KeyDown. The KeyPress event occurs when the user presses a key that represents an ASCII character. The specific key can be determined with property KeyChar of the event handler's KeyPressEventArgs argument. ASCII is a 128-character set of alphanumeric symbols, a full listing of which can be found in Appendix D.

The KeyPress event does not indicate whether modifier keys (e.g., Shift, Alt and Ctrl) were pressed when a key event occurred. If this information is important, the KeyUp or KeyDown events can be used. The KeyEventArgs argument for each of these events contains information about modifier keys. Often, modifier keys are used in conjunction with the mouse to select or highlight information. Figure 13.39 lists important key event information. Several properties return values from the Keys enumeration, which provides constants that specify the various keys on a keyboard. Like the FontStyle enumeration (Section 13.7), the Keys enumeration is a System.FlagAttribute, so the enumeration's constants can be combined to indicate multiple keys pressed at the same time.

Figure 13.39. Keyboard events and event arguments.

Keyboard events and event arguments

Key Events with Event Arguments of Type KeyEventArgs

KeyDown

Generated when a key is initially pressed.

KeyUp

Generated when a key is released.

Key Event with Event Argument of Type KeyPressEventArgs

KeyPress

Generated when a key is pressed.

Class KeyPressEventArgs Properties

KeyChar

Returns the ASCII character for the key pressed.

Handled

Indicates whether the KeyPress event was handled.

Class KeyEventArgs Properties

Alt

Indicates whether the Alt key was pressed.

Control

Indicates whether the Ctrl key was pressed.

Shift

Indicates whether the Shift key was pressed.

Handled

Indicates whether the event was handled.

KeyCode

Returns the key code for the key as a value from the Keys enumeration. This does not include modifier-key information. It is used to test for a specific key.

KeyData

Returns the key code for a key combined with modifier information as a Keys value. This property contains all information about the pressed key.

KeyValue

Returns the key code as an int, rather than as a value from the Keys enumeration. This property is used to obtain a numeric representation of the pressed key. The int value is known as a Windows virtual key code.

Modifiers

Returns a Keys value indicating any pressed modifier keys (Alt, Ctrl and Shift). This property is used to determine modifier-key information only.

Figure 13.40 demonstrates the use of the key-event handlers to display a key pressed by a user. The program is a Form with two Labels that displays the pressed key on one Label and modifier key information on the other.

Figure 13.40. Demonstrating keyboard events.

 1 // Fig. 13.40: KeyDemoForm.cs
 2 // Displaying information about the key the user pressed.
 3 System;
 4 System.Windows.Forms;
 5
 6 // Form to display key information when key is pressed
 7 KeyDemoForm : Form
 8 {
 9 // default constructor
10 KeyDemoForm()
11 {
12 InitializeComponent();
13 } // end constructor
14
15 // display the character pressed using KeyChar
16 KeyDemoForm_KeyPress( sender, KeyPressEventArgs e )
17 {
18 charLabel.Text = "Key pressed: " + e.KeyChar;
19 } // end method KeyDemoForm_KeyPress
20
21 // display modifier keys, key code, key data and key value
22 KeyDemoForm_KeyDown( sender, KeyEventArgs e )
23 {
24 keyInfoLabel.Text =
25 "Alt: " + ( e.Alt ? "Yes" : "No" ) + '
' +
26 "Shift: " + ( e.Shift ? "Yes" : "No" ) + '
' +
27 "Ctrl: " + ( e.Control ? "Yes" : "No" ) + '
' +
28 "KeyCode: " + e.KeyCode + '
' +
29 "KeyData: " + e.KeyData + '
' +
30 "KeyValue: " + e.KeyValue;
31 } // end method KeyDemoForm_KeyDown
32
33 // clear Labels when key released 34 KeyDemoForm_KeyUp( sender, KeyEventArgs e ) 35 { 36 charLabel.Text = ""; 37 keyInfoLabel.Text = ""; 38 } // end method KeyDemoForm_KeyUp 39 } // end class KeyDemoForm

Initially, the two Labels (charLabel and keyInfoLabel) are empty. Control charLabel displays the character value of the key pressed, whereas keyInfoLabel displays information relating to the pressed key. Because the KeyDown and KeyPress events convey different information, the Form (KeyDemoForm) handles both.

The KeyPress event handler (lines 1619) accesses the KeyChar property of the KeyPressEventArgs object. This returns the pressed key as a char and which we then display in charLabel (line 18). If the pressed key is not an ASCII character, then the KeyPress event will not occur, and charLabel will not display any text. ASCII is a common encoding format for letters, numbers, punctuation marks and other characters. It does not support keys such as the function keys (like F1) or the modifier keys (Alt, Ctrl and Shift).

The KeyDown event handler (lines 2231) displays information from its KeyEventArgs object. The event handler tests for the Alt, Shift and Ctrl keys by using the Alt, Shift and Control properties, each of which returns a bool valuetrue if the corresponding key is pressed and false otherwise. The event handler then displays the KeyCode, KeyData and KeyValue properties.

The KeyCode property returns a Keys enumeration value (line 28). The KeyCode property returns the pressed key, but does not provide any information about modifier keys. Thus, both a capital and a lowercase "a" are represented as the A key.

The KeyData property (line 29) also returns a Keys enumeration value, but this property includes data about modifier keys. Thus, if "A" is input, the KeyData shows that both the A key and the Shift key were pressed. Lastly, KeyValue (line 30) returns the key code of the pressed key as an int. This int is the key code, which provides an int value for a wide range of keys and for mouse buttons. The Windows virtual key code is useful when one is testing for non-ASCII keys (such as F12).

The KeyUp event handler (lines 3438) clears both Labels when the key is released. As we can see from the output, non-ASCII keys are not displayed in charLabel, because the KeyPress event is not generated. However, the KeyDown event still is generated, and keyInfoLabel displays information about the key that is pressed. The Keys enumeration can be used to test for specific keys by comparing the key pressed to a specific KeyCode.

Software Engineering Observation 13 3

To cause a control to react when a particular key is pressed (such as Enter), handle a key event and test for the pressed key. To cause a Button to be clicked when the Enter key is pressed on a Form, set the Form's AcceptButton property.


Preface

Index

    Introduction to Computers, the Internet and Visual C#

    Introduction to the Visual C# 2005 Express Edition IDE

    Introduction to C# Applications

    Introduction to Classes and Objects

    Control Statements: Part 1

    Control Statements: Part 2

    Methods: A Deeper Look

    Arrays

    Classes and Objects: A Deeper Look

    Object-Oriented Programming: Inheritance

    Polymorphism, Interfaces & Operator Overloading

    Exception Handling

    Graphical User Interface Concepts: Part 1

    Graphical User Interface Concepts: Part 2

    Multithreading

    Strings, Characters and Regular Expressions

    Graphics and Multimedia

    Files and Streams

    Extensible Markup Language (XML)

    Database, SQL and ADO.NET

    ASP.NET 2.0, Web Forms and Web Controls

    Web Services

    Networking: Streams-Based Sockets and Datagrams

    Searching and Sorting

    Data Structures

    Generics

    Collections

    Appendix A. Operator Precedence Chart

    Appendix B. Number Systems

    Appendix C. Using the Visual Studio 2005 Debugger

    Appendix D. ASCII Character Set

    Appendix E. Unicode®

    Appendix F. Introduction to XHTML: Part 1

    Appendix G. Introduction to XHTML: Part 2

    Appendix H. HTML/XHTML Special Characters

    Appendix I. HTML/XHTML Colors

    Appendix J. ATM Case Study Code

    Appendix K. UML 2: Additional Diagram Types

    Appendix L. Simple Types

    Index



    Visual C# How to Program
    Visual C# 2005 How to Program (2nd Edition)
    ISBN: 0131525239
    EAN: 2147483647
    Year: 2004
    Pages: 600

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