OOP with Microsoft Visual Basic . NET and Microsoft Visual C# Step by Step by Robin A. Reynolds-Haertle Microsoft Press © 2002 (393 pages) ISBN: 0735615683 This intuitive, self-paced learning title is designed to help you master the basics of object-oriented programming with Visual Basic. NET or Visual C#. Table of Contents OOP with Microsoft Visual Basic . NET and Microsoft Visual C# . NET Step by Step Introduction Chapter 1 - Writing Your First Object-Oriented Program Chapter 2 - Creating Class Instances with Constructors Chapter 3 - Creating Fields and Properties
Chapter 4 - Working with Methods Chapter 5 - Using Inheritance to Create Specialized Classes Chapter 6 - Designing Base Classes as Abstract Classes Chapter 7 - Responding to Changes with Events and Exceptions Chapter 8 - Putting It All Together with Components Chapter 9 - Providing Services Using Interfaces Chapter 10 - Using Classes Interchangeably Through Polymorphism Chapter 11 - Using Shared and Static Members Chapter 12 - Overloading Operators with Visual C# Chapter 13 - Saving Instance Data Chapter 14 - Reducing Complexity by Design
Appendix - Additional Resources Index Height Gage List of Sidebars OOP with Microsoft Visual Basic . NET and Microsoft Visual C# . NET Step by Step PUBLISHED BY Microsoft Press A Division of Microsoft Corporation One Microsoft Way Redmond, Washington 98052-6399 Copyright © 2002 by Robin A. Reynolds-Haertle All rights reserved. No part of the contents of this book may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Library of Congress Cataloging-in-Publication Data Reynolds-Haertle, Robin A. , 1959-
OOP with Microsoft Visual Basic . NET and Microsoft Visual C# Step by Step / Robin A. Reynolds-Haertle. p. cm. Includes bibliographical references and index. ISBN 0-7356-1568-3 1. Object-oriented programming (Computer science). 2. Microsoft Visual BASIC. 3. C# (Computer program language) I. Title. QA76. 64 . R495 2001 005. 2’768—dc21 2001052122 Printed and bound in the United States of America. 1 2 3 4 5 6 7 8 9 QWT 7 6 5 4 3 2 Distributed in Canada by Penguin Books Canada Limited. A CIP catalogue record for this book is available from the British Library.
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About the Author Robin A. Reynolds-Haertle Robin’s interest in computing began when she taught herself to program in C to fulfill a programming language requirement for her master’s degree in biomathematics at the University of Washington. Fascinated by the subject, Robin attended as many computer science classes as her schedule would permit, and took a position as a programmer with the University of Washington after graduation. Robin spent several years in the biotechnology industry, writing data applications in various database management systems, C, and Microsoft Visual Basic.
Not content to just read computer science and software engineering books, she then pursued and completed a master’s degree in software engineering at Seattle University. During these years, Robin presented training sessions on software engineering topics to her peers. After so many years in the classroom, Robin wanted to try teaching, and jumped at the opportunity to teach object- oriented programming with Visual Basic for the University of Washington Outreach program. Here she discovered she loved writing instructional materials and sample projects for her students. This led Robin to her current position as a programmer/writer at Microsoft, writing conceptual documentation for Visual Basic and Microsoft Visual C#.
When not at the computer, Robin is trying to make peace with her abandoned husband and sons. After she completes this book, they look forward to Mom’s attention to Cub Scouts, Boy Scouts, hiking, and watching BattleBots. After catching up with the family, Robin hopes to sew a few quilts. Acknowledgments First I’d like to thank the team at Microsoft Press that turned my writing into a book. Without Jack Beaudry, the technical editor, I never would have gotten any sleep. His meticulous reviews saved me time and saved readers from much frustration. Kathleen Atkins, the project editor, kept everything running smoothly and improved my text considerably.
Credit is also due to Danielle Bird, acquisitons editor; Rebecca McKay (Becka), manuscript editor; Cheryl Penner and Rebecca Wendling (Becky), copy editors; Gina Cassill, compositor; and Michael Kloepfer, electronic artist. I also want to thank my colleagues at Microsoft who listened sympathetically to my complaints about deadlines and beta software. Editors Roger Haight and Meredith Waring made me a better writer. Mike Pope reminded me to put the reader ahead of being clever. Megan Shult and Ann Morris, my managers, were supportive even when writing consumed all my after-hours energy. Much of what I learned about . NET came from the material written by my team members Jina Chan, Seth Grossman, Steve Hoag, Steve Stein, and Matt Stoecker. And thank you to Diana Rain, my office mate.
I’d also like to thank Ruth McBride, my longtime manager, and my instructors at Seattle University. I appreciate their patience with my often experimental approaches to their assignments over the years. I also have to mention the friends that still call to check on me, even though I haven’t called them in months. Jennifer Wirt, Lisa Wiken, Molly Potteiger, and Julie Brinkley have been true friends. This book would never have been written without the support of my husband, Bruce. He completely ran my life for the seven months I was writing this book. I worked and wrote; he did everything else. My friends are still laughing about how he RSVPs for me.
Lastly, I thank my sons for just being there and for being proud of me. Introduction Microsoft Visual Basic developers have long clamored for complete objectoriented language support. Microsoft Visual Basic . NET supports all the features of an object- oriented language. In addition, the entire Microsoft . NET Framework, which includes the development support for Microsoft Windows applications, Web applications, Web services, graphics, and data access, is designed according to object-oriented principles. Developers who have a firm grasp of object-oriented principles will be the strongest . NET developers.
Also new to developers is C#, a C-based language that gives developers a language choice for developing with the . NET Framework. Some C, Java, and C++ development will move to C# to take advantage of . NET’s features. Visual Basic programmers looking to learn a C language might also move to C#. Visual Basic . NET and C# both support object-oriented development with the . NET Framework. No matter what language you choose for development, being able to read code in either language will double your access to Microsoft Visual Studio documentation, . NET books, magazine articles, and other developer resources. System Requirements
You’ll need the following hardware and software to complete the exercises in this book: ¦ ¦ Microsoft Visual Studio . NET Professional edition. The Visual Studio . NET software isn’t included with this book. You must purchase it separately and install it before you can complete the exercises in this book. A computer capable of running Microsoft Visual Studio . NET. The following hardware configuration is recommended by the Microsoft Visual Studio . NET Web site, at http://msdn. microsoft. com/vstudio/nextgen/ Computer/Processor PC with a Pentium II–class processor, 450 megahertz (MHz); Pentium III–class processor, 600 MHz recommended
Operating System Microsoft Windows 2000, Server or Professional Microsoft Windows XP Home or Professional Microsoft Windows NT 4. 0 Server Memory Windows 2000 Professional, 96 megabytes (MB) of RAM; 128 MB recommended Windows 2000 Server, 192 MB of RAM; 256 MB recommended Windows XP Professional, 128 MB of RAM; 160 Recommended Hard Disk 500 MB on System Drive and 3. 0 gigabyte (GB) on installation drive Drive CD-ROM drive Display VGA or higher–resolution monitor Computer/Processor Input Device Microsoft Mouse or compatible pointing device Finding Your Best Starting Point This book is designed to teach you the fundamentals of object-oriented programming.
You can use this book if you have a basic knowledge of Visual Basic 6, Visual Basic . NET, Visual C#, or another Windows programming language. The exercises in this book assume you can already perform the following tasks: ¦ Create a new Windows Application project, build it, and run it. ¦ Add Windows Forms controls to a Windows Form. ¦ Create a method to respond to the Click event of a Button control. ¦ Create a simple method (called a Sub or Function in Visual Basic . NET). ¦ Declare and use variables. For an introduction to Visual Basic . NET, read Microsoft Visual Basic . NET Step by Step by Michael Halvorson (Microsoft Press, 2002).
For an introduction to Visual C# , read Microsoft Visual C# . NET Step by Step by John Sharp and Jon Jagger (Microsoft Press, 2002). Use the following table to find your best starting point in this book. If you are New To object- oriented programming Migrating From Visual Basic 6 Switching From another object- oriented programming Follow these steps Install the practice files as described in the following section, “Installing and Using the Practice Files” Work through the chapters sequentially for a complete introduction to object-oriented programming. Chapters 1 through 7, 9, and 11 concentrate on the mechanics of object- riented programming, while the other chapters cover the concepts in more depth. Install the practice files as described in “Installing and Using the Practice Files” on the next page. Work through the chapters sequentially for a complete introduction to object- oriented programming with Visual Basic .NET. Chapters 1 through 7, 9, and 11 concentrate on the mechanics of object- oriented programming, while the other chapters cover the concepts in more depth. Install the practice files as described in “Installingand Using the Practice Files. ” If you are New language. Referencing The book after working through the exercises Follow these steps Complete Chapter 1 to learn the basic yntax of properties and methods. Read the Quick Reference sections at the end of the chapters for information about specific class constructs. Use the index or the Table of Contents to find information about particular subjects. Read the Quick Reference at the end of each chapter to find a brief review of the syntax and techniques presented in the chapter. Installing and Using the Practice Files The companion CD inside the back cover of this book contains the practice files that you’ll use as you perform the exercises in the book. For example, when you’re learning to create class events, you’ll use a bitmap file named Train. bmp.
By using the practice files, you won’t waste time creating objects that aren’t relevant to the exercise. Instead, you can concentrate on learning object-oriented programming with Visual Basic . NET and Visual C# . NET. The files and the step-by-step instructions in the lessons also let you learn by doing, which is an easy and effective way to acquire and remember new skills. Important Install the practice files Before you break the seal on the OOP with Microsoft Visual Basic . NET and Microsoft Visual C# Step by Step companion CD package, be sure that this book matches your version of the software.
This book is designed for use with Microsoft Visual Studio . NET Professional Edition for the Windows operating systems. To find out what software you’re running, you can check the product package or you can start the software, and then click About Microsoft Development Environment in the Help menu at the top of the screen. Follow these steps to install the practice files on your computer’s hard disk so that you can use them with the exercises in this book. 1. Remove the companion CD from the package inside the back cover of this book and insert the CD in your CD-ROM drive. 2. Double-click the My Computer icon on the Desktop.
Tip On some computers, the startup program might run automatically when you close the CD-ROM drive. In this case, skip steps 2 through 5 and follow the instructions on the screen. 3. Double-click the icon for your CD-ROM drive. 4. Double-click StartCD. exe 5. Click Install Sample Code. The setup program window appears with the recommended options preselected for you. For best results in using the practice files with this book, accept these preselected settings. 6. When the files have been installed, remove the CD from your CD- ROM drive and replace it in the package inside the back cover of the book.
A folder called OOPVBCS has been created on your hard disk, and the practice files have been placed in that folder. Using the Practice Files Each lesson in this book explains when and how to use any practice files for that lesson. The practice files contain the complete source listings for all the applications created in this book, as well as any resources, such as bitmaps and databases, that you’ll need to complete the exercises. For those of you who like to know all the details, here’s a list of the Visual Basic and Visual C# projects on the practice disk: Project Chapter 1 ReadBooks Chapter 2 ReadMoreBooks Chapter 3 CodeAnalysis CodeAnalysis2 Chapter 4
DeckOfCards Chapter 5 TheBank ARoundButton Chapter 6 ABetterBank ABetterLibrary Variations Description This simple program demonstrates the basics of creating, instantiating, and using a class. This program expands on the ReadBooks program and adds constructors. These two applications demonstrate different approaches to using class properties, and the interaction of class properties and the DataGrid control. This application explores class methods by using dynamic creation of Windows Forms controls and drag-and-drop operations. This simple application demonstrates the basics of class inheritance. This small project shows how easy it is to derive rom a Windows Forms control and redefine its drawing. This adaptation of Chapter 5’s TheBank application uses an abstract class as a base class. This improvement on Chapter 1’s ReadBooks application uses a strongly typed collection. This application contains code snippets demonstrating variations on inheritance. Chapter 7 TrainGame ThrowSystemException PersonList Project Chapter 8 GamesLibrary Memory Chapter 9 MoveIt Points Chapter 10 PatternMaker Chapter 11 BetterCard SortablePoint Singleton Chapter 12 VectorAlgebra Chapter 13 Serialize DataSetExercise Chapter 14 This application introduces delegates, events, and user-drawn controls in the context of a simple game.
This small program throws a system exception and recovers by using exception handling. This application creates and throws a custom application exception. Description The GamesLibrary project creates a component library containing objects used to develop the simple Memory card game This application covers the basics of creating and implementing an interface. The Points project contains objects that implement the IComparable, IFormattable, and IEnumerable interfaces. Moving beyond the basics of inheritance, the PatternMaker program makes extensive use of inheritance and polymorphism. This improvement on the Card class from Chapter 4 uses static methods to liminate the project’s dependency on file locations. The SortablePoint application from Chapter 10 is made more flexible through static properties. Static fields are used to implement the Singleton design pattern. The mathematical concept of vectors is used to demonstrate the definition and use of operator overloading in Visual C#. The Serialize application demonstrates the use of binary and XML serialization of data. This very simple ADO. NET application reads data from an Access database. PatternMaker Uninstall the practice files This example uses the PatternMaker exercise from Chapter 10 to demonstrate the way to make design changes after the i nitial development of an application.
If you are using the Windows XP Professional operating system, follow these steps to remove the practice files from your computer. If you are using a different version of Windows, refer to your Windows Help documentation for removing programs. 1. Click Start, and then click Control Panel. 2. In Control Panel, click Add Or Remove Programs. 3. In the Add Or Remove Programs window, click OOP Visual Basic And C# . NET Code in the Currently Installed Programs list. 4. Click Change/Remove. The Confirm File Deletion dialog appears. 5. Click Yes to delete the practice files. 6. Click Close to close the Add Or Remove Programs window. 7. Close Control Panel.
Conventions and Features in this Book This book uses conventions designed to make the information more readable and easier to follow. The book also includes features that contribute to a deeper understanding of the material. Conventions ¦ Each exercise is a series of tasks. Each task is presented as a series of numbered steps. If a task has only one step, the step is indicated by a round bullet. ¦ Notes labeled “tip” provide more information for completing a step successfully. ¦ Notes labeled “important” alert you to information you need to check before continuing. The book uses typographic styles to help organize the information presented. The following table describes the styles used. Style Code Italics Used for Code that you type in Method argument or parameter Event Procedure Field Fully Qualified Name Keyword Method Property value Example ‘ Visual Basic Public Class Book End Class // Visual C# public class Book { } aBook showPage_Click m_shelf SomeBook. Text Public, public, If, if GetPage listOfBooks Roman Other Features Boolean values Class name Control type Data type Event Form Namespace Parameter type Property True, true, False, false Book, Library, Train ListBox, TextBox String, string, Integer, int Click Form1 ReadBooks String, string, Integer, int Name
Shaded sidebars throughout the book provide more in-depth information about the exercise. The sidebars might contain debugging tips, design tips, or topics you might want to explore further. Each chapter ends with a Quick Reference section. The Quick Reference provides a brief review of the syntax and techniques presented in the chapter. Corrections, Comments, and Help Every effort has been made to ensure the accuracy of this book and the contents of the practice files on the companion CD. Microsoft Press provides corrections and additional content for its books through the World Wide Web at http://www. microsoft. com/mspress/support/
If you have problems, comments, or ideas regarding this book or the companion CD, please send them to Microsoft Press. Send e-mail to mspinput@microsoft. com Or send postal mail to Microsoft Press Attn: Step by Step Series Editor One Microsoft Way Redmond, WA 98052-6399 Please note that support for the Visual Studio . NET software product itself is not offered through the preceding address. For help using Visual Studio . NET, visit http://support. microsoft. com. Visit the Microsoft Press World Wide Web Site You are also invited to visit the Microsoft Press World Wide Web site at http://www. microsoft. com/mspress/
You’ll find descriptions for the complete line of Microsoft Press books, information about ordering titles, notice of special features and events, additional content for Microsoft Press books, and much more. You can also find out the latest in Visual Studio . NET software developments and news from Microsoft Corporation at http://msdn. microsoft. com/vstudio/nextgen/ Check it out! Chapter 1: Writing Your First Object-Oriented Program Overview ESTIMATED TIME 2 hr. 30 min. In this chapter, you’ll learn how to ¦ Decide which classes to implement in your program. ¦ Create a class with fields, properties, and methods. Use a class in an application. ¦ Use Microsoft Visual Studio . NET tools to create a class definition. Classes are the building blocks of object-oriented programs. Object-oriented program design is driven by the objects in the problem you need to solve. If your goal is to automate class registration, you might create classes for the instructor, student, and class schedule objects. Objects also have properties that describe them and their behavior. These are implemented as properties and methods of a class. Just as an instructor has a name, so does the Instructor class have a Name property.
To assign a student to a class, you’d need to find an open section in the schedule. So your ClassSchedule class might implement a FindOpenSection method. The method would likely check the variable, called a field, in the class in which you’ve stored information about sections. In this chapter, you’ll learn how to identify the objects in your problem domain and their properties and behaviors (methods). Using this analysis, you’ll design and implement the classes using property and method programming constructs. You’ll then declare and initialize the variables of the classes you’ve coded.
Finally, you’ll implement the solution to your problem by calling the properties and methods of the class variables. Reading Books: Your First Object-Oriented Program Your task in this chapter is to implement a program that displays large text files in page- size pieces. Typically, your task begins with a specification, perhaps complete, perhaps not. The specification for Chapter 1 follows: You have downloaded on your computer the text of several books. You want to be able to select a book and read one particular page at a time. You also want to be able to set the length of a page. You’ve already decided which user interface you want to use; it’s shown here:
As you look at the user interface, you can see that you need to add some controls to a Windows form: a ListBox, a RichTextBox, two NumericUpDown controls, and some labels. How will you store the texts of the books? How will you fill the list? How will you retrieve the correct page of the book that you want to read? You can use object-oriented programming to answer these questions. Designing the Classe s Before you can implement your classes, you must decide which classes you need. First you look for the objects in the problem. Having found the objects, you then look for properties, which are characteristics or qualities that belong to the object, and methods, which are behaviors of the object.
You can choose from many ways to design the classes in an application. The approach presented here begins with a textual analysis of the problem. The nouns are selected as candidates for classes, while verbs become candidates for the methods. In the course of the analysis, you’ll eliminate many of the nouns as candidates, and you might discover classes that aren’t among the nouns in the specification. After you determine the objects, properties, and methods that belong to your classes, you can then write the class specification that you’ll use in the implementation. Find the classes . Read the problem statement, and find all the nouns. You have downloaded on your computer the text of several books. You want to be able to select a book and read one particular page at a time. You also want to be able to set the length of a page. 2. Eliminate candidates. Reasons to eliminate a candidate include ¦ The class based on the noun would have only properties or only methods. ¦ The class based on the noun wouldn’t be relevant to the problem. ¦ The class based on the noun wouldn’t represent one object.
You can eliminate the irrelevant candidates: computer and time. Length (of a page) is merely an integer value and wouldn’t generate enough behavior to qualify as a class. The same is true of text in this example—the only thing to be done with it is to display a piece of it, a page. By the same reasoning, page is also not a class. That leaves book and books. Books is just the plural of book , so you are left only with book as a potential class. But you aren’t finished yet. 3. Search for missing candidates. Consider this specification, “The dealer deals four cards to each player. There’s no mention of a deck of cards, although deck is a likely class in that problem. Remember eliminating books? Another class does, in fact, represent the properties and behavior of a group of books. You can call this class Library. The library concept is different from the book concept. A book has a title and text and can be read. A library contains many books, which can be checked out and returned. Left with the Book and Library classes, you can now search for properties and methods. Find the methods and properties 1. Read the problem statement, and find all the verbs. You can leave out the helping verbs, such as is, was, and have.
As in the case of the nouns, textual analysis of verbs is just the starting point for finding the methods. You have downloaded on your computer the text of several books. You want to be able toselect a book and read one particular page at a time. You also want to be able to set the length of a page. 2. Consider each verb. Is it a method, or does it indicate a method? Is it relevant to the problem? Downloaded and want are clearly irrelevant to the problem. Select is an operation of the Library class. In a real library, this action would correspond to finding a book on the shelf and checking it out. So the Library has a CheckOut method.
There’s also a hidden property here because a book needs a title. Read is an operation of the Book class. This method allows you to read one particular page, so it can be named GetPage. The verb set indicates that a property needs to be changed, and that property is the length of a page, PageLength. 3. The same nouns that you eliminated as classes might in fact be properties of those classes. Text, length (of a page), and page were eliminated as classes. A book does need text, so Text becomes a property of Book. You discovered that PageLength is a property in considering the verb set.
Page represents one section of the text and represents the result of the GetPage operation, so it isn’t a property. 4. Look for missing properties and methods. If you’re going to check books out of the library, you need a way to add books to the library and return the checked-out books. A CheckIn method will handle this. Testing the Class Design Reread the problem, and determine whether your classes, with their properties and methods, provide the functionality necessary to solve the problem. You have downloaded on your computer the text of several books. Do you have a way of storing and organizing several books?
Yes, you can create one Book for each book and one Library to store them all. You want to be able to select a book and read one particular page at a time. Can you select one book and read one page? Yes, books can be selected by their titles, and the GetPage method retrieves one page. You also want to be able to set the length of a page. Can you set the length of a page? Yes, the Book class has a PageLength property. The results are shown in the following table. The methods are shown as they might be declared in Visual Basic. Class Book Properties Integer PageLengt h Methods GetPage (pageNumber As Integer) As String Class Library Creating the Book Class Properties
String Text String Title Methods CheckIn (aBook As Book) CheckOut(title As String) As Book The following exercise covers the basics of class implementation using the Book class as an example. To implement the Library class, you’ll use some of the development tools provided by the Microsoft Visual Studio . NET integrated development environment (IDE). Create the book class 1. In the IDE, click the File menu, point to New, and then click Project. The New Project dialog box opens. 2. Select Visual Basic Projects or Visual C# Projects in the Project Types tree, click Windows Application in the Templates list. 3.
Enter ReadBooks in the Name box, and click OK. 4. Display the Solution Explorer by selecting Solution Explorer on the View menu. Click the ReadBooks project in the Solution Explorer. 12. On the Project menu, click Add Class. The Add New Item dialog box appears, as shown here: 13. Enter either Book. vb or Book. cs in the Name box, depending on the language you are using. Note that the class name begins with a capital letter and is singular. 14. Click Open. The IDE adds a file to your project. The file includes the basic definition of a class, as shown in the following two screen shots.
The Visual Basic class contains the minimum for a class declaration. Here’s the syntax for declaring a class in Visual Basic: Class ClassName End Class In this case, the class is named Book. The IDE adds the Public modifier that’s shown to indicate that the class can be used throughout the project. The Visual C# class contains the class declaration as well as a constructor. Here’s the syntax for declaring a class in C#: class ClassName {} A constructor contains code to initialize the fields of a class and perform other class initialization fun‘ctions. In C#, it has the same name as the class. A constructor isn’t required.
I’ll talk more about constructors in Chapter 2, “Creating Class Instances with Constructors. ” Add the Text and PageLength fields A field is a variable declared in a class block. Fields can be any . NET data type, such as Integer or Boolean; . NET class, such as TextBox or ListBox; or any class that you have created. 1. Locate the beginning of the class definition. In Visual Basic, the class definition begins immediately after the line that shows the class name. In Visual C#, the class definition begins after the opening curly brace of the class. 2. Add the following code inside the class to create Text and PageLength fields. 3. ‘ Visual Basic
Public Text As String = "" Public PageLength As Integer = 10 // Visual C# public string Text = "";p public int PageLength = 10; Tip By convention, the initial letters of names of public members (fields, properties, methods, and events) of a class are capitalized (Textfield) or are intercapitalized (PageLength field). According to the code, you have specified initial values for the fields: the empty string for Text and 10 for PageLength. A basic tenet of object-oriented programming is that an object should maintain a consistent state. That means that the state of the object (the values of its fields) should represent a usable state.
If you didn’t initialize the fields, values would default to “” for the Text field and 0 for the PageLength field. If those were acceptable values for a book, you could leave them uninitialized. But because compilers and their default values change, you can prevent maintenance problems by initializing the fields. Your client code (the code that uses a Book object) is able to read and write to any field declared with the public keyword (Public in Visual Basic and public in Visual C#). Providing direct access to the class data is a violation of the object-oriented principle of information hiding, which stipulates that the client has no knowledge of the underlying data structure of an object.
In the next section, you’ll learn how to allow the client code to get and set the Title of the Book without giving away details about the implementation. Add the Title property A property is a programming construct that allows your code to get and set a value. Typically, the code in the property constructor will get and set the value of a private field in the class. In client code, a public field and a property are used in the same way—for example, SomeBook. Text and SomeBook. Title. 1. Add the following code to the Book class after the Text and PageLength declarations. 2. Private m_title As String ‘ Visual Basic private string m_title; // Visual C#
This code creates a private field in the Book class. Client code doesn’t have access to this property. Tip Private fields of a class are declared using the m_ prefix to identify them as member data. Private field names aren’t capitalized. 3. Add the following code to the Book class, after the m_title declaration. 4. ‘ Visual Basic 5. Public Property Title() As String 6. Get 7. Return m_title 8. End Get 9. Set(ByVal value As String) 10. m_title = value 11. End SetEnd Property 12. // Visual C# 13. public string Title { 14. get { 15. return m_title; 16. } 17. set { 18. m_title = value; 19. }} These syntax blocks define class properties.
The Title property appears in the IntelliSense drop-down list just like any other property, such as the familiar TextBox. Text or Form. Backcolor. The property block allows you to control access to the property. You can add validation code to the Set block to ensure that only reasonable values are assigned to the underlying m_title field. Note Please notice an important difference between fields and properties. A place is reserved in memory for fields. They contain the actual data of the class. Properties provide access to the data but are not data themselves. In this book, I use the word set to mean changing a property. I use the word get to mean retrieving the value of a property.
The Get and Set blocks of a property can be called getters and setters, or accessors . The property block is more flexible than you’ve seen here. Properties can be public or private, read/write, read-only, or write-only. In Visual Basic, the property statements can even take a parameter. By the way, I cover properties in detail inChapter 2, but I need to talk about them at least a little bit in this chapter. A Little Bit About Properties We can use the word properties, in a general object-oriented sense, to mean the descriptive information about an object. We can also use properties to mean the particular syntactic construct provided by Visual Basic and C#.
The particular meaning of the word can be determined by context. Use properties to validate class data and hide class implementation. You have to make a strong case for using public fields in a class. The addition of a property to a class to control access to the underlying data requires minimal effort. The benefit of this practice is that you can easily add validation or change the implementation if you need to without affecting clients already using your objects. Add the GetPage method ¦ Add the GetPage method to the class definition after the field declarations. ‘ Visual Basic
Public Function GetPage(ByVal pageNumber As Integer) As String Dim start As Integer = (pageNumber -1) * PageLength If (start < Text. Length) And (start >= 0) Then If (start + PageLength) < Text. Length Then Return Text. Substring(start, PageLength) Else Return Text. Substring(start, Text. Length - start) End If Else Return "" End IfEnd Function // Visual C# public string GetPage(int pageNumber) { int start = (pageNumber - 1) * PageLength; if ((start < Text. Length) && (start >= 0)) { if ((start + PageLength) < Text. Length) { return Text.
Substring(start, PageLength); } else { return Text. Substring(start, Text. Length - start); } } else { return ""; }} In Chapter 3, “Fields and Properties,” you’ll see how we can replace the GetPage method with a construct known as an indexer in Visual C# or with a default Item method in Visual Basic. The complete class definitions for our project are shown here: ‘ Visual Basic Public Class Book Public Text As String = "" Public PageLength As Integer = 10 Private m_title As String Public Property Title() As String Get Return m_title End Get Set(ByVal Value As String) m_title = Value End Set End Property
Public Function GetPage(ByVal pageNumber As Integer) As String Dim start As Integer = (pageNumber - 1) * PageLength If (start < Text. Length) And (start >= 0) Then If (start + PageLength) < Text. Length Then Return Text. Substring(start, PageLength) Else Return Text. Substring(start, Text. Length - start) End If Else Return "" End If End FunctionEnd Class // Visual C#using System;namespace ReadBooks{ /// /// Summary description for Book. /// public class Book { public string Text = ""; public int PageLength = 10; private string m_title; public Book() { // // TODO: Add constructor logic here // } public string Title { get { return m_title; } set { m_title = value; } } ublic string GetPage(int pageNumber) { int start = (pageNumber - 1) * PageLength; if ((start < Text. Length) && (start >= 0)) { if ((start + PageLength) < Text. Length) { return Text. Substring(start, PageLength); } else { return Text. Substring(start, Text. Length - start); } } else { return ""; } } }} Fields, properties, methods, and constructors can appear in any order in a class definition. Good organization benefits future readers of your code.
Here’s a common organization and, in fact, the one I used in this book: ¦ Field declarations ¦ Constructors ¦ Properties ¦ Methods Using the Book Class in an Application You’ve just finished implementing the Book class. The class definition is just a template for an object. To put data in the fields and properties, you have to create an instance of the class in memory; this action is known as instantiation. When you create an instance, a section of memory is set aside to hold the fields of the object. If you create another instance of the class, another section of memory is set aside for its fields. You aren’t going to implement the full solution yet.
First you need to write some code to test your class. You’ll create two instances of the Book class in the ReadBooks project, and you’ll display the fourth page of each book. (These will be very short books. ) You’ll create a cookbook and a book of fairy tales, so you’ll need to create two separate instances of the Book class. Instead of creating a fancy interface, you’ll write just enough code to see whether your class is working as you expected. Test Drivers A short program to test a class is called a driver. It’s a good idea to exercise your class a bit with a driver before adding the class to a larger program.
Use the driver to test your class without the interference of other code in the program. Create an instance of Book 1. In the Solution Explorer, double-click Form1 to open it in the Windows form designer. If Form1 is opened in the code editor, select View, Designer. 2. Drag a button from the Toolbox onto Form1. If the Toolbox isn’t visible, select View, Toolbox. 3. Right -click the button, and click Properties on the shortcut menu. In the Properties window, set the Name property of the button to showPage and set the Text property to Show Page. The button on the Windows form is created from the Button class.
Name and Text are properties of the Button class. So we can talk about getting and setting these properties. Form1 is a class as well, and the button you just created is a field of the Form1 class. 4. Double-click the button to create the Click event method. 5. Add the following code in boldface to the Click event to create a book of fairy tales. 6. ‘ Visual Basic 7. Private Sub showPage_Click(ByVal sender As System. Object, _ 8. ByVal e As System. EventArgs) Handles showPage. Click 9. Dim fairyTales As Book 10. fairyTales = New Book() 11. End Sub 12. // Visual C# 13. rivate void showPage_Click(object sender, System. EventArgs e) { 14. Book fairyTales; 15. fairyTales = new Book(); } 16. Add the following code to set the Text, PageLength, and Title properties immediately after the code you entered in step 5: 17. ‘ Visual Basic 18. fairyTales. Text = "Once upon a time there was a bear. " 19. fairyTales. PageLength = 8fairyTales. Title = "Fairy Tales" 20. // Visual C# 21. fairyTales. Text = "Once upon a time there was a bear. "; 22. fairyTales. PageLength = 8; fairyTales. Title = "Fairy Tales";
When the instance of Book is created, its fields contain the values specified in the class definition. The Text field is an empty string, the page length is 10, and the title is blank. Notice that it makes no difference in the client code whether you use a field or a property. 23. Add the following code after the fairyTales code to create another instance of the Book class. (This instance will be a recipe book. ) 24. ‘ Visual Basic 25. Dim cookies As Book = New Book() 26. cookies. Text = "Chocolate chip cookies are the most delicious co okies. " 27. ookies. PageLength = 8 28. cookies. Title = "Cookie Recipes" 29. // Visual C# 30. Book cookies = new Book(); 31. cookies. Text = "Chocolate chip cookies are the most delicious co okies. "; 32. cookies. PageLength = 8; cookies. Title = "Cookie Recipes"; In this case, you used a different syntax for declaring and initializing a variable of the Book class. Visual Basic and Visual C# allow declaration and initialization in the same statement. Declaring and initializing in the same statement has the following advantages: ¦ Programmers are less likely to forget to initialize the variable. When a class defines a constructor with parameters, the fields can be initialized at the same time. (You’ll create constructors with parameters in Chapter 3. ) Use an instance of the Book class 1. Add the following code after the cookies code to display some of the text of the two books. In later chapters, you’ll learn other ways to return the text of a particular page in the book. 2. ‘ Visual Basic 3. Dim page As Integer = 3 4. Dim report As String 5. report = "Page " & page. ToString() & ControlChars. CrLf _ 6. & fairyTales. Title & ": " & fairyTales. GetPage(page) _ 7. ControlChars. CrLf _ 8. & "Cookies: " & cookies. GetPage(page) 9. MessageBox. Show(report) 10. report = "Titles: " + fairyTales. Title & " and " & cookies. Title 11. MessageBox. Show(report) 12. // Visual C# 13. int page = 3; 14. string report; 15. report = "Page " + page. ToString() + " " 16. + fairyTales. Title + ": " + fairyTales. GetPage(page) + " " 17. + cookies. Title + ": " + cookies. GetPage(page); 18. MessageBox. Show(report); 19. report = "Titles: " + fairyTales. Title + " and " + cookies. Title; MessageBox. Show(report);
This bit of code demonstrates that there are two separate instances of the Book class. We can refer to these instances using the variables fairyTales and cookies . The object-oriented concept that permits each instance to be referred to separately is known as identity. You’ll see in later chapters that the identity principle doesn’t mean that you have to create a variable for each instance. Creating so many variables is unwieldy if you need hundreds of instances of a class. Identity does mean that you can refer to each instance separately when you need to.
Notice that when you created an instance of Book, the fields of fairyTales were changed and the GetPage method was called. Later on we retrieved the value of the Title property. The value of Title was unchanged after the GetPage method was called. The fact that the value was unchanged demonstrates the concept of object state, the idea that the fields retain their values between method calls. Compare the way the GetPage method works with a method that has variable declarations. After the GetPage method ends, the variables go out of scope and their values are lost to the application. 20. Press F5 to run the code. Click the Show Page button.
The results are shown here: Click OK, and the book titles are displayed in a message box as shown here: Click OK, and then close the application. You’ve now created a class, Book, and two instances of it. Your code sent a message to the Book class through the GetPage method to ask for the third page of the text. In the next sections, you’ll implement another class, Library. This time, however, you’ll let some of the IDE tools do some of the syntactic work for you. Using the Class View The IDE provides a Class View that displays a tree view of the class structure of the project, namespaces, and classes.
The Class View can share the same window as the Solution Explorer. On the View menu, click Class View to open the Class View. The expanded Class View is shown below for Visual Basic and Visual C#, respectively. The highest-level node represents the project, ReadBooks. The next level of nodes represents the namespaces in the project. A project can contain several namespaces; in this case, there’s only one. The project namespace contains two classes: the class that we created, Book, and the class for the Windows form, Form1. The Book class contains two public fields, PageLength and Text, epresented by blue blocks, and one private field, m_ti
