C# ПІДРУЧНИКИ / c# / MS Press - Oop With MS Vb Net And C# Step By Step (2002)

Uninstall the practice files

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.

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

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 pagesize 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

1.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 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.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 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;

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.

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.

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 in Chapter 2, but I need to talk about them at least a little bit in this chapter.