1. Chapter 11: Classes and Objects
Programming with Microsoft Visual Basic .NET, Second Edition

2. Classes and Objects Lesson A Objectives
Define a class
Add properties to a class
Instantiate an object from a class that you define
Programming with Microsoft Visual Basic .NET, Second Edition

3. Defining a Class
Classes that you define must specify the properties and methods of the objects they create
Properties describe the characteristics of the objects
Methods specify the tasks that the objects can perform
Programming with Microsoft Visual Basic .NET, Second Edition

4. Defining a Class (continued)
Use the Class statement to define a class in Visual Basic .NET
Enter the Class statement in a class file
After defining a class, you can use it to create objects
Programming with Microsoft Visual Basic .NET, Second Edition

5. Defining a Class (continued)
Figure 11-3: Syntax and an example of the Class statement
Programming with Microsoft Visual Basic .NET, Second Edition

6. Defining a Class (continued)
Figure 11-4: Procedure for adding a class file to a project
Programming with Microsoft Visual Basic .NET, Second Edition

7. Defining a Class (continued)
Figure 11-7: Syntax and examples of creating an object from a class
Programming with Microsoft Visual Basic .NET, Second Edition

8. Using a Class That Contains Properties Only
The sales manager at Sweets Unlimited wants an application to allow him to save each salesperson’s name, quarterly sales amount, and quarterly bonus amount in a sequential access file
The bonus amount is calculated by multiplying the sales amount by 5%
Programming with Microsoft Visual Basic .NET, Second Edition

9. Using a Class That Contains Properties Only (continued)
Figure 11-8: Sample run of the Sweets Unlimited application
Programming with Microsoft Visual Basic .NET, Second Edition

10. Using a Class That Contains Properties Only (continued)
The Salesperson class contains three properties: Name, Sales, and Bonus
Rules for naming properties
A name should be composed of one or more words, with the first letter of each word being capitalized
Use nouns and adjectives to name a property
Programming with Microsoft Visual Basic .NET, Second Edition

11. More on Classes and Objects Lesson B Objectives
Add Property procedures to a class
Create constructors
Add methods to a class
Programming with Microsoft Visual Basic .NET, Second Edition

12. Using a Class That Contains Properties and Methods
You will learn how to create a class named Square and use the class in the Area application
Square class
Contains one property and two methods
Creates an object that calculates and returns the area of a square, using the side measurement provided by the application
Programming with Microsoft Visual Basic .NET, Second Edition

13. Using a Class That Contains Properties and Methods (continued)
Figure 11-11: Sample run of the Area application
Programming with Microsoft Visual Basic .NET, Second Edition

14. Using a Class That Contains Properties and Methods (continued)
An application cannot directly refer to a Private variable in a class; it must refer to the variable indirectly, through the use of a Public property
You create a Public property using a Property procedure
Programming with Microsoft Visual Basic .NET, Second Edition

15. Using a Class That Contains Properties and Methods (continued)
Figure 11-13: Syntax and an example of creating a Property procedure
Programming with Microsoft Visual Basic .NET, Second Edition

16. Using a Class That Contains Properties and Methods (continued)
Figure 11-13: Syntax and an example of creating a Property procedure (continued)
Programming with Microsoft Visual Basic .NET, Second Edition

17. Using a Class That Contains Properties and Methods (continued)
Within the Property procedure you define a Get block of code and a Set block of code
Code in the Get block allows an application to retrieve the contents of the Private variable associated with the property
Code in the Set block allows the application to assign a value to the Private variable associated with the property
Programming with Microsoft Visual Basic .NET, Second Edition

18. Using a Class That Contains Properties and Methods (continued)
The Get block uses the Get statement, which begins with the keyword Get and ends with the keywords End Get
The Set block uses the Set statement, which begins with the keyword Set and ends with the keywords End Set
Programming with Microsoft Visual Basic .NET, Second Edition

19. Constructors
A constructor is a method whose instructions the computer processes, automatically, each time an object is created (instantiated) from the class
The purpose of a constructor is to initialize the variables of the class
Programming with Microsoft Visual Basic .NET, Second Edition

20. Constructors (continued)
Figure 11-14: Syntax and an example of creating a constructor
Programming with Microsoft Visual Basic .NET, Second Edition

21. Constructors (continued)
Every class should have at least one constructor
A constructor that has no parameters is called the default constructor
Each constructor included in a class has the same name, New, but its parameters (if any) must be different from any other constructor in the class
Programming with Microsoft Visual Basic .NET, Second Edition

22. Methods Other Than Constructors
Methods, other than constructors, included in a class can be either Sub procedures or Function procedures
Rules for naming methods
Names should be composed of one or more words, with the first letter of each word being capitalized
The first word in a name should be a verb; subsequent words should be nouns and adjectives
Programming with Microsoft Visual Basic .NET, Second Edition

23. Methods Other Than Constructors (continued)
Figure 11-15: Syntax and an example of creating a method that is not a constructor
Programming with Microsoft Visual Basic .NET, Second Edition

24. Coding the Cornwall Calendars Application Lesson C Objectives
Create a class that contains more than one constructor
Include data validation in a class
Programming with Microsoft Visual Basic .NET, Second Edition

25. Using a Class That Contains Two Constructors and Data Validation
MyDate class
Contains more than one constructor
Performs data validation
Creates an object that returns a month number, followed by a slash, and a day number
MyDate class is used in the Personnel application
Programming with Microsoft Visual Basic .NET, Second Edition

26. Using a Class That Contains Two Constructors and Data Validation (continued)
Figure 11-18: Sample run of the Personnel application
Programming with Microsoft Visual Basic .NET, Second Edition

27. Using a Class That Contains Two Constructors and Data Validation (continued)
MyDate class contains:
Two Private variables: monthNum and dayNum
Two Property procedures: Month and Day
Three methods: two named New and one named GetNewDate
Programming with Microsoft Visual Basic .NET, Second Edition

28. Coding the Cornwall Calendars Application
Created for Jesse Washington, the manager of the Accounts Payable department at Cornwall Calendars
Should allow Jesse to record (in a sequential access file) the check number, date, payee, and amount of each check written by his department
Programming with Microsoft Visual Basic .NET, Second Edition

29. Coding the Cornwall Calendars Application (continued)
Figure 11-21: Interface for the Cornwall Calendars application
Programming with Microsoft Visual Basic .NET, Second Edition

30. Coding the Cornwall Calendars Application (continued)
Figure 11-22: TOE chart for the Cornwall Calendars application
Programming with Microsoft Visual Basic .NET, Second Edition

31. Coding the Cornwall Calendars Application (continued)
Figure 11-22: TOE chart for the Cornwall Calendars application (continued)
Programming with Microsoft Visual Basic .NET, Second Edition

32. Creating the Check Class
The Check class will contain four properties and two methods
The Cornwall Calendars application will use the Check class to create a Check object
It will store the user input in the object’s properties
It will use the object’s methods to initialize the Private variables and save the check information to a sequential access file
Programming with Microsoft Visual Basic .NET, Second Edition

33. Creating the Check Class (continued)
Figure 11-23: Pseudocode for the Check class
Programming with Microsoft Visual Basic .NET, Second Edition

34. Coding the uiSaveButton Click Event Procedure
Figure 11-31: Pseudocode for the uiSaveButton’s Click event procedure
Programming with Microsoft Visual Basic .NET, Second Edition

35. Coding the uiSaveButton Click Event Procedure (continued)
The selection structure in the pseudocode determines whether the user entered the check information—in this case, the check number, date, payee, and amount
If the user neglected to enter one or more of the items, the selection structure’s false path should display an appropriate message
Programming with Microsoft Visual Basic .NET, Second Edition

36. Coding the uiSaveButton Click Event Procedure (continued)
Figure 11-34: Message box that appears when the user does not enter all of the check information
Programming with Microsoft Visual Basic .NET, Second Edition

37. Summary
Two versions of the syntax used to create (instantiate) an object from a class:
Version 1: {Dim | Private} objectVariable As class objectVariable = New class
Version 2: {Dim | Private} objectVariable As New class
To access the properties of an object, use the syntax: objectVariable.property
Programming with Microsoft Visual Basic .NET, Second Edition

38. Summary (continued)
To create a Public property, use a Property procedure
The Get block allows an application to retrieve the contents of the Private variable associated with the property
The Set block allows an application to assign a value to the Private variable associated with the property
Programming with Microsoft Visual Basic .NET, Second Edition

39. Summary (continued)
A constructor that has no parameters is called the default constructor
To include more than one constructor in a class, each constructor’s parameters must be different from any other constructor in the class
To include data validation in a class, place the data validation in the Set block of a Property procedure
Programming with Microsoft Visual Basic .NET, Second Edition