1. Chapter 3: Using Variables and Constants
Programming with Microsoft Visual Basic .NET, Second Edition

2. Creating Variables and Named Constants Lesson A Objectives
Create a procedure-level and module-level variable
Select an appropriate data type for a variable
Select an appropriate name for a variable
Programming with Microsoft Visual Basic .NET, Second Edition

3. Creating Variables and Named Constants Lesson A Objectives (continued)
Assign data to an existing variable
Explain the scope and lifetime of a variable
Create a named constant
Programming with Microsoft Visual Basic .NET, Second Edition

4. Previewing the Completed Application
To preview the completed Skate-Away Sales application:
Use the Run command on the Start menu to run the Skate (Skate.exe) file contained in the VBNET\Chap03 folder
An order form similar to the one that you created in Chapter 2 appears on the screen
Programming with Microsoft Visual Basic .NET, Second Edition

5. Using Variables to Store Information
Besides storing data in the properties of controls, a programmer also can store data, temporarily, in memory locations inside the computer
The memory locations are called variables, because the contents of the locations can change as the program is running
Programming with Microsoft Visual Basic .NET, Second Edition

6. Using Variables to Store Information (continued)
One use for a variable is to hold information that is not stored in a control on the user interface
You can also store the data contained in a control’s property in a variable
Programming with Microsoft Visual Basic .NET, Second Edition

7. Selecting a Data Type for a Variable
Memory Required
Byte
1 byte
Short
2 bytes
Char
Integer
4 bytes
Boolean
Long
8 bytes
Decimal
16 bytes
Single
Double
String
Varies
Date
Object
Programming with Microsoft Visual Basic .NET, Second Edition

8. Selecting a Data Type for a Variable (continued)
Short, Integer, Long
Store whole numbers
Single, Double
Store floating-point numbers
Decimal
Stores numbers with a decimal point
Boolean
Stores True and False
Char
Stores one Unicode character
Byte
Stores 8-bits of data
Date
Stores date and time information
String
Stores a sequence of characters
Programming with Microsoft Visual Basic .NET, Second Edition

9. Selecting a Name for a Variable
The naming convention used in this book:
The name indicates only the variable’s purpose and is entered using lowercase letters
Use camel casing: if a variable’s name contains two or more words, you capitalize the first letter in the second and subsequent words
The name assigned to a variable must follow the rules listed in Figure 3-4
Programming with Microsoft Visual Basic .NET, Second Edition

10. Selecting a Name for a Variable (continued)
Figure 3-4: Rules for variable names along with examples of valid and invalid names
Programming with Microsoft Visual Basic .NET, Second Edition

11. Declaring a Variable
You use a declaration statement to declare, or create, a variable
Syntax: {Dim | Private | Static} variablename [As datatype][= initialvalue]
Examples:
Dim hoursWorked As Integer
Dim dataOk As Boolean = True
Dim name As String, age As Integer
Programming with Microsoft Visual Basic .NET, Second Edition

12. Assigning Data to an Existing Variable
You use an assignment statement to assign a value to a variable while an application is running
Syntax: variablename = value
Examples:
quantityOrdered = 500
firstName = “Mary”
state = Me.uiStateTextBox.Text
discountRate = .03
Programming with Microsoft Visual Basic .NET, Second Edition

13. Assigning Data to an Existing Variable (continued)
A literal constant is an item whose value does not change while the application is running
String literal constants are enclosed in quotation marks, but numeric literal constants and variable names are not
A literal type character forces a literal constant to assume a data type other than the one its form indicates
Programming with Microsoft Visual Basic .NET, Second Edition

14. Assigning Data to an Existing Variable (continued)
Figure 3-7: Literal type characters
Programming with Microsoft Visual Basic .NET, Second Edition

15. Assigning Data to an Existing Variable (continued)
A variable can store only one item of data at any one time
When you use an assignment statement to assign another item to the variable, the new data replaces the existing data
After data is stored in a variable, you can use the data in calculations
Programming with Microsoft Visual Basic .NET, Second Edition

16. The Parse Method
Every numeric data type in Visual Basic .NET has a Parse method that can be used to convert a string to that numeric data type
Syntax: numericDataType.Parse(string)
Example: Dim sales As Decimal sales = Decimal.Parse(Me.uiSalesTextBox.Text)
Programming with Microsoft Visual Basic .NET, Second Edition

17. The Convert Class
The Convert class contains methods to convert a numeric value to a specified data type
Syntax: Convert.method(value)
Example: Dim purchase As Double = Dim tax As Decimal tax = Convert.ToDecimal(purchase) * .03D
Programming with Microsoft Visual Basic .NET, Second Edition

18. The Convert Class (continued)
Figure 3-9: Most commonly used methods contained in the Convert class
Programming with Microsoft Visual Basic .NET, Second Edition

19. The Scope and Lifetime of a Variable
A variable’s scope indicates which procedures in an application can use the variable
The scope is determined by where the Dim, Public or Private statement is entered
When you declare a variable in a procedure, the variable is called a procedure-level variable and is said to have procedure scope
Programming with Microsoft Visual Basic .NET, Second Edition

20. The Scope and Lifetime of a Variable (continued)
When you declare a variable in the form’s Declarations section, it is called a module-level variable and is said to have module scope
Block-level variables are declared within specific blocks of code, such as within If...Then...Else statements or For...Next statements
Programming with Microsoft Visual Basic .NET, Second Edition

21. The Scope and Lifetime of a Variable (continued)
Creating a procedure-level variable
Created with the Dim keyword
The Dim statement is entered in an object’s event procedure
Only the procedure in which it is declared can use the variable
Removed from memory when the procedure ends
Programming with Microsoft Visual Basic .NET, Second Edition

22. The Scope and Lifetime of a Variable (continued)
Creating a module-level variable
Created with the Private keyword
Entered in a form’s Declarations section
Can be used by any of the procedures in the form
Removed from memory when the application ends or the form is destroyed
Programming with Microsoft Visual Basic .NET, Second Edition

23. Named Constants
A memory location whose contents cannot be changed while the program is running
You create a named constant using the Const statement
Syntax: Const constantname [As datatype] = expression
Example:
Const PI As Double =
Programming with Microsoft Visual Basic .NET, Second Edition

24. Option Explicit and Option Strict
Visual Basic .NET provides a way to prevent you from using undeclared variables in your code
Enter the statement Option Explicit On in the General Declarations section of the Code Editor window
Programming with Microsoft Visual Basic .NET, Second Edition

25. Option Explicit and Option Strict (continued)
To eliminate the problems that occur as a result of implicit type conversions
Enter the Option Strict On statement in the General Declarations section of the Code Editor window
Programming with Microsoft Visual Basic .NET, Second Edition

26. Option Explicit and Option Strict (continued)
Type conversion rules used when the Option Strict On statement is used:
Strings will not be implicitly converted to numbers, and numbers will not be implicitly converted to strings
Lower-ranking data types will be implicitly promoted to higher-ranking types
Higher-ranking data types will not be implicitly demoted to lower-ranking data types; rather, a syntax error will occur
Programming with Microsoft Visual Basic .NET, Second Edition

27. Modifying the Skate-Away Sales Application Lesson B Objectives
Include a procedure-level and module-level variable in an application
Concatenate strings
Get user input using the InputBox function
Include the ControlChars.NewLine constant in code
Designate the default button for a form
Programming with Microsoft Visual Basic .NET, Second Edition

28. Storing Information Using Variables
You need to revise the Skate-Away Sales application’s TOE chart and the pseudocode for the Calculate Order button
The uiCalcButton control’s Click event procedure now has two more tasks to perform:
It must calculate the sales tax
It must display the message, sales tax, and salesperson’s name in the uiMessageLabel control
Programming with Microsoft Visual Basic .NET, Second Edition

29. Storing Information Using Variables (continued)
Two additional objects (OrderForm and uiMessageLabel) are included in the revised TOE chart
The OrderForm’s Load event procedure is responsible for getting the salesperson’s name when the application is started
The uiMessageLabel control will display the message, sales tax, and salesperson’s name
Programming with Microsoft Visual Basic .NET, Second Edition

30. Storing Information Using Variables (continued)
As the revised TOE chart indicates, you need to:
Change the code in the uiCalcButton’s Click event procedure
Code the form’s Load event procedure
Programming with Microsoft Visual Basic .NET, Second Edition

31. Modifying the Calculate Order Button’s Code
You will first remove the existing code from the Calculate Order button’s Click event procedure
You then will recode the procedure using variables in the equations
Figure 3-18 shows the revised pseudocode for the Calculate Order button’s Click event procedure (changes made to the original pseudocode are shaded in the figure)
Programming with Microsoft Visual Basic .NET, Second Edition

32. Modifying the Calculate Order Button’s Code (continued)
Figure 3-18: Revised pseudocode for the Calculate Order button’s Click event procedure
Programming with Microsoft Visual Basic .NET, Second Edition

33. Concatenating Strings
Connecting strings together is called concatenating
Use the concatenation operator, which is the ampersand (&), to concatenate strings in Visual Basic .NET
When concatenating strings, be sure to include a space before and after the concatenation operator
Programming with Microsoft Visual Basic .NET, Second Edition

34. Concatenating Strings (continued)
Example
Result
firstName & lastName
SueChen
firstName & “ “ & lastName
Sue Chen
lastName & “, “ & firstName
Chen, Sue
“She is “ & Convert.ToString(age) & “!”
She is 21!
“She is “ & age & “!”
Programming with Microsoft Visual Basic .NET, Second Edition

35. The InputBox Function
The InputBox function displays one of Visual Basic .NET’s predefined dialog boxes
Syntax: InputBox(prompt[, title][, defaultResponse])
Use sentence capitalization for the prompt, and book title capitalization for the title
Has limitations: can’t control appearance and allows user to enter only one piece of data
Programming with Microsoft Visual Basic .NET, Second Edition

36. The InputBox Function (continued)
Figure 3-29: Example of a dialog box created by the InputBox function
Programming with Microsoft Visual Basic .NET, Second Edition

37. The NewLine Character
The NewLine character, which is Chr(13) & Chr(10), instructs the computer to issue a carriage return followed by a line feed
The ControlChars.NewLine constant advances the insertion point to the next line on the screen
The ControlChars.NewLine constant is an intrinsic constant, which is a named constant that is built into Visual Basic .NET
Programming with Microsoft Visual Basic .NET, Second Edition

38. Designating a Default Button
Can be selected by pressing the Enter key even when the button does not have the focus
Set the form’s AcceptButton property to the desired button
If used, it is typically the first button
If a button’s action is destructive and irreversible, then it should not be the default button
Programming with Microsoft Visual Basic .NET, Second Edition

39. Modifying the Skate-Away Sales Application’s Code Lesson C Objectives
Include a static variable in code
Code the TextChanged event procedure
Create a procedure that handles more than one event
Programming with Microsoft Visual Basic .NET, Second Edition

40. Modifying the Code in the Load and uiCalcButton Click Procedures
Mr. Cousard would like to have the order form ask for the salesperson’s name each time an order is calculated
Before making modifications, you should review the application’s documentation and revise the necessary documents
Figure 3-44 shows the revised pseudocode for the Calculate Order button’s Click event procedure
Programming with Microsoft Visual Basic .NET, Second Edition

41. Modifying the Code in the Load and uiCalcButton Click Procedures (continued)
Figure 3-44: Revised pseudocode for the Calculate Order button
Programming with Microsoft Visual Basic .NET, Second Edition

42. Static Variables
A static variable is a local variable that retains its value when the procedure in which it is declared ends
Syntax: Static variablename [As datatype] [= initialvalue]
Removed from memory when application ends or form is removed from memory
Programming with Microsoft Visual Basic .NET, Second Edition

43. Coding the TextChanged Event Procedure
A control’s TextChanged event occurs when the contents of a control’s Text property change
This can happen as a result of either the user entering data into the control, or the application’s code assigning data to the control’s Text property
Programming with Microsoft Visual Basic .NET, Second Edition

44. Associating a Procedure with Different Objects or Events
The keyword Handles appears in a procedure header and indicates the object and event associated with the procedure
You can associate a procedure with more than one object and event
To do so: list each object and event, separated by commas, in the Handles section of the procedure header
Programming with Microsoft Visual Basic .NET, Second Edition

45. Summary
The syntax of a variable declaration statement is: {Dim | Private | Static} variablename [As datatype][= initialvalue]
To create a procedure-level variable, enter the variable declaration statement in a procedure
To create a module-level variable, enter the variable declaration statement in a form’s Declarations section
Programming with Microsoft Visual Basic .NET, Second Edition

46. Summary (continued)
To concatenate strings, use the concatenation operator —the ampersand (&)
To display a dialog box containing a prompt, an input area, an OK button, and a Cancel button, use the InputBox function, whose syntax is InputBox(prompt[, title][, defaultResponse])
To make a button the default button, set the form’s AcceptButton property to the name of the button
Programming with Microsoft Visual Basic .NET, Second Edition

47. Summary (continued)
To create a static variable, use a declaration statement that follows the syntax: Static variablename [As datatype] [= initialvalue]
To process code when the contents of a control have changed, enter the code in the control’s TextChanged event
To create a procedure for more than one object or event, list each object and event after the Handles keyword in the procedure
Programming with Microsoft Visual Basic .NET, Second Edition