1. Chapter 3: Variables, Constants, and Calculations
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Chapter 3: Variables, Constants, and Calculations
Objectives
Distinguish between variables, constants, and controls.
Differentiate among the various data types.
Apply naming conventions and indicating the data type.
Declare variables and constants.
Select the appropriate scope for a variable.
Convert text input to numeric values.
Perform calculations using variables and constants.
Convert between numeric data types using implicit and explicit conversions.
Round decimal values using the Decimal.Round method.
Format values for output using the ToString method.
Use Try/Catch blocks for error handling.
Display message boxes with error messages.
Accumulate sums and generate counts.
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2. Chapter 3: Data - Variables & Constants
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Variable
Memory locations that hold data that can be changed during project execution
Example: customer’s name
Named Constant
Memory locations that hold data that cannot be changed during project execution
Example: sales tax rate
When you declare a variable or a named constant, VB reserves an area of memory and assigns it a name, called an identifier.
Identifier names are specified according to the rules of Basic as well as some recommended naming conventions.
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3. Chapter 3: Data - Variables & Constants
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In Visual Basic, when you declare a Variable or Named Constant
An area of memory is reserved
A name is assigned called an Identifier
Follow rules and naming conventions
Use Declaration Statements to establish Variables and Constants,
Assign name and data type,
Not executable unless initialized on same line
Example:
' Declare a string variable.
Dim NameString As String
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4. Chapter 3: Data Types REV 01 Data Type Use For Storage Size in bytes
Boolean
True or False value 2
Byte
0 to 255, binary data 1
Clear
Single Unicode character
Date
1/1/0001 through 12/31/9999 8
Decimal
Decimal fractions, such as dollars/cents 16
Single
Single precision floating-point numbers with six digits of accuracy 4
Double
Double precision floating-point numbers with 14 digits of accuracy
Short
Small integer in the range -32,768 to 32,767
Integer
Whole numbers in the range -2,147,483,648 to +2,147,483,647
Long
Larger whole numbers
String
Alphanumeric data: letters, digits, and other characters
Varies
Object
Any type of data
The data type of a variable or constant indicates what type of information will be stored in the allocated memory space.
The data type charts displays the kind of data each type of data types hold, and the amount of memory allocated.
The most common types of variables and constants are String, Integer, and Decimal.
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5. Chapter 3: Naming Variables and Constants
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Chapter 3: Naming Variables and Constants
Must follow Visual Basic Naming Rules
Should follow Naming Conventions
Meaningful names consisting of letters, digits, and underscores; must begin with a letter and no spaces or periods. Include class (data type) of variable (QUOTA_Integer)
Use mixed case for variables and uppercase for constants (quantityInteger).
Cannot use reserved words or keywords to which Basic has assigned a meaning, such as print, name, and value
A programmer has to name (identify) the variables and named constants that will be used in a project.
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6. Chapter 3: Constants Named Intrinsic
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Chapter 3: Constants
Named
User assigned name, data type, and value
Use CONST keyword to declare.
Intrinsic
System defined within Visual Studio
Const COMPANY_ADDRESS_String As String = "101 S. Main Street"
Const SALES_TAX_RATE_Decimal As Decimal = .08D
Constants provide a way to use words to describe a value that doesn’t change.
Constants are declared using the keyword and are given a name, a data type, and a value.
Once a value is declared as a constant, its value can’t be changed during the execution of the project.
Data type declared and data type of the value must match.
Many sets of intrinsic constants (key term) are declared in system class libraries and are available for use in VB programs.
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7. Chapter 3: Assigning Values to Constants
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Chapter 3: Assigning Values to Constants
Declare the data type of numeric constants by appending a type-declaration character.
Decimal D
Decimal – D
Double R
Double – R
Integer I
Integer – I
Long L
Long – L
Short S
Single F
Single – F
If a type-declaration character is not appended, any whole number is assumed to be Integer and any fractional value is assumed to be Double.
Use two quotes with a string literal to avoid confusion.
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8. Chapter 3: Declaring Variables
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Chapter 3: Declaring Variables
Declared inside a procedure using a Dim statement
Declared outside a procedure using Public, Private, or Dim statements
Always declare the variable’s data type.
May declare several variables with one statement.
Use IntelliSense to assist in writing statements.
Inside a procedure you must use the Dim statement.
Declaration Statements—General Form:
Public|Private|Dim Identifier [As Datatype]
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9. Chapter 3: Declaration Statement Examples
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Chapter 3: Declaration Statement Examples
Dim customerNameString
As String
Private totalSoldInteger
As Integer
Dim temperatureSingle
As Single
Dim priceDecimal
As Decimal
Private priceDecimal
VB’s IntelliSense feature helps you enter Private, Public, and Dim statements—after you type the space that follows VariableName As, a list pops up and displays the possible entries for data type to complete the statement.
If you begin to complete the statement the list automatically scrolls to the correct section; when the correct entry is highlighted press Enter, Tab, or the spacebar to select the entry, or double-click if using the mouse.
The reserve word Dim is really short for dimension, which means size. When declaring a variable, the amount of memory reserved depends on its data type.
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10. Chapter 3: Scope and Lifetime of Variables
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Chapter 3: Scope and Lifetime of Variables
Visibility of a variable is its scope.
Scope may be
Namespace
Module level
Local
Block level
Lifetime of a variable is the period of time the variable exists.
A variable may exist and be visible for an entire project, for only one form, or for only one procedure.
Visibility really means “this variable can be used or ‘seen’ in this location.”
Namespace — Available to all procedures of project
Module — Available to all procedures within that module (often a form)
Use Public or Private keywords
Local — Available only to the procedure in which it is declared
Block — Available only in block of code inside a procedure where declared
Previous versions of VB and some other programming languages refer to namespace variables as global variables.
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11. Chapter 3: Module Level Variable Declaration Example
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Chapter 3: Module Level Variable Declaration Example
Code module-level declarations in the Declaration section at the top of the code.
To enter module-level declarations, you must be in the Editor window at the top of your code. Place the Private and Const statements after the
Class declaration but before your first procedure.
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12. Chapter 3: Calculations
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Chapter 3: Calculations
Calculations can be performed with variables, constants, properties of certain objects, and numeric literals.
Do not use strings in calculations.
Values from Text property of Text Boxes
Are strings, even if they contain numeric data
Must be converted to a numeric data type before performing a calculation
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13. Chapter 3: Converting Strings to a Numeric Data Type
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Chapter 3: Converting Strings to a Numeric Data Type
Use Parse methods to convert the Text property to its numeric form before it’s used in a calculation.
Each numeric data type class has a Parse method.
Parse method returns a value that can be used in calculations.
Parse method fails if user enters nonnumeric data or leaves data blank.
The class that you use depends on the data type of the variable to which you are assigning a value; for example, to convert text to an integer, use the Integer.Parse method, to convert to a decimal value, use Decimal.Parse. Pass the text string that you want to convert as an argument of the Parse method.
Example:
' Convert input values to numeric variables.
QuantityInteger = Integer.Parse(QuantityTextBox.Text)
PriceDecimal = Decimal.Parse(PriceTextBox.Text)
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14. Chapter 3: Converting to String
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Chapter 3: Converting to String
Values assigned to string variables or Text properties must be string.
Convert any numeric data type to string using .ToString method.
Examples:
ResultTextBox.Text = ResultDecimal.ToString()
CountTextBox.Text = CountInteger.ToString()
IDString = IDInteger.ToString()
When you assign a value to a variable, you must take care to assign like types; you assign an integer value to an Integer variable and a decimal value to a Decimal variable.
There are some exceptions to the rule about assigning only like types — See “Implicit Conversions.”
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15. Chapter 3: Conversion Methods
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Chapter 3: Conversion Methods
Method Convert To
Integer.Parse Integer
Decimal.Parse Decimal
.ToString String
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16. Chapter 3: Conversion Examples
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Chapter 3: Conversion Examples
QuantityInteger =Integer.Parse(quantityTextBox.Text)
PriceDecimal =Decimal.Parse(priceTextBox.Text)
WholeNumberInteger =Integer.Parse(digitString)
ResultTextBox.Text =ResultDecimal.ToString( )
CountTextBox.Text =CountInteger.ToString( )
IDString =IDInteger.ToString( )
The Parse methods examine the value stored in the argument and attempts to convert it to a number in a process called parsing, which means to pick apart, character by character, and convert to another format.
In case you need to convert to Long, Single, or Double, VB has a Parse method for each of those data type classes.
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17. Chapter 3: Arithmetic Operations
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Chapter 3: Arithmetic Operations
Operator Operation
+ Addition
– Subtraction
* Multiplication
/ Division
\ Integer Division
Mod Modulus – Remainder of division
^ Exponentiation
The arithmetic operations you can perform in VB include addition, subtraction, multiplication, division, integer division, modulus, and exponentiation. The first four operations are self explanatory, but you may not be familiar with Integer Division, Modulus, and/or exponentiation.
Integer division — use to divide one integer by another giving an integer result, truncating (dropping) any remainder
Modulus — returns the remainder of a division problem
Exponentiation — raises a number to the power specified and returns (produces) a result of the Double data type.
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18. Chapter 3: Order of Operations
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Chapter 3: Order of Operations
Hierarchy of operations, or order of precedence, in arithmetic expressions from highest to lowest
1. Any operation inside parentheses
2. Exponentiation
3. Multiplication and division
4. Integer division
5. Modulus
6. Addition and subtraction
To change the order of evaluation, use parentheses.
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19. Chapter 3: Evaluation of Expression
1. All operations within parentheses.
2. All exponentiation. Multiple exponentiation operations are performed from left to right.
3. All multiplication and division. Multiple operations are performed from left to right.
4. All integer division. Multiple operations are performed from left to right.
5. Mod operations. Multiple operations are performed from left to right.
6. All addition and subtraction are performed from left to right.
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20. Chapter 3: Mathematical Examples
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Chapter 3: Mathematical Examples
Note the use of parentheses to control order of precedence.
3+4*2 = 11 Multiply then add
(3+4)*2 = 14 Parentheses control: add then multiply
8/4*2 = 4 Same level, left to right: divide then multiply
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21. Chapter 3: Using Calculations in Code
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Chapter 3: Using Calculations in Code
Perform calculations in assignment statements.
What appears on right side of assignment operator is assigned to item on left side.
Assignment operators — allows shorter versions of code =, +=, -=, *=, /=, \=, &=
The assignment operators that you will use most often are += and – =.
‘Accumulate a total.
TotalSalesDecimal += salesDecimal
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22. Chapter 3: Option Explicit and Option Strict
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Chapter 3: Option Explicit and Option Strict
Option Explicit forces variables to be declared before using.
Option Strict
Makes VB a strongly typed language like C++, Java and C#
Does not allow implicit conversions from a wider data type to a narrower one or between String and numeric data types
Best practice to always turn on either in code or in Project Properties dialog box
VB provides two options that can significantly change the behavior of the editor and compiler.
Not using either Option Explicit and/or Option Strict can make coding somewhat easier, but provides opportunities for hard-to-find errors and very sloppy programming.
When Option Explicit is turned off, programmers can use any variable name without first declaring it, which is a throwback to the older version of Basic and programmers spent many hours debugging programs that had small typos in a variable name.
Programmers should always program with Option Explicit turned on-in VB.NET the option is turned on by default for all new projects. Programmers will need to place an additional code line before the first line of code in a file to turn it off.
When Option Strict is turned on, the editor and compiler try to help keep programmers from making hard-to-find mistakes.
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23. Chapter 3: Converting Between Numeric Data Types
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Chapter 3: Converting Between Numeric Data Types
Implicit (automatic) conversion
Converts value from narrower data type to wider type where no danger of losing precision exists
Explicit conversion (casting)
Uses methods of Convert class to convert between data types
Convert Class has methods that begin with “To” for each of the data types.
In VB, you can convert data from one numeric data type to another—some conversions can be performed implicitly (automatically) and some you must specify explicitly.
Implicit:
Example:
BigNumberDouble = SmallNumberInteger
does not generate any error message, assuming that both variables are properly declared
Explicit:
Examples:
NumberDecimal = Convert.ToDecimal(NumberSingle)
ValueInteger = Convert.ToInt32(ValueDouble)
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24. Chapter 3: Performing Calculations with Unlike Data Types
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Chapter 3: Performing Calculations with Unlike Data Types
VB performs the calculations using the wider data type.
Use a cast if converting the result to a different data type.
Eg:
Convert.ToInt32(CountInteger / NumberDecimal) or Convert.ToSingle(CountInteger / NumberDecimal).
VB does not convert to a different data type until it is necessary.
The expression countInteger / 2* amount Decimal is evaluated as integer division for countInteger /2, producing an integer intermediate result; then the multiplication is performed on the integer and decimal value (amountDecimal), producing a decimal result.
The methods of the Convert class use the CLR data types rather than the VB data types. Use Int32 for Integer, Int16 for Short, and Int64 for Long.
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25. Chapter 3: Rounding Numbers
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Chapter 3: Rounding Numbers
Round decimal fractions
Decimal.Round method returns a decimal result rounded to a specified number of decimal positions.
Decimal.Round and Convert methods use technique called “rounding toward even.”
See the Appendices for additional mathematical, financial, and string functions
Example:
' Round to two decimal positions.
ResultDecimal = Decimal.Round(AmountDecimal, 2)
Decimal Value to Round
Number of Decimal Positions
Results
1,455 2
1.46
1.44
1.5
2.5
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26. Chapter 3: Formatting Data for Display
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Chapter 3: Formatting Data for Display
To display numeric data in a label or text box, first convert value to string.
Use ToString method
Format the data using formatting codes.
Specifies use of dollar sign, percent sign, and commas
Specifies number of digits that appear to right of decimal point
When wanting to display numeric data in the Text property of a label or text box, the value must first be converted to string—the data can be formatted, which controls the way the output will look.
DisplayTextBox.Text = NumberInteger.ToString()
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27. Chapter 3: Using Format Specifier Codes
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Chapter 3: Using Format Specifier Codes
"C" code
Currency — String formatted with dollar sign, commas separating each group of 3 digits and 2 digits to the right of decimal point
"N" code
Number — String formatted with commas separating each group of 3 digits and 2 digits to the right of decimal point
Can specify number of decimal positions
Example: "C0" zero digits
The format specifier codes format the display of output and are predefined.
The default format of each of the formatting codes is based on the computer’s regional setting.
Format specifier codes are displayed on the next slide (Slide 30) and examples are shown on the following slide (Slide 31).
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28. Chapter 3: Format Specifier Codes
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Chapter 3: Format Specifier Codes
Format Specifier Codes
Name
C or c
Currency
F or f
Fixed-point
N or n
Number
D or d
Digits
P or p
Percent
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29. Chapter 3: Format Specifier Code Examples
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Chapter 3: Format Specifier Code Examples
Variable
Value
Code
Output
totalDecimal
"C"
$1,125.67
"N0"
1,126
pinInteger
123
"D6"
000123
rateDecimal
0.075
"P"
7.50%
"P3"
7.500%
"P0" 8%
valueInteger
-10
($10.00)
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30. Chapter 3: Date Specifier Code
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Chapter 3: Date Specifier Code
Format DateTime values using format codes and ToString method.
Date codes are case sensitive.
You can use methods of the DateTime structure for formatting dates:ToLongDateString, ToShortDateString, ToLongTimeString,
ToShortTimeString.
See Appendix B or MSDN for additional information.
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31. Chapter 3: Handling Exceptions
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Chapter 3: Handling Exceptions
Use structured exception handling to easily catch errors before run-time error occurs.
Catching exceptions is referred to as error trapping.
Coding to handle exception is called error handling.
Error handling in Visual Studio.NET is standardized for all languages using the Common Language Runtime, CLR, which improves on previous versions of VB.
When users input numbers and use those numbers in calculations, lots of things can go wrong—for example, the Parse methods, Integer.Parse and Decimal.Parse fail if the user enters nonnumeric data or leaves the text box blank.
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32. Chapter 3: Try/Catch Blocks
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Chapter 3: Try/Catch Blocks
Enclose statements that might cause an error within Try/Catch block.
If an exception occurs while statements in the Try block are executing, program control is transferred to the Catch Block.
If a Finally statement is included, the code in that section executes last, whether or not an exception occurred.
The Try-Block General Form is shown on the next slide (Slide 35).
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33. Chapter 3: Try Block - General Form
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Chapter 3: Try Block - General Form
Try
‘statements that may cause an error
Catch [VariableName As ExceptionType]
‘statements for action when an exception occurs
[Finally
‘statements that always execute before exit of the Try block]
End Try
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34. Chapter 3: Try Block - Example Catches Any Exception
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Chapter 3: Try Block - Example Catches Any Exception
Try
QuantityInteger = Integer.Parse(QuantityTextBox.Text)
QuantityTextBox.Text = QuantityInteger.ToString( )
Catch
MessageLabel.Text = "Error in input data."
End Try
The Catch will catch any exception. You also can specify the type of exception that you want to catch, and even
write several Catch statements, each to catch a different type of exception. For example, you might want to display one message for bad input data and a different message for a calculation problem.
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35. Chapter 3: Try Block - Example Catches Specific Exception
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Chapter 3: Try Block - Example Catches Specific Exception
This Catch statement catches bad input data that cannot be converted to numeric.
To catch bad input data that cannot be converted to numeric, write the above Catch statement.
Catch theException As FormatException
MessageLabel.Text="Error in input data."
End Try
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36. Chapter 3: Common Exception Classes
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Chapter 3: Common Exception Classes
Each exception is an instance of the Exception class. The properties of this class allow you to determine the code location of the error, the type of error, and cause.
*Refer to the Table 3.2 for Common Exception Classes examples.
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37. Chapter 3: Try Block - Example Handling Multiple Exceptions
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Chapter 3: Try Block - Example Handling Multiple Exceptions
Catch TheException As FormatException
' Statements for nonnumeric data.
Catch TheException As ArithmeticException
' Statements for calculation problem.
Catch TheException As Exception
' Statements for any other exception.
If wanting to trap for more than one type of exception, multiple Catch blocks (handlers) can be included. When an exception occurs, the Catch statements are checked in sequence.
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38. Chapter 3: MessageBox Object
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Chapter 3: MessageBox Object
The MessageBox is an overloaded method.
Signatures correspond to the argument list.
There are multiple signatures to choose from.
Do not reverse, transpose, or leave out any of the arguments.
IntelliSense displays argument list (also called signatures).
Two sample message boxes created with the MessageBox clas.
MessageBox.Show (TextMessage, TitlebarText, _ MessageBoxButtons, MesssageBoxIcon)
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39. Chapter 3: MessageBox Object
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Chapter 3: MessageBox Object
TextMessage string
String literal or variable that displays message
Title Bar text
String that appears in title bar of message box
MessageBox Buttons
OK, OKCancel, RetryCancel, YesNo, YesNoCancel, AbortRetryIgnore
MessageBox Icons
Asterisk, Error, Exclamation, Hand, Information, None, Question, Stop, Warning
General Form:
MessageBox.Show(TextMessage)
MessageBox.Show(TextMessage, TitlebarText)
MessageBox.Show(TextMessage, TitlebarText, MessageBoxButtons)
MessageBox.Show(TextMessage, TitlebarText, MessageBoxButtons, MessageBoxIcon)
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40. Chapter 3: Using Overloaded Methods
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Chapter 3: Using Overloaded Methods
This OOP feature allows the Show method to act differently for different arguments.
Each argument list is called a signature so the Show method has several signatures.
Supplied arguments must exactly match one of the signatures provided by the method.
IntelliSense in Visual Studio editor helps when entering arguments so that they don’t need to be memorized.
When you call the Show method, the arguments that you supply must exactly match one of the signatures provided by the method.
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41. Chapter 3: Testing Multiple Fields
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Chapter 3: Testing Multiple Fields
Each input field presents an opportunity for an exception.
To indicate specific fields that caused the exception, use nested Try/Catch blocks.
Pinpoints specific errors, and after error, sets focus back to field in error
Use SelectAll method of text box to make text appear selected to aid user.
When you have more than one input field, each field presents an opportunity for an exception.
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42. Chapter 3: Counting & Accumulating Sums
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Chapter 3: Counting & Accumulating Sums
Declare module-level variables, since local level variables reset to 0 each time the procedure is called.
Summing Numbers
Counting
Calculating an Average
DiscountedPriceSumDecimal += DiscountedPriceDecimal
Programs often need to calculate the sum of numbers.
The technique for summing is to declare a module-level variable for the total.
If you want to count something, you need another module-level variable.
To calculate an average, divide the sum of the items by the count of the items.
Private saleCountInteger As Integer
saleCountInteger += 1
AverageDiscountedSaleDecimal = DiscountedPriceSumDecimal / SaleCountInteger
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