t u t o r i a l 2009 pearson education, inc. all rights reserved. 1 15 fund raiser application...

T U T O R I A L

2009 Pearson Education, Inc. All rights reserved.

1

15Fund Raiser

ApplicationIntroducing Scope,

Pass-by-Reference andOption Strict


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Outline

15.1 Test-Driving the Fund Raiser Application

15.2 Constructing the Fund Raiser Application

15.3 Passing Arguments: Pass-by-Value vs.Pass-by-Reference

15.4 Option Strict


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In this tutorial you will learn: ■ Create variables that can be used in all the Form’s procedures.

■ Pass arguments by reference, using ByRef, so that the called procedure can modify the caller’s variables.

■ Eliminate subtle data-type errors by enabling Option Strict in your projects.

■ Change a value from one data type to another, using methods of class Convert.

Objectives

Application Requirements


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15.1 Test-Driving the Fund Raiser Application

An organization is hosting a fund raiser to collect donations. A portion of each donation is used to cover the operating expenses of the organization—the rest of the donationgoes to the charity. Create an application that allows the organization to keep track of the total amount of money raised. The application should deduct 17% of each donation for operating costs—the remaining 83% is given to the charity. The application should display the amount of each donation after the 17% for operating expenses isdeducted—it also should display the total amount raisedfor the charity for all donations up to that point.


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Test-Driving the Fund Raiser Application

■ Run the completed application (Fig. 15.1)

Figure 15.1 | Fund Raiser application’s Form.


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Test-Driving the Fund Raiser Application (Cont.)

■ The application calculates the amount of the donation after the operating expenses have been deducted and displays the result in the After expenses: field (Fig. 15.2).

Figure 15.2 | Fund Raiser application’s Form with first donation entered.


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Test-Driving the Fund Raiser Application (Cont.)

■ Enter more donations, and note that the total raised increases with each additional donation (Fig. 15.3).

Figure 15.3 | Making further donations.

Total of all donations(minus expenses)


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When the user changes the current donation amount in the TextBox:

Clear Label that displays amount of current donation that goes toward

charity

When the user clicks the Make Donation Button:

Obtain amount of current donation from TextBox

Call function CalculateDonation to calculate amount of current donation that

goes toward charity (amount after operating costs)

Display amount of current donation that goes toward charity

Update total amount raised for charity (from all donations received)

Display total amount raised for charity

When the CalculateDonation procedure gets called:

Calculate operating costs (multiply the donated amount by the operating-cost percentage)

Calculate amount of donation that goes toward charity (subtract operating costs from donated amount)

15.2 Constructing the Fund Raiser Application


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■ Now use an ACE table to convert the pseudocodeto Visual Basic (Fig. 15.4).

Figure 15.4 | Fund Raiser application’s ACE table. (Part 1 of 2.)

Action/Control/Event Table for theFund Raiser Application

Action Control Event/Method

Label all the application’s controls donationLabel, donatedLabel, totalLabel

Application is run

donationTextBox TextChanged

Clear Label that displays amount of

current donation that goes toward

charity

donatedValueLabel

donateButton Click

Obtain amount of current donation from

TextBox donationTextBox

Call function CalculateDonation to

calculate amount of current donation

that goes toward charity


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Action Control Event/Method

Display amount of current donation that

goes toward charity

donatedValueLabel

Update total amount raised for charity

Display total amount raised for charity totalValueLabel

CalculateDonation

Calculate operating costs

Calculate amount of donation that goes

toward charity

Figure 15.4 | Fund Raiser application’s ACE table. (Part 2 of 2.)

Action/Control/Event Table forthe Fund Raiser Application (Cont.)


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Examining Scope with the Fund Raiser Application

■ Open the template application (Fig. 15.5).

Figure 15.5 | Fund Raiser template application’s Form.


12Examining Scope with the

Fund Raiser Application (Cont.)

■ Add lines 2-3 of Fig. 15.6 to FundRaiser.vb.

Figure 15.6 | Declaring an instance variable in class FundRaiserForm.


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■ An instance variable is a variable declared inside a class, but outside any of the class’s procedure definitions.

– All procedures in the same class have access to this variable and can modify its value.

– Instance variables have module scope. Module scope begins at the identifier after keyword Class and terminates at the End Class statement.

Examining Scope with theFund Raiser Application (Cont.)




■ Double click the Make Donation Button to generate its Click event handler (Fig. 15.7).

Figure 15.7 | Adding a Click event handler to the application.




■ Add lines 30-31 of Fig. 15.8 to the event handler.

Figure 15.8 | Declaring local variables in the donateButton_click event handler.

■ Variable donation stores the original donation amount.■ Variable afterCosts stores the donation amount after

the operating expenses have been deducted.


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■ Identifiers that are declared inside a procedure (but outside a control statement) have procedure scope.

– Procedure scope begins at the identifier’s declarationand ends at the last statement of the procedure.

– Identifiers with procedure scope cannot be referenced outside of the procedure in which they are declared.

– A procedure’s parameters also have procedure scope.

■ Identifiers declared inside control statements (such as inside an If...Then statement) have block scope.

– Block scope begins at the identifier’s declaration and endsat the enclosing block’s final statement.



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■ Variables with either procedure scope or block scope are called local variables.

– These variables cannot be referenced outside the procedure or block in which they are declared.

– If a local variable has the same name as an instance variable, the instance variable is hidden by the local variable.

– You can still access the instance variable by precedingits name with the keyword Me and a dot (.).



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Error-Prevention Tip

Hidden variable names can sometimes lead to subtle logic errors. Use unique names for all variables, regardless of scope, to prevent an instance variable from becoming hidden.




■ The constant COSTS, which stores the operating-cost percentage, is “local” to this procedure (Fig. 15.9) and cannot be used elsewhere.

Figure 15.9 | Function procedure CalculateDonation providedin the template application.

Parameter donatedAmount has procedure scope because it is

declared in the procedure header

Local variable netDonation has procedure scope because it is

declared in the procedure body


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■ Replace the constant COSTS with the variable donation, which is declared as a local variablein donateButton_Click.

– Note the jagged line under donation to indicatean error (Fig. 15.10).

– Variables with procedure scope can be accessed and modified only in the procedure in which they are defined.


Figure 15.10 | Demonstrating procedure scope.




■ This code (Fig. 15.11) obtains the donation amount from the donationTextBox.

Figure 15.11 | Obtaining the donation amount.




■ This code (Fig. 15.12) invokes procedure CalculateDonation with the amount donated.

■ The donation amount after operating costs is formatted as a currency string.

Figure 15.12 | Calculating and displaying the donation amount after operating expenses.




■ Add the display code (Fig. 15.13) to the event handler.

Figure 15.13 | Updating and displaying the total amount raised for charity.


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■ Instance variable totalRaised has modulescope, and therefore maintains its value between procedure calls.

■ Variables with procedure scope, such as donation, do not retain their values between procedure calls.

– These must be reinitialized each time their procedureis invoked.





■ When the user enters data into the TextBox, the TextChanged event (Fig. 15.14) occurs and line 23 should clear the previous donation from the After expenses: Label.

Figure 15.14 | Clearing the Donation: TextBox.


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■ The keyword ByVal indicates that an argument will be passed by value.

– The application makes a copy of the argument’s valueand passes the copy to the called procedure.

– Changes made to the copy in the called proceduredo not affect the original variable’s value.

■ The keyword ByRef indicates that an argumentwill be passed by reference.

– The original variable in the calling procedure can be accessed and modified directly by the called procedure.

– This is useful in some situations, such as when a procedure needs to return more than one result.

15.3 Passing Arguments: Pass-by-Valuevs. Pass-by-Reference


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■ Replace line 37 in the event handler with line 37of Fig. 15.15.

■ Note that the second argument is flagged as a compilation error.

Passing Arguments with ByRef in theFund Raiser Application

Figure 15.15 | Passing variable afterCosts by reference.


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■ Delete the CalculateDonation Function procedure (Fig. 15.16) so it can be rewritten.

Figure 15.16 | Function procedure CalculateDonation to be removed.

Passing Arguments with ByRef in theFund Raiser Application (Cont.)

Delete these lines of code


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■ Keyword ByRef (line 7 of Fig. 15.17) indicates that variable netDonation is passed by reference.

– Any changes made to variable netDonation in CalculateDonation affect donateButton_Click’s local variable afterCosts.

– Since CalculateDonation no longer needs to return a value, CalculateDonation is now created as a Sub procedure.


Figure 15.17 | CalculateDonation Sub procedure.


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■ Assigning the calculation result (Fig. 15.18) to variable netDonation actually assigns the valueto local variable afterCosts in donateButton_Click.


Figure 15.18 | Calculating the donation that goes toward charity afteroperating costs have been deducted.


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■ Data types in Visual Basic are divided into two categories:

– A variable of a value type (such as Integer) simply contains a value of that type.

Dim count AsInteger = 7

– A variable of a reference type contains the location wherean object is stored in memory.

– Reference type instance variables are initialized by defaultto the value Nothing.

– Except for type String, primitive types are value types.

15.3 Passing Arguments: Pass-by-Valuevs. Pass-by-Reference (Cont.)


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■ To interact with an object, you must use a variable that references the object:

– to invoke the object’s methods

– to access the object’s properties.

currentTimeLabel.Text=String.Format("{0:hh:mm:sstt}",Date.Now)

■ By default, arguments are passed by value.



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■ When a reference-type variable is passed to a procedure by value, a copy of the object’s location is passed.

■ In effect, the variable is passed by value, but the object to which the variable refers is passed by reference.

■ If you want to change which object a reference-type variable refers to, you could pass that variable to a procedure by reference.



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■ When a computer accesses data, it needs to know its type in order for the data to make sense.

– Imagine you are purchasing a book from an online store that ships internationally.

– You notice that the price for the book is 20, but no currency is associated with the price.

– If the currency is different from the one that you normally use, you need to perform a conversion to get the price.

15.4 Option Strict


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■ Similar conversions occur in an application.– Visual Basic can convert one data type to another, as

long as the conversion “makes sense.”

– Assign an Integer value to a Decimal variable is allowed without writing additional code.

– These types of assignments perform so-called implicit conversions.

– When an attempted conversion does not make sense, such as assigning "hello" to an Integer variable, an error occurs.

15.4 Option Strict (Cont.)


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■ Figure 15.19 lists some of Visual Basic’s data types and their allowed implicit conversions.

Figure 15.19 | Some data types and their allowed implicit conversions.


Data Type Can be implicitly converted to these (larger) types

Boolean Object

Byte Short, Integer, Long, Decimal, Single, Double or Object

Char String or Object

Date Object

Decimal Single, Double or Object

Double Object

Integer Long, Decimal, Single, Double or Object

Long Decimal, Single, Double or Object

Object none

Short Integer, Long, Decimal, Single, Double or Object

Single Double or Object

String Object


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■ Placing a smaller data type into a larger one is called an implicit widening conversion.

■ When a larger type is assigned to a smaller type either a runtime error occurs or the assignment is permitted.Dim value1 As Double = 4.6Dim value2 As Integer = value1

■ Variable value2 will be assigned 5—the result of implicit conversion.

– Such conversions are called narrowing conversions.– The actual value being assigned may be altered without

your being aware of it.



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■ Visual Basic provides a project setting called Option Strict.

– This setting disallows implicit narrowing conversions.

– You can override this by performing narrowing conversions explicitly.



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■ Right click the project name in the Solution Explorer.

■ Select Properties to open the property pages tab (Fig. 15.20).

Figure 15.20 | FundRaiser’s property page tab.

Enabling Option Strict


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■ On the left side of the tab, select the Compile category (Fig. 15.21).

Figure 15.21 | Selecting Compile in the FundRaiser’s property pages.

Enabling Option Strict (Cont.)

Compile category

ComboBox containing value for Option Strict, which is

set to Off by default


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■ Select On in the ComboBox labeled Option Strict: (Fig. 15.22).

Figure 15.22 | Setting Option Strict to On.


On option for Option Strict


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■ You can also set Option Strict to On programmatically by adding the following statement as the first line of code in asource-code file: OptionStrictOn



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■ Select Tools > Options..., then expand the Projects and Solutions node and select VB Defaults.

■ Select On in the ComboBox labeled Option Strict: (Fig. 15.23).

Figure 15.23 | Setting default for Option Strict to On.



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■ When Option Strict is On, you must write code to perform narrowing conversions explicitly.

– This helps avoid subtle errors that could result from implicit conversions.

– Class Convert (Fig. 15.24) helps you perform conversions when Option Strict is On.


Figure 15.24 | Three of class Convert’s methods.

Convert To Use Convert Method Sample Statement

Integer ToInt32 value = Convert.ToInt32( _ inputTextBox.Text)

Decimal ToDecimal

value = Convert.ToDecimal( _ Pmt(monthlyInterest, _ months,-loanAmount))

Double ToDouble rate = Convert.ToDouble( _ rateTextBox.Text) / 100


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■ Line 12 in Fig. 15.25 is underlined, indicating that Option Strict prohibits an implicit conversion from Double to Decimal.

■ This conversion is not allowed by Option Strict because converting from Double to Decimal could result in data loss.

Figure 15.25 | Option Strict prohibits implicit narrowing conversions.

Using Class Convert in theFund Raiser Application


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■ Method Convert.ToDecimal (Fig. 15.26) converts the Double value to a Decimal value.

■ This error also could be corrected by declaring the COSTS constant as a Decimal.

Figure 15.26 | Explicitly performing a narrowing conversion with Convert.ToDecimal.

Using Class Convert in theFund Raiser Application (Cont.)


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■ The error at line 32 (Fig. 15.27) indicates that Option Strict prohibits an implicit conversion from Double to Decimal.

Figure 15.27 | Option Strict prohibits a narrowing conversionfrom type Double to type Decimal.



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■ Method Convert.ToDecimal explicitly converts donationTextBox.Text to a Decimal.

■ After this change is made, the jagged line disappears (Fig. 15.28).

Figure 15.28 | Explicitly converting a Double to type Decimal with Convert.ToDecimal.


■ Note that each method in class Convert has multiple versions for converting various types to the type specified in the method name.


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1 Public Class FundRaiserForm

2 ' instance variable stores total raised for charity

3 Dim totalRaised As Decimal = 0

4

5 ' returns donation amount after operating expenses

6 Sub CalculateDonation(ByVal donatedAmount As Decimal, _

7 ByRef netDonation As Decimal)

8

9 Const COSTS As Double = 0.17

10

11 ' calculate amount of donation for charity

12 netDonation = Convert.ToDecimal(donatedAmount - _

13 (donatedAmount * COSTS))

14 End Sub ' CalculateDonation

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Instance variable declaration

Procedure CalculateDonation determines the amount of donation after operating costs— parameter netDonation is modified directly (using ByRef)

Converting the calculation result to type Decimal

■ Figure 15.29 presents the source code forthe Fund Raiser application.

Outline

(1 of 3 )


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16 ' handles Donation: TextBox’s TextChanged event

17 Private Sub donationTextBox_TextChanged(ByVal sender As _

18 System.Object, ByVal e As System.EventArgs) _

19 Handles donationTextBox.TextChanged

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21 donatedValueLabel.Text = "" ' clear After expenses: field

22 End Sub ' donationTextBox_TextChanged

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24 ' handles Make Donation Button’s Click event

25 Private Sub donateButton_Click(ByVal sender As System.Object, _

26 ByVal e As System.EventArgs) Handles donateButton.Click

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28 Dim donation As Decimal ' amount donated

29 Dim afterCosts As Decimal ' amount for charity

30

31 ' get donation amount

32 donation = Convert.ToDecimal(donationTextBox.Text)

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Convert donation amount from a Double to a Decimal value

Outline

(2 of 3 )


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34 ' obtain donation amount after operating costs deduction

35 CalculateDonation(donation, afterCosts)

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37 ' display amount of donation after costs

38 donatedValueLabel.Text = String.Format("{0:C}", afterCosts)

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40 ' update total amount of donations received

41 totalRaised += afterCosts

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43 ' display total amount collected for charity

44 totalValueLabel.Text = String.Format("{0:C}", totalRaised)

45 End Sub ' donateButton_Click

46 End Class ' FundRaiserForm

afterCosts is passedby reference

Outline

(3 of 3 )

t u t o r i a l 2009 pearson education, inc. all rights reserved. 1 15 fund raiser application...

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