T U T O R I A L 2009 Pearson Education, Inc. All rights reserved. 1 5 Completing the Inventory...

T U T O R I A L

2009 Pearson Education, Inc. All rights reserved.

1

5Completing the

InventoryApplication

Introducing Programming


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Outline

5.1 Test-Driving the Inventory Application

5.2 Introduction to Visual Basic Code

5.3 Inserting an Event Handler

5.4 Performing a Calculation and Displaying the Result

5.5 Using the IDE to Eliminate Compilation Errors


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In this tutorial you will learn: ■ Add an event handler for a Button control.■ Insert code into an event handler.■ Access a property’s value by using Visual Basic

code.■ Use the assignment and multiplication operators.■ Use the Visual Basic IDE to fix compilation errors.

Objectives


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■ You will add functionality to the Inventory application that you designed in Tutorial 4.

■ The term functionality describes the actions an application can execute.

■ GUI events, represent user actions, such as clickinga Button or altering a value in a TextBox.

■ Event handlers are pieces of code that execute when such events occur.

■ Events and event handlers are crucial to programming Windows applications.

Introduction

Application Requirements


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5.1 Test-Driving the Inventory Application

A college bookstore receives cartons of textbooks.In each shipment, all cartons contain the same number of textbooks. The inventory manager wants to use a computer to calculate the total number of textbooks arriving at the bookstore for each shipment. The inventory manager will enter the number of cartons received and the fixed number of textbooks in each carton for each shipment; then the application will calculate the total number of textbooks in the shipment.


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Test-Driving the Inventory Application

■ Open the directory C:\Examples\Tutorial05\CompletedApplication\Inventory2. Double click Inventory2.sln to open the application.

■ Select Debug > Start Debugging to run the application (Fig. 5.1).

Figure 5.1 | Inventory application with quantities entered.


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Test-Driving the Inventory Application (Cont.)

■ Click the Calculate Total button. ■ The application multiplies the two numbers you

entered and displays the result in the Label to the right of Total: (Fig. 5.2).

Figure 5.2 | Result of clicking the Calculate Total Buttonin the Inventory application.

Result of calculation


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Customizing the IDE

■ Enable the IDE’s capability to show line numbers in your code:– Select Tools > Options... (Fig. 5.3).

– In the Text Editor Basic category, select the Editor category and locate the Interaction group of CheckBoxes.

– Make sure the CheckBox next to Line numbers is checked.

Figure 5.3 | Options dialog.

Text Editor Basic category

Show all settings CheckBox


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Customizing the IDE (Cont.)

■ Use proper spacing when writing code.

■ Indenting code improves program readability.

■ In the Options dialog (Fig. 5.4), enter 3 for both the tab size and indent size, or use the Smart indenting feature.

Figure 5.4 | General settings page for Visual Basic text editor.

Line numbers CheckBox(checked)

Smart indenting feature

Editor Category

Text Editor Basic category


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Customizing the IDE (Cont.)

■ In the Environment category, select the Fontsand Colors category.

■ Click the Use Defaults Button (Fig. 5.5).

■ Click the OK Button to apply any changes.

Figure 5.5 | Examining the Fonts and Colors page.

Use Defaults Button

Fonts and Colors category


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Common Programming Practice

You can change the font and color settings if you prefer a different appearance for your code. To remain consistent with this book, however, we recommend that you not change the default font and color settings.


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Introducing Visual Basic Code

■ Double click the Inventory.vb file in the Solution Explorer window.

■ Switch to Code view (Fig. 5.6) by selectingView > Code or pressing F7.

Figure 5.6 | IDE showing code for the Inventory application.

Inventory.vb tabbed window

Class definition


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■ Most Visual Basic programs consist of pieces called classes, which simplify application organization.

■ Classes contain groups of code statements that perform tasks and return information when the tasks are completed.

– These lines collectively are called a class definition.

■ Most Visual Basic applications consist of acombination of code written by programmers and preexisting classes written and provided by Microsoft in the .NET Framework Class Library.

Introducing Visual Basic Code (Cont.)


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■ Line 1 begins the class definition.

■ The Class keyword introduces a class definitionin Visual Basic and is followed by the class name.– Example: Public Class InventoryForm

■ The name of the class is an identifier– An identifier is a series of characters consisting of letters,

digits and underscores.

– Identifiers cannot begin with a digit and cannot contain spaces.

■ The class definition ends at line 3 with the keywords End Class.



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Capitalize the first letter of each class identifier, such as the Form name.


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■ Keywords (or reserved words) are reserved for use by Visual Basic.

■ Keywords appear in blue by default in the IDE.

■ A complete list of Visual Basic keywords can be found in Appendix E, Keyword Chart.

■ Visual Basic keywords and identifiers are not casesensitive.

■ The IDE applies the correct case to each letter of a keyword and identifier, so when you type clasS, it changes to Class when you press the Enter key.



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Adding a Button’s Click Event Handler

■ Click the Inventory.vb [Design] tab to view theWindows Form Designer. Then double click the Calculate Total Button to enter Code view.

■ The code for the application, which now includes the new event handler in lines 3–5 of Fig. 5.7, is displayed.

Figure 5.7 | Event handler calculateButton_Clickbefore you add your program code.

Asterisks indicate unsavedchanges to application

Empty event handler


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■ Double clicking the Calculate Total Buttoncaused the Visual Basic IDE to generate theButton’s Click event handler.

– The event handler is the code that executes when the user clicks the Calculate Total Button.

– When you double click a control, the IDE inserts an event handler for that control.

Adding a Button’s Click Event Handler (Cont.)


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■ At the end of each event handler’s first line, Visual Basic inserts a Handles clause. Scroll to the right in Code view to see the Handles clause for the Calculate Total Button’s Click event handler (line 3).

Handles calculateButton.Click

■ This Handles clause indicates that the event handler is called when the calculateButton’sClick event occurs.



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■ In Visual Basic, event handlers follow the naming scheme controlName_eventName.

■ This convention mimics the event handler’sHandles clause.

■ The word controlName refers to the name of the control provided in its Name property (in this case, calculateButton).

■ The word eventName represents the name of the event (in this case, Click) raised by the control.

– When event eventName occurs on the controlcontrolName, event handler controlName_eventName executes.



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■ Run the application (Fig. 5.8).

Figure 5.8 | Running the application without functionality.

Close box



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■ Select View > Code to view the application’s code (Fig. 5.9).

Figure 5.9 | Code added to the Calculate Total Button’s event handler.

Event handler

Adding Code to an Empty Event Handler

Type this code


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■ A single-quote character ('), indicates that the remainder of the line is a comment.

– You insert comments in programs to improve the readability of your code.

– Comments explain the code so that other programmers who need to work with the application can understand it more easily. By default, comments are displayed in green.

– Comments also help you read your own code.

■ Comments can be placed either on their own lines (these are called “full-line comments”) or at the end of a line of Visual Basic code.

■ Comments do not cause the computer to perform any actions when your applications run.

Adding Code to an Empty Event Handler (Cont.)


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Comments written at the end of a line should be preceded by a space, to enhance program readability.


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■ A Visual Basic statement performs an action.

■ By default, statements end when the current line ends.

■ In Visual Basic, properties are accessed in code by placing a period between the control name.

– This period is called the member-access operator (.),or the dot operator.



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■ When the control name and member-access operator are typed, a window appears listing that object’s members (Fig. 5.10).


Selected member

Description of selected member

IntelliSense

Figure 5.10 | IntelliSense activating while entering code.


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■ This feature, known as IntelliSense, displays items that can be used in the current context.

– The IntelliSense window can also be opened by pressingCtrl + Space.

– Click the member name once to display a description of that member; double click it to add the member’s name to your application. You can also press Enter or Tab to insert the member

– The Common tab shows the most commonly used members. The All tab shows every member that can appear to the right of the dot operator.


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■ The “=” symbol is known as the assignment operator.

■ The expressions on either side of the assignment operator are referred to as its operands.

– This assignment operator assigns the value on the right of the operator (the right operand) to the variable on the left of the operator (the left operand).

■ The assignment operator is known as a binary operator.



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■ The entire statement is called an assignment statement because it assigns a value to the left operand.

■ Note that the right operand is unchanged by the assignment statement.

■ Run the application and test its output (Fig. 5.11).

Figure 5.11 | Running the application with the event handler.

Result of clicking Calculate Total Button



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■ The underscore character used in Figure 5.12 isknown as the line-continuation character.

– At least one space character must precede each line-continuation character.

■ A whitespace character is a space, tab or newline.

Completing the Inventory Application

Figure 5.12 | Using multiplication in the Inventory application.

Modified Inventory application code


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A lengthy statement may be spread over several lines. If a single statement must be split across lines, choose breaking points that make sense, such as after an operator. If a statement is split across two or more lines, indent all subsequent lines with one “level” of indentation.


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Common Programming Error

Splitting a statement over several lines without including the line-continuation character is a compilation error.


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Common Programming Error

Placing non-whitespace characters, including comments, to the right of a line-continuation character is a compilation error.


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■ The asterisk (*) is known as the multiplication operator.

– Its left and right operands are multiplied together.

■ The Val function prevents nonnumeric inputs from terminating the application. A function is a piece of code that performs a task when called and returns a value.

– The values returned by Val become the values used inthe multiplication expression.

■ Note: data entered in Text Boxes is considered Text. To perform calculations with that data, the numbers entered by the user (which are considered text) should be converted to values using the Val function.

Completing the Inventory Application (Cont.)


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■ Call functions by typing their name followed by parentheses.

■ Any values inside the parentheses are known as function arguments.

– Arguments are inputs to the function.

– In this case, the argument specifies which value you want to send to function Val.



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■ Function Val obtains a value from a string of characters.

■ Val reads its argument one character at a time until it encounters a character that is not a number.

■ Val recognizes the decimal point as a numeric character, as well as the plus and minus signs when they appear at the beginning of the string.



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■ Once a nonnumeric character is read, Val returns the number it has read up to that point.

– Val ignores whitespace characters.

– Val does not recognize symbols such as commas and dollar signs.

– If function Val receives an argument that cannot be converted to a number, it returns 0.



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■ Be careful when using Val—although the value returned is a number, it is not always the value the user intended (see Fig. 5.13).

■ If incorrect data is entered by the user, Val makes no indication of the error.


Figure 5.13 | Val function call examples.

Val Function call examples Results

Val( "16" ) 16 Val( "–3" ) –3 Val( "1.5" ) 1.5 Val( "67a4" ) 67 Val( "8+5" ) 8 Val( "14 Main St." ) 14 Val( "+1 2 3 4 5" ) 12345 Val( "hello" ) 0


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1 Public Class InventoryForm

2

3 Private Sub calculateButton_Click( _

4 ByVal sender As System.Object, ByVal e As System.EventArgs) _

5 Handles calculateButton.Click

6

7 ' multiply values input and display result in Label

8 totalResultLabel.Text = _

9 Val(cartonsTextBox.Text) * Val(itemsTextBox.Text)

10 End Sub ' calculateButton_Click

11 End Class ' InventoryForm

■ Figure 5.14 presents the Inventory application’s code.

Outline


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■ Debugging is the process of fixing errors in an application.

■ There are two types of errors — compilation errors and logic errors.

– Compilation errors occur when code statements violate the grammatical rules of the programming language or when code statements are simply incorrect in the current context.

– Compilation errors include: misspellings, failure to use the line-continuation character when splitting a statement across multiple lines or using an identifier in the wrong context.

5.5 Using the IDE to Eliminate Compilation Errors


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– Syntax errors are compilation errors that are violations of the grammatical rules of the programming language.

– Logic errors do not prevent the application from compiling successfully, but do cause the application to produce erroneous results.

■ The Visual Basic IDE contains a debugger that allows you to analyze the behavior of your application.

5.5 Using the IDE to EliminateCompilation Errors (Cont.)


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■ You can compile an application without executing it by selecting Build > Build [Project Name].

■ The Output window (Fig. 5.15) displays the result of the compilation.

– Select Debug > Windows > Output to view it while debugging.

Figure 5.15 | Results of successful build in the Output window.



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■ Compilation errors appear in the Error List window along with a description of each error.

■ Figure 5.16 displays the error that appears when the line-continuation character is left out of a multiple-line statement.

■ Double click the error in the Error List window to go the location of the error in the code.

■ For additional information on a compilation error, right click the error statement in the Error List window and select Show Error Help.


Figure 5.16 | Error List lists compilation errors.


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■ The Visual Basic IDE provides real-time error checking (Fig. 5.17).

Figure 5.17 | IDE with compilation errors.

Using the IDE to Eliminate Compilation Errors

Indicates compilation error


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■ Open the Error List window (Fig. 5.18) by selectingView > Error List.

Figure 5.18 | Error List displaying the compilation errors.

Using the IDE to Eliminate Compilation Errors (Cont.)

■ These features notify you of possible errors and give you achance to fix them before compiling the application.

■ The IDE refuses to run your modified application until allcompilation errors have been corrected.


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■ Double clicking an error in the Error List window selects the code containing that error.

■ Placing the cursor over the compilation error displays the error message (Fig. 5.19).

Figure 5.19 | Highlighting the code where a compilation error occurs.


Highlighted compilation error


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■ Additional help regarding the compilation error is also available through the Error List item’s context menu, which you can access by right clicking an item (Fig. 5.20).

Figure 5.20 | Getting additional help.


Context help


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■ This displays a helpful reference page (Fig. 5.21).

Figure 5.21 | Error Help window.


Suggested solutions to misspelled identifier

(note that the debuggerinterprets the

misspellingas a new, undeclared

identifier)


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■ When you correct an error, note that the jagged line does not disappear immediately.

■ When you move the cursor to another line, the IDE rechecks the code for errors and removes the jagged underline.



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Figure 5.22 | Using the Error Correction window to fix a compilation error.


■ Hover over the small red rectangle located at a compilation error.

– The Error Correction Options icon ( ) appears.

– Click this icon to open the Error CorrectionOptions window (Fig. 5.22).

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