microsoft vb 2005: reloaded, advanced chapter 5 input validation, error handling, and exception...

Microsoft VB 2005: Reloaded, Advanced

Chapter 5Input Validation, Error Handling, and

Exception Handling

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Objectives

• Perform input validation using a variety of techniques

• Describe the differences between runtime errors and exceptions

• Resolve runtime errors in a simple application

• Interpret error messages created by the Common Language Runtime

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Objectives (continued)

• Perform classic error handling in an application

• Discuss the inheritance hierarchy of Visual Basic exception classes

• Perform structured error handling in an application

• Create programmer-defined exceptions

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Input Validation

• Input validation– Validating user data before it is used by the program

• Techniques– Proper interface design– Trapping keystrokes– The Validating event handler– The MaskedTextBox control– The ErrorProvider component

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Error Prevention Through Proper Interface Design

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Input Validation by Trapping Keystrokes

• In some cases it is advisable to check each individual keystroke– To determine immediately if the character is valid

• And refrain from displaying it

• .NET Framework KeyPress event– Can be used to check each keystroke made in a

specific control– Properties: Handled and KeyChar

• Char class has several methods to check for keystroke acceptability– IsDigit, IsLetter, and IsWhiteSpace

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Input Validation by Trapping Keystrokes (continued)

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Input Validation with the Validating Event Handler

• Validate the entire contents of a text box when the Leave event occurs– Event fires when the user Tabs away from the control

or clicks on another control– A Validating event also fires if the CausesValidation property of the control is True

• Validating event enables you to write an event procedure to validate the entire contents of a control

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Input Validation with the Validating Event Handler (continued)

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Using the MaskedTextBox Control for Input Validation

• MaskedTextBox control– An enhanced TextBox that uses the Mask property

to specify valid user input

• You can create a character mask that specifies:– Required and optional characters– Positions of such characters– Types of required characters – Values and positions of literal characters

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Using the MaskedTextBox Control for Input Validation (continued)

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Using the MaskedTextBox Control for Input Validation (continued)

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Using the MaskedTextBox Control for Input Validation (continued)

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Using the MaskedTextBox Control for Input Validation (continued)

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Using the MaskedTextBox Control for Input Validation (continued)

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Using the MaskedTextBox Control for Input Validation (continued)

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Using the ErrorProvider Component for Input Validation

• ErrorProvider component– Notifies the user that a data entry error has occurred

in a particular control• Without using a MessageBox

– Displays a default error icon next to the control where the error occurred

– When the user positions the mouse over the error icon, a ToolTip error message displays

• Informing the user of the problem

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Using the ErrorProvider Component for Input Validation

(continued)

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Using the ErrorProvider Component for Input Validation

(continued)

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Runtime Errors and Exceptions

• Objectives– Learn about kinds of errors that can occur within

programs• And different ways that you can deal with such errors

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Types of Errors

• Compile-time errors– Errors that occur while the program is being compiled– Usually due to syntax violation

• Runtime errors– Errors generated by the program during execution

• Logic errors– Errors made by the programmer in designing or

implementing the logic of a program

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Types of Errors (continued)

• Classic error handling– Refers to the way Visual Basic internally handles an

error object called Err– Error object is automatically generated by a Visual

Basic program when a runtime error occurs

• Exception– Any exceptional, unusual, or abnormal condition that

occurs during program execution– Exception objects are created by your application

– All Visual Basic runtime errors are exceptions• Not all exceptions are Visual Basic runtime errors

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Types of Errors (continued)

• Structured exception handling– Process for managing Exception objects that uses

special program elements– Usually preferred over classic error handling

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Types of Errors (continued)

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Runtime Errors in a Simple Application

• Division application– Allows the user to divide one number (the dividend)

by another (the divisor), resulting in the answer (the quotient)

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Runtime Errors in a Simple Application (continued)

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Runtime Errors in a Simple Application (continued)

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Runtime Errors in a Simple Application (continued)

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Runtime Errors in a Simple Application (continued)

• Types of exceptions– FormatException

• An input string was not in the correct format

– DivideByZeroException

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Runtime Errors in a Simple Application (continued)

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Runtime Errors in a Simple Application (continued)

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Runtime Errors in a Simple Application (continued)

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Runtime Errors in a Simple Application (continued)

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Classic Error Handling in Visual Basic

• Accomplished using the On Error statement– Of which there are several variations

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On Error GoTo <line>

• On Error GoTo <line> statement– Used to place Visual Basic in error-handling mode

• A state of a Visual Basic application that enables the features of classic error handling

– Then jumps to an indicated <line> in the program, which begins the error-handling code

• Known as the error handler

• Will usually display a message to the user and exit the Sub procedure

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On Error GoTo <line> (continued)

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Alternatives to Exit Sub

• Exit Sub– Exits error-handling mode and immediately leaves the Sub procedure

• Exit Function– Performs error handling within a Function

• Keyword Resume– Repeats execution of the same statement that caused

the error• Resume Next

– Resumes execution with the statement after the one that caused the error

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On Error Resume Next

• Allows a program to simply skip a statement that causes an error– And resume normal flow with the next statement

• Errors can then be handled using traditional If...Then logic

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Disadvantages of Classic Error Handling

• Classic error handling can be challenging

• It is very difficult to understand how a program handles errors just by looking at the program code

• You cannot nest error-handling code

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Exception Classes in the .NET Framework

• Structured exception handling is the preferred way of dealing with runtime errors and other exceptions in a Visual Basic application

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Runtime Errors and Exceptions

• A runtime error creates an object called Err• Every runtime error in a Visual Basic program also

generates a specific kind of exception object

• Exception-handling code– Handles special exceptions objects– Can provide important custom error messages to the

user and keep the application running

• Visual Basic’s runtime environment can automatically handle exceptions for you– By displaying standard error messages and

subsequently shutting down the program

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Runtime Errors and Exceptions (continued)

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The Inheritance Hierarchy of Exception Classes

• Hierarchy begins with the Object class

• Method-call stack– Sequence of method calls within an application that

leads to the creation of an exception object

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The Inheritance Hierarchy of Exception Classes (continued)

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Structured Exception Handling in Visual Basic .NET

• Objective– Learn how to handle exception objects in a precise

way using structured exception handling• Applications are more robust and fault tolerant

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The Try…Catch…Finally Statement

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The Try…Catch…Finally Statement (continued)

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Definitions of Try, Catch, and Finally Blocks

• Try block– Section of code that could possibly throw (create or

generate) an exception object– Including code that should not execute if an exception is

thrown

• Exception thrower or exception propagator– The method that contains the Try block

• Catch block– Code in a Try...Catch...Finally statement

designed to catch (handle or process) an exception object

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Definitions of Try, Catch, and Finally Blocks (continued)

• Exception catcher or exception handler– The method that contains a Catch block

• Finally block– Contains program code that performs final actions after

a Try block or Catch block fully executes– Finally block will always execute whether or not an

exception is thrown– Often used to release any resources created in the Try

block

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How Try, Catch, and Finally Blocks Work

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A Simple Try…Catch…Finally Example

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Multiple Catch Blocks

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Multiple Catch Blocks (continued)

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More About the Finally Block

• Finally block– It is always executed, regardless of what happens in the Try or Catch blocks

– Block is well suited for closing any application resources that had been previously opened

• Or performing any other kind of program cleanup tasks

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Programmer-Defined Exceptions

• Programmer-defined exceptions– Exceptions not associated with standard Visual Basic

runtime errors– Allow your application to handle many additional

unusual situations

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Creating a Programmer-Defined Exception Class

• Microsoft recommendations for creating your own exception classes– Give a programmer-defined exception class a

meaningful name that ends with Exception– Class should derive from ApplicationException

• To provide the application with the required exception-handling functionality

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Creating a Programmer-Defined Exception Class (continued)

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Throwing Programmer-Defined Exceptions

• Use keyword Throw• Example

– Throw New IncorrectAgeException(“Negative age not allowed”)

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Throwing Programmer-Defined Exceptions (continued)

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Using the InnerException Property

• Display additional information about a particular exception object

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Using the InnerException Property (continued)

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Summary

• Minimize errors through the proper design of the user interface– .NET Framework KeyPress event– MaskedTextBox control– ErrorProvider component

• Types of errors– Compile-time errors– Runtime errors– Logic errors

• In Visual Basic, a runtime error creates an object called Err, which has certain properties

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Summary (continued)

• An exception is any kind of unusual or infrequent problem that occurs during program execution, including runtime errors

• Structured exception handling manages program problems using exception objects

• Classic error handling is the Visual Basic process for managing error objects generated by the program when standard runtime errors occur

• Visual Basic structured exception handling uses the Try...Catch...Finally statement

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Summary (continued)

• The inheritance hierarchy for .NET Framework exception classes begins with the Object class– And continues at the next lower level with the Exception class

• You can define your own custom exception classes, called programmer-defined exceptions