Microsoft VB 2005: Reloaded, Advanced

1
Microsoft VB 2005 Reloaded, Advanced

• Chapter 5
• Input 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

4
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 ltlinegt

• On Error GoTo ltlinegt 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 ltlinegt 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 ltlinegt (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 Basics 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 TryCatchFinally Statement
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The TryCatchFinally 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 TryCatchFinally 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