Chapter11_PPT

– Sever problems
• Prevent normal execution – Notification – Termination
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11.2
Exception Handling Overview
• Program detects error
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11.2
• Finally block
Exception Handling Overview
– Appears after last Catch handler – Optional (if one or more catch handlers exist) – Encloses code that always executes
The appropriate error message dialog is displayed for the user
2002 Prentice Hall.
All rights reserved.
Outline
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DivideByZeroTest .vb
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' Convert.ToInt32 generates FormatException if argument ' is not an integer Dim numerator As Integer = _ Convert.ToInt32(txtNumerator.Text) Dim denominator As Integer = _ Convert.ToInt32(txtDenominator.Text) ' division generates DivideByZeroException if ' denominator is 0 Dim result As Integer = numerator \ denominator lblOutput.Text = result.ToString() ' process invalid number format Catch formattingException As FormatException MessageBox.Show("You must enter two integers", _ "Invalid Number Format", MessageBoxButtons.OK, _ MessageBoxIcon.Error) ' user attempted to divide by zero Catch dividingException As DivideByZeroException MessageBox.Show(dividingException.Message, _ "Attempted to Divide by Zero", _ MessageBoxButtons.OK, MessageBoxIcon.Error) End Try End Sub ' cmdDivide_Click End Class ' FrmDivideByZero
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11.2
Exception Handling Overview
• Catch block (Catch handler)
– Appears after Try block
• Parameter included – Handles specific exception type • Parameterless – Handles all exception types
Outl .vb
Type of exception that the Catch block can handle
If either exception occurs the Try block expires
If the denominator is zero the CLR throws a DivideByZeroException
– Code executes whether exception occurs or not
• Optional
– Not required if one or more catch handlers exist
• Why use Finally block?
– Typically releases resources acquired in Try block
2002 Prentice Hall. All rights reserved.
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11.1 Introduction
• Exception handling
– Clear, robust and more fault-tolerant programs
• Continue normal execution
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Chapter 11 – Exception Handling
Outline
11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 Introduction Exception Handling Overview Example: DivideByZeroException .NET Exception Hierarchy Finally Block Exception Properties Programmer-Defined Exception Classes Handling Overflows
2002 Prentice Hall. All rights reserved.
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11.1 Introduction
• Exception
– – Indication of problem during program’s execution Although problem can occur, it occurs infrequently
2002 Prentice Hall. All rights reserved.
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11.4
.NET Exception Hierarchy
• Benefit of hierarchy
– Inheritance
• If handling behavior same for base and derived classes – Able to catch base class – Otherwise catch derived classes individually
2002 Prentice Hall. All rights reserved.
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' Fig. 11.1: DivideByZeroTest.vb ' Basics of Visual Basic exception handling. Imports System.Windows.Forms.Form Public Class FrmDivideByZero Inherits Form ' label and TextBox for specifying numerator Friend WithEvents lblNumerator As Label Friend WithEvents txtNumerator As TextBox ' label and TextBox for specifying denominator Friend WithEvents lblDenominator As Label Friend WithEvents txtDenominator As TextBox ' button for dividing numerator by denominator Friend WithEvents cmdDivide As Button Friend WithEvents lblOutput As Label ' output for division ' Windows Form Designer generated code ' obtain integers from user and divide numerator by denominator Private Sub cmdDivide_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles cmdDivide.Click lblOutput.Text = "" ' retrieve user input and call Quotient Try
– ApplicationException
• Can create exception data types specific to applications
– SystemException
• Runtime exceptions – Can occur anytime during execution – Avoid with proper coding
2002 Prentice Hall. All rights reserved.
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11.2