Chapter 12: Advanced Topics: Exception Handling Visual Basic.NET Programming: From Problem Analysis...

Chapter 12: Advanced Topics: Exception Handling

Visual Basic .NET Programming:

From Problem Analysis to Program Design

Visual Basic .NET Programming: From Problem Analysis to Program Design 2

Objectives

• Examine the VB .NET exception-handling model

• Explore the supplied Exception classes

• Write user-defined Exception classes

• Add data validation code to methods

• Explore the debugging tool


Examining the VB .NET Exception-Handling Model

• Exception– Object instance of Framework Class Library

(FCL)-supplied Exception class• Or one of its subclasses

– Used by VB .NET to notify you of:• Errors

• Problems

• Other unusual conditions that may occur while your system is running


Causing an Exception

• Create deliberate error

• Exception is thrown

– Message dialog indicates that code did not deal with exception

– Execution is interrupted


Example 12-2: Invoking the String Insert Method with an Invalid Argument (excerpt)

4. Sub Main()

5. Dim s1 As String = “Hello Again”

6. Dim s2 As String = s1.Insert(16, “There “)

7. Console.WriteLine(s2)

8. End Sub


Exception-Handling Syntax

• When exceptions arise

– Server method can create exception instance containing information about situation

– Server then sends exception instance to invoking client

– Client must be prepared to receive exception and take appropriate action


Exception-Handling Syntax (continued)

• Exception-related keywords:

– Try

– Catch

– Finally

– End Try

– Throw



• Try block

– Code that may invoke exception placed in Try block

– Begins with keyword Try

– Ends with keyword End Try

• Throw keyword

– Used by server to pass exception to client



• Catch block

– Client catches exception instance

– Executes statements to deal with exception

• Finally block

– Optional

– Will execute regardless of whether exception is caught


Example 12-3: Invoking the Insert Method Using Try-

Catch (excerpt)

4. Sub Main()

5. Dim s1 As String = "Hello Again"

6. Try

7. Dim s2 As String = s1.Insert(16, "There ")



Example 12-3: Invoking the Insert Method Using Try-

Catch (continued)9. Catch e As Exception

10. Console.WriteLine("Exception was caught:")

11. Console.WriteLine(e.Message)

12. Finally

13. Console.WriteLine("Finally block is always executed.")

14. End Try

15. End Sub


Exploring the Supplied Exception Classes

• Exception class

– Base class of all exceptions

– Contains:

• Constructor

• Message property

• ApplicationException

– Superclass for user-defined exceptions


Exploring the Supplied Exception Classes (continued)

• SystemException

– Base class for exceptions used by VB .NET

• Properties can also throw exceptions


Example 12-4: Triggering an Exception from the CLR

(excerpt)3. Module Chapter12Example4

4. Sub Main()

5. Dim s1, s2 As String

6. s1 = “Hello Again”

7. s2 = s2.Insert(6, “There ”)


9. End Sub

10. End Module


Example 12-5: Instantiating the Supplied Exception Classes

(excerpt)

Server module:

6. If b = 0 Then

7. e = New DivideByZeroException(“You cannot divide by zero”)

8. Throw e


Example 12-5: Instantiating the Supplied Exception Classes

(excerpt)

Client module:

6. Try

…

11. Catch x As DivideByZeroException

12. Console.WriteLine(x.Message)

13. End Try


Writing User-Defined Exception Classes

• Essentially same as writing any class definition

• Include:

– Attribute definitions

– Accessor methods

– Constructors

• Naming convention:

– Name includes word “Exception”


Adding Data Validation Code to Methods

• Data validation code

– Added to methods to ensure valid arguments are passed

– Can throw exception when invalid data is used


Example 12-7: InvalidPhoneTypeException

Class Definition (excerpt)

1. Public Class InvalidPhoneTypeException

2. Inherits ApplicationException

3. Private phoneTypeSubmitted As String

…

11. End Class


Example 12-8: Phone Class Definition with Data Validation

(excerpt)

15. Select Case aPhoneType

16. Case "Home", "home", "HOME"

17. typeOK = True

18. aPhoneType = "Home“

…

28. End Select


Example 12-8: Phone Class Definition with Data Validation

(continued)29. If typeOK Then

30. phoneType = aPhoneType

31. Else

32. e = New InvalidPhoneTypeException(aPhoneType,

"Phone type must be Home, Mobile, Office, or Fax")

33. Throw e

34. End If


Exploring the Debugging Tool

• Debugger

– Software tool

– Provided to assist in finding errors that keep program from running as intended

• Watch window

– View contents of variables and expressions


Setting a Breakpoint

• Breakpoint

– Flag in program

– Tells debugger to pause execution of program at specific line of code

• View and alter contents of variables

• Alter sequence of statement execution

– By specifying next statement to be executed


Setting a Breakpoint (continued)

• Hit count

– Number of times breakpoint is encountered before stopping

– Default value is 1

• Attach logical expression to breakpoint

– Execution will stop if expression evaluates true


Creating a Watch Window

• Watch window

– Display contents of variables while program is suspended at breakpoint

– Available only when execution of program is suspended at breakpoint


Creating a Watch Window (continued)

• To display Watch Window:

– Click Debug on the menu bar

– Point to Windows

– Point to Watch

– Click Watch 1


Using the Step Feature

• Step

– Execute one line of code at a time

– Options:

• Step into

• Step over

• Step Into

– Executes line of code and stops


Using the Step Feature (continued)

• Step Over

– Executes next line of code

– If code invokes a procedure

• All statements in procedure are executed

• Then debugger stops


Programming Example: Data Validation with User-Defined

Exception Classes

• Extends Chapter 7 end-of-chapter programming example

• Demonstrates implementation of user-defined Exception classes used in data validation


Programming Example: Data Validation with User-DefinedException Classes (continued)

• Problem analysis and algorithm design:

– Server module consists of a single method named ComputeWithHolding

– Method throws exceptions:

• InvalidMaritalStatusException

• InvalidWagesException


Summary

• Exceptions notify you of errors, problems, and other unusual conditions

• Exception class– Superclass for all exceptions

• Server methods can create exception instances – Containing information about the errors

• Try block– Code that may throw an exception should be

placed inside


Summary (continued)

• Throw keyword

– Used by server to send exception to client

• Catch block

– Client code to process exception

• Finally block

– Optional

– If included will execute regardless of whether exception is caught


Summary (continued)

• Base class of all exceptions is the Exception class

• Visual Studio debugger

– Software tool provided to assist in finding errors

– Components:

• Breakpoints

• Watch window

Chapter 12: Advanced Topics: Exception Handling Visual Basic.NET Programming: From Problem Analysis...

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