IS437: Fall 2004

Instructor: Dr. Boris Jukic

Exceptions and Exception Handling

Message Boxes

•Exceptions: objects created by the system when an error event takes place during the code execution

•Three general types of Errors:

•User errors: inconsistent data type of input, for example

•Resource Failures: out of memory, for example

•Failures of programming logic

• Examples of exception messages: file not found, division by zero, disk full, disk not ready, out of memory, …

• Unhandled exceptions lead to abrupt termination of programs without warnings and explanations.

•

Exceptions

•Exception handling means interpreting and reacting to the exceptions created by errors.

•The goals of exception handling are to anticipate potential problems and provide opportunities to address the problems when they occur.

•Structured Exception handling uses a control structure to create exception handling mechanisms that differentiate between different types of errors

•Try...Catch...Finally control structure tests a piece of code and reacts differently based on the different types of errors

Exception Handling

Try/Catch Blocks

…

…

Try

statements that may cause error

Catch

what to do if an error occurs

[Finally

do this regardless of whether error occurs or not]

End Try

…

…

A Simple Example

Private Sub Button1_Click(…) Handles Button1.Click

Dim x, y, z As Short x = 1000 y = 1000 Try z = x * y Catch ‘An exception will be thrown if x value is outside of range for Short MessageBox.Show("Error has occurred", "Error") End Try

End Sub

Exception Classes and Properties

Exceptions are objects belonging to different classes

General Exception class Use title “Exception”

Specific Exception classes: FormatException, ArithmeticException, etc.

Exception objects have several properties: Message property holds a string with a text message

about the nature of the error that caused the exception

Catch … As

Exception

ArithmeticException

IOException DataException

OverflowException

DivisonByZeroException

DiskNotReadyException

…

FileNotFoundException

MissingPrimaryKeyException

…

…

…

General Exception:

Catch ex as Exception

‘create ex as an object of Exception class

Specific Exceptions:

“Catch …As” example

Private Sub Button1_Click(…) Handles Button1.Click

Dim x, y, z As Short x = 1000 y = 1000 Try z = x * y Catch ex As Exception MessageBox.Show(ex.Message, "Error") End Try End Sub

Message property of exException. ex is an instance ofOverflowException sub class ofArithmenticException class.

Multiple Catch Clauses

Try

‘the code of the procedure goes here

Catch err As ArithmeticException ‘more specific exception

‘what to do

Catch err As Exception ‘most general exception

‘what to do

End Try

•Sequential search for error

•Only the block that first catches an error executes.

•Always list the most specific exception statement fist

Incorrect Catch Clause: an Example

Try

‘the code of the procedure goes here

Catch err As Exception ‘most general exception

‘what to do

Catch err As ArithmeticException ‘more specific exception

‘what to do

End Try

Catch err as ArithmeticException can never execute in this example.

“Catch .. As .. When” Clause: an Example

Try

varAvailableSeats = varAuditoriumSeats - varNumberOfGuests

Catch ex As Exception When varAuditoriumSeats = 0

MessageBox.Show("Auditorium lacks chairs!")

Catch ex As Exception When varAvailableSeats < 0

MessageBox.Show("There are no more available seats.")

Catch ex As Exception

‘all other exceptions

Finally MessageBox.Show("Thank you for your interest in our concert.")

End Try

With when clause, the programmer can define what constitutes an exception

Message Boxes

Dialog Boxes displaying message to the users (often as a result of exceptions)

Invoked by a MessageBox.Show() method MessageBox.Show is a so called

“overloaded method” Accepts many different combinations of

arguments:

MessageBox.Show Method Arguments

“Text to be displayed” in the body of the message box “Text to be displayed” in the title of the message box Type of a button combination: MessageBoxButton

argument Type of an icon displayed: MessageBoxIcon argument

Message Box Button Combinations

MessageBoxButton.OK Displays the OK button MessageBoxButton.OKCancel Displays the OK and

Cancel buttons MessageBoxButton.AbortRetryIgnore Displays Abort,

Retry, and Ignore buttons MessageBoxButton.YesNoCancel Displays the Yes, No,

and Cancel buttons MessageBoxButton.YesNo Displays the Yes and No

buttons MessageBoxButton.RetryCancel Displays the Retry and

Cancel buttons

Message Box Icons

MessageBoxIcon.Exclamation MessageBoxIcon.Question MessageBoxIcon.Error MessageBoxIcon.Information