Topik 4.4 Exception Handling

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F5227VISUAL BASIC .NET PROGRAMMING

Topic 4.4 :

Understand the concepts of Exception

Handling


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Course Learning Outcome (CLO) Upon completion of this course, students should be able to :

1. Create a simple VB.NET based application based on the

Windows Application template.

2. Describe on essential terminology including memory,data types and graphical user interface.

3. Apply object oriented programming techniques to

create classes, add methods and add properties.

4. create a simple VB.NET

based Web forms applicationthat uses an XML Web Service and manipulate data in

database by using Microsoft ADO.NET.


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Course Learning Outcome:

Topic 4.0

1. Apply object oriented programming techniques to

create classes, add methods and add properties. (CLO3)


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Topic 4.0

Topic 4.1 : Use Classes, Object and Methods

Topic 4.2 : Apply Inheritance And

Polymorphism

Topic 4.3 : Construct Program Using String

Methods

Topic 4.4 : Understand the concepts ofException Handling


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Error & Exception Handling

There will be problems with your code or your

application.

Some problems will come from you.

Some problems will be caused by users.

And some problems will be caused by neither

you nor your users.


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Types of Errors

Syntax

Run-time

Logic


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Syntax Error

A syntax error comes from your mistyping a

word or forming a bad expression in your

code.

It could be that you misspelled a keyword

such as ByVel instead ofByVal.

It could also be a bad expression such as 524+

+ 62.55.

It could be a "grammar" error


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Run-time Error

For example, in your code, you indicate that a

picture would be loaded and displayed to the

user but you forget to ship the picture or the

directory of the picture indicated in your code

becomes different when a user opens your

application.

In this case, when you compiled and executedthe application in your machine, everything

was fine. This is a type ofrun-time error.


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Run-time Error

Run-time errors are mostly easy to fix because

you will know what problem is occurring and

why.


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Logic Error

These are errors that don't fit in any of the abovecategories.

They could be caused by the user misusing your

application, a problem with the computer on whichthe application is running while the same application

is working fine in another computer.

Because logic errors can be vague, they can also be

difficult (even, to the extreme, impossible) to fix.


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Exception Handling

Exception handling is a technique of handling

a situation which is occur at the run time

when program abort without execution and

the cause of that situation is called exception.


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Types of Exception Handling

Exception can be handled by two ways in

VB.NET .

Structured exception handling (SEH).

Unstructured exception handling (unSEH).


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Unstructured exception handling

(unSEH)

On Error GoTo

On Error GoTo system provides a global

variable named Err.

Err allows you to identify the error and its

description.

Because an error depends on what caused it

and why, the values of the Err variable also

depend and are not always the same.


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Structured exception handling

(SEH)

SEH is based on two main keywords: Try and

Catch.

An exception handling section starts with the

Try keyword and stops with the End Try

statement.

Between Try and End Try, there must by at

least one Catch section.


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VB.NET provides three keywords try, catch and

finally to do exception handling.

The try encloses the statements that might

throw an exception whereas catch handles an

exception if one exists.

The finally can be used for doing any clean up

process.


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Syntax

Try

' Statement which can cause an exception.

Catch x As Type

' Statements for handling the exception

Finally

End Try 'Any cleanup code


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If any exception occurs inside the try block,

the control transfers to the appropriate catch

block and later to the finally block.

But in VB.NET, both catch and finally blocks

are optional.

The try block can exist either with one or

more catch blocks or a finally block or with

both catch and finally blocks.


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If there is no exception occurred inside the try

block, the control directly transfers to finally

block.

We can say that the statements inside the

finally block is executed always.

Note that it is an error to transfer control out

of a finally block by using break, continue,

return or goto.


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Try

' Code to Execute In Case Everything is Alright

Catch' If Something Bad happened, deal with it here

End Try


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Example

Module Module1Sub Main()

Dim a = 0, b = 1, c As Integer

Try

c = b / a

Console.WriteLine("C is " & c)

Catch e As Exception

Console.WriteLine(e)

Console.WriteLine("0 can't be divided by any number")

Console.ReadLine()End Try

End Sub

End Module


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Output

In this code a number is divided by zero which result is infinity.This error is thrown by Try Block and Catch block shows the exception after

catching it.


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Exception class

To support exception handling, the .NET

Framework provides the Exception class.

Once the compiler encounters an error, the

Exception class allows you to identify the type

of error and take appropriate action.


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Syntax

Try

Catch ex As Exception

End Try

By default, an exception is first of type Exception


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Exception

There are two types of exceptions:

exceptions generated by an executing program

exceptions generated by the common language

runtime.

System.Exception is the base class for all

exceptions in VB.NET.

The user-defined exception classes must

inherit from either Exception class or one of

its standard derived classes.


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Exception class

If you declare the exception as an Exception

type, this class will identify the error.

One of the properties of the Exception class is

called Message.

This property contains a string that describes

the type of error that occurred.

You can then use this message to display an

error message if you want.


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Public Class CustomException

Inherits ApplicationException

Public Sub New(ByVal message As String)MyBase.New(message)

End Sub

End Class


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Example of Exception

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As

System.EventArgs) Handles Button1.Click

Dim Number As Double

Dim Twice As Double

TryNumber = Me.TextBox1.Text

Twice = Number * 2

Me.TextBox2.Text = Twice

Catch ex As Exception

MsgBox(ex.Message)

End Try

End Sub


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Output Message

Date post:	04-Apr-2018
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Topik 4.4 Exception Handling

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