vb .net tutorial - 11

8/14/2019 VB .net tutorial - 11

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Exception Handling and Debugging

NIIT Exception Handling and Debugging/Lesson 11/Slide 1 of 30

Objectives

In this lesson, you will learn to:

Identify the mechanism to handle run time errors

Differentiate between structured and unstructured exceptionhandling

Handle exceptions

Debug an application


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Problem Statement 11.D.1

While executing the application for adding order details, if theserver returns an error or there is insufficient memory toexecute the application, an appropriate message should bedisplayed and the application should not terminate abruptly.


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Task List

Identify the mechanism to handle errors.

Write the code to handle errors.

Execute the application to verify the results of exceptionhandling.


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Task 1: Identify the mechanism to handle errors.

An exception is termed as an abnormal conditionencountered by an application during execution.

Exception handling

Is the process of providing an alternate path to beexecuted when the application is unable to execute inthe desired way.


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Task 1: Identify the mechanism to handle errors.(Contd.)

Structured exception handling

All the exceptions are derived from theSystem.Exception class.

System.Exception

System.ApplicationException

System.Windows.Forms.AxHost.InvalidActiveXStateException

System.Runtime.Remoting.MetadataServices.SUDSParserException

System.IO.IsolatedStorage.IsolatedStorageException

System.SystemException

System.Runtime.Remoting.MetadataServices.SUDSGeneratorException


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Structured exception handling (Contd.)

Divides an application into blocks of code that havethe probability of raising an error.

Uses the TryCatchFinally statement.

Errors are filtered in the Catch block.


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Just a Minute

In the TryCatchFinally statement, when is the codegiven in each of the blocks executed?


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User-defined exceptions

Can be created by deriving a class from theApplicationException class.

Unstructured exception handling

Is done by placing an On Error statement in a block of

code. Uses the On Error GoTo statement to pass the control

to a specific error handler when an error occurs.

Uses the On Error Resume Next statement to passcontrol to the next executable statement when an erroroccurs.

Uses the OnErrorGoTo0 to disable an error handler.


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Result:

Since unstructured exception handling code is difficult tomaintain, you will use structured exception handling to trap

and handle errors for the given problem statement.


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Just a Minute

What is the difference between the On ErrorGoTo0, OnErrorResumeNext, and OnErrorGoToErrorHandlerstatements and when is each of them used?


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Task 2: Write the code to handle errors.

Task 3: Execute the application to verify the results

of exception handling.


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Problem Statement 11.P.1

A weekly order details report and a weekly query handlingreport have been created. Based on the users choice, thecorresponding report should be displayed. If the report to beopened gives a loading error, an appropriate message shouldbe displayed and the application should not end abruptly.


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An application has been created to display the product ID for aproduct that costs more than $3000.

A Windows Form has been designed with a ListBox controland a Button control.

A data adapter has been created and the Base_Cost andProdID columns of the table Product have been retrieved. A

dataset has been generated for the data adapter.

The code has been written for the Load event of the form todisplay the ProductID for the products with cost more than$3000 in the list box.

However, the application does not give the expected output.The list box displays one product ID multiple times.


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Task List

Identify the mechanism to trap the error.

Identify the starting point to search for errors.

Implement error trapping.

Run the application and trace the error.

Rectify the code.

Verify the output of the code.


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Task 1: Identify the mechanism to trap the error.

Tracing and rectifying an error is termed as debugging.

In order to assist you in debugging logical and run-time

errors, Visual Basic .NET provides extensive debuggingtools. You can access these tools by using:

Debug toolbar

Debug menu


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Task 1: Identify the mechanism to trap the error.(Contd.)

In an application, by using the debugging tools, you can:

Start execution

Break execution

Stop execution

Result:

You will use the debugging tools provided by VisualStudio .NET to trap and rectify the error.


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Task 2:Identify the starting point to search for errors.

Result:

Since the list box contains a product ID, it indicates that

there is no error in the database connection. The error trapshould be set starting from the line containing

OleDbDAProduct.Fill(DsProduct1)command as theerror occurs after the connection is opened.

Task 3: Implement error trapping.

Task 4: Run the application and trace the error.

Two options in the Debug menu to start an application:

The Start option

The Start Without Debugging option


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Task 4: Run the application and trace the error.(Contd.)

You may select the Debug menu option and select any ofthe following, to instruct Visual Basic .NET regarding theexecution of the code.

Step Into

Step Over

Step Out

Task 5: Rectify the code.

Task 6: Verify the output of the code.


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An application has been developed to connect to the databaseand retrieve customer details from the database. The codehas been added in the Load event of the form, which willdisplay customer details.

However, the application generates an error, as it is not ableto connect to the database.


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Task List

Identify the mechanism to debug the code.

Implement debugging.

Rectify the code.



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Task 1: Identify the mechanism to debug the code.Visual Basic provides you with some tools to debug your

application during the break mode. Two of the debuggingtools are listed below:

The Localswindow The Immediatewindow

Result:

As per the problem statement, the connection with thedatabase server has failed. The connection to the database

is being made with the help of variables that form part of theconnection string. The connection to the database can fail ifthe values in the variables are not correct.

To solve this problem, you need to use the Immediatewindow to see the value that the user enters and is stored inthe variable db.


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Task 2: Implement debugging.




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An application has been developed which displays thecorresponding base cost and the product name of the productID selected by the user from a list box. The general level

declarations have been made and the code has been added.

However, the application displays the product name and thebase cost of the product ID P014 regardless of the ID selectedby the user.


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Task List

Identify the mechanism to debug the code.

Implement debugging.

Rectify the code.



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Task 1: Identify the mechanism to debug the code.

The following windows are used for debugging anapplication:

Watch window Call Stack window

The Output window


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Task 1: Identify the mechanism to debug the code.(Contd.)

Result:

Since the variable prodid stores the selected item, its valuehas to be traced. Therefore, you will use the Watch

window to trace the behavior of prodid.

Task 2: Implement debugging.




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Summary

In this lesson, you learned that:

The types of errors that can occur in an application are

syntax errors, run-time errors, and logical errors.An exception is an abnormal or condition that an application

encounters during execution.

Exception handling is the process of providing an alternatepath to be executed when the application is not able toexecute in the desired way.


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Summary (Contd.)

Visual Basic .NET provides two methods to handleexceptions:

Structured exception handling Unstructured exception handling

In order to assist you in debugging logical and runtimeerrors, Visual Basic .NET provides extensive debuggingtools. You can use the Visual Basic .NET Debug toolbar toaccess these tools.


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Summary (Contd.)

By using the debugging tools, you can perform the followingactions:

Start execution Break execution

While in break mode, the following windows can beused to trace and debug an application:

Call Stack window

Watch window

Locals window Immediate window


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Summary (Contd.)Output window

Step through execution

Stop execution

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