Visual Basic 2010 Lesson 20 – Errors Handling

[Lesson 19] << [CONTENTS] >> [Lesson 21]

20.1 Introduction

Error handling is an essential procedure in Visual Basic 2010 programming that helps make a program error-free. An error-free program can run smoothly and efficiently, and the user does not have to face all sorts of problems such as program crashes or system hangs.Errors often occur due to incorrect input from the user. For example, the user might make the mistake of attempting to enter text (string) to a box that is designed to handle only numeric values such as the weight of a person, the computer will not be able to perform arithmetic calculation for text therefore will create an error. These errors are known as synchronous errors.

Therefore a good programmer should be more alert to the parts of program that could trigger errors and should write errors handling code to help the user in managing the errors. Writing errors handling code is a good practice for Visual Basic 2010 programmers, so do not  try to finish a program fast by omitting the errors handling code. On the other hand, there should not be too many errors handling code in the program as it create problems for the programmer to maintain and troubleshoot the program later. VB2010 has improved a lot in its built-in errors handling capabilities compared to older version versions of VB. For example, when the user attempts to divide a number by zero, Vb2010 will not return an error message but gives the ‘infinity’ as the answer (although this is mathematically incorrect, because it should be undefined)



20.2 Using On Error GoTo Syantax

Example 20.1: Division by Zero

In this example, we will deal with the error of entering non-numeric data into the textboxes that suppose to hold numeric values. The program_label here is error_hanldler. when the user enter a non-numeric values into the textboxes, the error message will display the text”One of the entries is not a number! Try again!”. If no error occurs, it will display the correct answer. Try it out yourself.

The Code

Public Class Form1

Private Sub CmdCalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CmdCalculate.Click

Lbl_ErrorMsg.Visible = False

Dim firstNum, secondNum As Double

On Error GoTo error_handler

firstNum = Txt_FirstNumber.Text

secondNum = Txt_SecondNumber.Text

Lbl_Answer.Text = firstNum / secondNum

Exit Sub ‘To prevent error handling even the inputs are valid

error_handler:

Lbl_Answer.Text = “Error”

Lbl_ErrorMsg.Visible = True

Lbl_ErrorMsg.Text = ” One of the entries is not a number! Try again!”

End Sub

End Class

The Output





20.3 Errors Handling using Try…..Catch….End Try Structure

VB2010 has adopted a new approach in handling errors known as exceptions handling. It is supposed to be more efficient than the old On Error Goto method, where it can handles various types of errors within the Try…Catch…End Try structure.

The structure looks like this

Try

statements

Catch exception_variable as Exception

statements to deal with exceptions

End Try

Example 20.2

This is a modification of Example 20.1. Instead of using On Error GoTo method, we use the Try…Catch…End Try method. In this example, the Catch statement will catch the exception when the user enters a non-numeric data and return the error message. If there is no exception, there will not any action from the Catch statement and the program returns the correct answer.

The code

Public Class Form1

Private Sub CmdCalculate_Click(ByVal sender As System.Object, ByVal eAs System.EventArgs) Handles CmdCalculate.Click

Lbl_ErrorMsg.Visible = False

Dim firstNum, secondNum, answer As Double

Try

firstNum = Txt_FirstNumber.Text

secondNum = Txt_SecondNumber.Text

answer = firstNum / secondNum

Lbl_Answer.Text = answer

Catch ex As Exception

Lbl_Answer.Text = “Error”

Lbl_ErrorMsg.Visible = True

Lbl_ErrorMsg.Text = ” One of the entries is not a number! Try again!”

End Try

End Sub

End Class

The output

[Lesson 19] << [CONTENTS] >> [Lesson 21]


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