Lesson 20: Errors Handling

20.1 Introduction

Error handling is an essential procedure in Visual Basic 2008 programming because it can help make the program error-free. An error-free program can run smoothly and efficiently, otherwise all sorts of problems occur such as program crash or system hang.

Errors often occur due to incorrect input from the user. For example, the user might make the mistake of attempting to enter a 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 should be considered a good practice for Visual Basic programmers, so do try to finish a program fast by omitting the errors handling code. However, 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.

Visual Basic 2008 has improved a lot in built-in errors handling compared to Visual Basic 6. For example, when the user attempts to divide a number by zero, Vb2008 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

Visual Basic 2008 still supports the VB6 errors handling syntax, that is the  On  Error GoTo  program_label structure. Although it has a more advanced error handling method, we shall deal with that later.We shall now learn how to write errors handling code in VB2008. The syntax for errors handling is

On Error GoTo   program_label

where program_label is the section of code that is designed by the programmer to handle the error committed by the user. Once an error is detected, the program will jump to the program_label section for error handling.

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 the text"One of the entries is not a number! Try again!". If no error occur, it will display the correct answer. Try it out yourself.

The Code

Public Class> Form1

Private Sub CmdCalculate_Click(ByVal 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/p>

End Class

The Output

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

VB2008 has adopted a new approach in handling errors, or rather 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 e As 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