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35.1
Introduction
Error
handling
is an essential procedure in Visual Basic programming because it can help make the
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 crash or system hang.
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Errors often occur due
to incorrect input from the user. For example, the user might make the
mistake of attempting to ask the computer to divide a number by zero
which will definitely cause system error. Another example is the user
might 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.
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35.2 Writing the
Errors Handling Code
We shall now learn how
to write errors handling code in Visual Basic. 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
35.1: Division by Zero
Private Sub CmdCalculate_Click()
Dim firstNum, secondNum As
Double
firstNum = Txt_FirstNumber.Text
secondNum = Txt_SecondNumber.Text
On Error GoTo error_handler
Lbl_Answer.Caption = firstNum / secondNum
Exit Sub
'To prevent error handling
even the inputs are valid
error_handler:
Lbl_Answer.Caption = "Error"
Lbl_ErrorMsg.Visible = True
Lbl_ErrorMsg.Caption = " You attempt to divide a number
by zero!Try again!"
End Sub
Private Sub Txt_FirstNumber_GotFocus()
Lbl_ErrorMsg.Visible =
False
End Sub
Private Sub
Txt_SecondNumber_GotFocus()
Lbl_ErrorMsg.Visible = False
End Sub
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The Output Window
Explanation:
In this example, you design the
interface as above. Name the first textbox as Txt_FirstNumber
and the second textbox as Txt_SecondNumber. Insert one command
button as label it as Calculate. Insert one label and name it as
Lbl_Answer to display the answers. If the user enter 0 in the
second textbox as shown above, the program will jump to the
label error_handler, and the procedure that is executed.
It will show an error in the Txt_Answer label and an error
message in the Lbl_ErrorMsg label.
Notice that Exit sub after the
division. It is important because it can prevent the program to
execute the error_handler code even though the user does not
enter zero in the second textbox.
Lastly, after the error message
appeared, the user will click on the textboxes again. When this
occur, the error message will disappea both from the answer
label and error message label. This is achieved by using he
event procedure GotFocu, as shown in the code.
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Example 35.2:
Nested Error Handling Procedure
By referring to
Example 28.1, we need to consider other errors probably will be made by
the user, such as entering non-numeric inputs like letters. Therefore,
we need to write error handling code for this error too. It should be
put in the first place as soon as the user input something in the
textboxes. And the error handler label error_handler1 for
this errror should be put after the error_handler2 label.
This means the second error handling procedure is nested within
the first error handling procedure. Notice that you have to put an Exit
Sub for the second error handling procedure to prevent to execute the
first error handling procedure again. The code is as follow:
Private Sub CmdCalculate_Click()
Dim firstNum, secondNum As Double
On Error GoTo error_handler1
firstNum = Txt_FirstNumber.Text
secondNum = Txt_SecondNumber.Text
On Error GoTo error_handler2
Lbl_Answer.Caption = firstNum / secondNum
Exit Sub 'To prevent errror handling even the inputs are valid
error_handler2:
Lbl_Answer.Caption = "Error"
Lbl_ErrorMsg.Visible = True
Lbl_ErrorMsg.Caption = " You attempt to divide a number by zero!Try
again!"
Exit Sub
error_handler1:
Lbl_Answer.Caption = "Error"
Lbl_ErrorMsg.Visible = True
Lbl_ErrorMsg.Caption = " You are not entering a number! Try again!"
End Sub
Private Sub Txt_FirstNumber_GotFocus()
Lbl_ErrorMsg.Visible = False
End Sub
Private Sub Txt_SecondNumber_GotFocus()
Lbl_ErrorMsg.Visible = False
End Sub
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The Output window
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