Dealing with errors in Visual Basic 6
There are often occasions when the code you are writing has a high potential for going wrong. An example might be opening a file - if the file isn't there a "File not found" error will occur.
The standard error messages in VB6 aren't really very helpful for users of your program that don't really understand computers very well so it would be helpful to them if you could create error messages that tells them what really went wrong.
Example code - a division by zero
In my example code below I have created two variables - iTotal and iEvilZero. As you are probably aware, you cannot divide by zero - it's a mathematical impossibility. If your program tries to divide by zero it will end with a nasty error message.
Private Sub Form_Load()
' The Form_Load event is triggered when the program starts and
' this form is loaded before showing it to the user.
' Declare a variable called iNumber to store whole numbers (integers)
Dim iNumber As Integer
' Declare a variable called iEvilZero to store whole numbers (integers)
Dim iEvilZero As Integer
'Store 432 in iNumber
iNumber = 432
' Store zero in iEvilZero
iEvilZero = 0
' Divide iNumber by iEvilZero and store the result in iNumber
iNumber = iNumber / iEvilZero
End Sub
Adding the error handler
Because you know that any division by a variable is dangerous (the variable could contain zero) it would be a good idea to make sure that you trap any errors that occur during the division.
To do this, just before the line that performs the division, enter the code below in the red dotted box:
iEvilZero = 0
On Error GoTo divideError
' Divide iNumber by iEvilZero and store the result in iNumber
iNumber = iNumber / iEvilZero
What this line of code does is tell VB6 that if an error occurs, to jump to a label called divideError and continue from there.
Labels are special markers in your programming code that can be jumped to. You will need to create the label to jump to.
Creating the label to jump to
Insert some empty lines just before the End Sub line and add the code in the red dotted box below. Don't add another End Sub - that's just there to show you where to add the code.
iNumber = iNumber / iEvilZero
' Prevent the error code being executed when there wasn't an error
Exit Sub
divideError:
' Code here should only be executed if an error happens
End Sub
The Exit Sub is there so that the sub-routine is exited nicely if nothing goes wrong in the code above. If it wasn't there, even if there wasn't an error, the code below the divideError label would still be executed.
Displaying a message when something goes wrong
Finally, you need to add some code to display a polite error message if an error does occur (which in my example code is 100% guaranteed).
Just before the End Sub, add the following lines of code in the dotted red box to display a message box:
' Display a polite message box with a critical icon
MsgBox "Sorry, I can't divide by zero.", vbCritical, "Error"
' Close the program (this error could be critical)
End
End Sub
Putting it all together
When your run the program it should display your polite message box because the program tries to divide by zero.
Here's what the completed code should look like (just in case you're not sure you did it right):
Option Explicit
Private Sub Form_Load()
' The Form_Load event is triggered when the program starts and
' this form is loaded before showing it to the user.
' Declare a variable called iNumber to store whole numbers (integers)
Dim iNumber As Integer
' Declare a variable called iEvilZero to store whole numbers (integers)
Dim iEvilZero As Integer
'Store 432 in iNumber
iNumber = 432
' Store zero in iEvilZero
iEvilZero = 0
' When an error occurs, jump to the divideError label.
On Error GoTo divideError
' Divide iNumber by iEvilZero and store the result in iNumber
iNumber = iNumber / iEvilZero
divideError:
' Display a polite message box with a critical icon
MsgBox "Sorry, I can't divide by zero.", vbCritical, "Error"
' Close the program (this error could be critical)
End
End