Q191474: HOWTO: Create a Generic Error Handler for your Application
Article: Q191474
Product(s): Microsoft Visual Basic for Windows
Version(s):
Operating System(s):
Keyword(s): kbGrpDSVBDB
Last Modified: 11-JAN-2001
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The information in this article applies to:
- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0
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SUMMARY
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The Visual Basic ON ERROR statement is designed to encapsulate error handling
for each procedure or module in an application. This behavior is by design, and
is intended to conform with object oriented programming (OOP) conventions. When
you create an application, you may wish to have a generic error handler that
traps all errors for the application, and handle exceptions to this routine as
they occur in various modules.
The ON ERROR statement does not allow the use of an application wide ON ERROR
routine. However, it is possible to create a generic routine that is called from
within each procedure or method in the application.
MORE INFORMATION
================
If the error routine is placed in a global module, it can be called manually
from each procedure or method in your application. The following code example
demonstrates how to set up a global error handling routine, and illustrates how
this can be combined with specific error handling in a procedure or method.
This example creates a project that contains two forms and one code module. Each
form contains command buttons that cause errors to occur. Some errors are
handled within the click method for the command button. Other errors are passed
to the generic error handler.
Steps to Reproduce Behavior
---------------------------
1. Create a new standard EXE project in Visual Basic. Form1 is created by
default.
2. Add the following code to the Load method of Form1. This code causes two
errors to occur when the form is loaded. The first error will be handled by
the form load method. The second error will be passed to the generic error
handler:
Private Sub Form_Load()
On Error GoTo FormLoadErr
Err.Raise 76
Err.Raise 70
Exit Sub
FormLoadErr:
Select Case Err.Number 'Evaluate Error Number
Case 76
MsgBox "Form_Load Error Handler. Form Does Not Exist"
Case Else
AppWideErr (Err.Number) 'Pass Error to generic module
End Select
End Sub
3. Add a CommandButton to the form. This code causes two errors to occur. The
first error will be handled by the click method. The second error will be
passed to the generic routine. Change the Caption property to "Cause Error 53
and 70." Add the following code to the Click method of the CommandButton:
Private Sub Command1_Click()
On Error GoTo Cmd1Err
Err.Raise 53 'Handled locally
Err.Raise 70 'Handled by generic module
Exit Sub
Cmd1Err:
Select Case Err.Number
Case 53
MsgBox "Command 1 Error Handler"
Case Else
AppWideErr (Err.Number)
End Select
Resume Next 'Process the next (70) error
End Sub
4. Add a second CommandButton to the form. If an error occurred in this method,
no error handler would be called. Change the Caption property to "Show Form
2." Add the following code to the Click event:
Private Sub Command2_Click()
'No error handling is coded in this method
'AppWideErr would not be called
Form2.Show 'Use VB's default error handling only
End Sub
5. Add a second form to the project. Add a CommandButton to the form. This
CommandButton passes all errors to the generic error handler and does not
perform any special processing for specific errors. Change the Caption
property of the button to "Cause Error 17." Add code to the Click method as
follows:
Private Sub Command1_Click()
On Error GoTo ThisSubErr
Err.Raise 17
Exit Sub
ThisSubErr:
AppWideErr (Err.Number)
End Sub
6. Add a module to the project. Add the following code to the module:
Public Sub AppWideErr(lnErrNumber)
Select Case lnErrNumber 'Evaluate error passed to routine
Case 70
'The following two lines of code should be typed on one line.
'Added title to MsgBox to make it clearer where error message
'came from.
MsgBox "Generic Routine. Access Denied. See Net Administrator."
, , "AppWideErr"
Exit Sub
Case Else
'The following two lines of code should be typed on one line.
'Added title to MsgBox to make it clearer where error message
'came from.
MsgBox "Generic Routine. Unhandled Error: " + Err.Description +
" # " & lnErrNumber , , "AppWideErr"
Exit Sub
End Select
End Sub
7. Save and run the project. Note that some errors will be handled by the error
routine specific to a method. The logic for each method invokes the generic
AppWideErr procedure for any errors not explicitly handled by the method. The
error number is passed as a numeric argument to AppWideErr.
REFERENCES
==========
Visual Basic 5.0 Books Online - Error Handling
Visual Basic 6.0 MSDN - Error Handling
Additional query words: kbVBp400 kbVBp500 kbVBp600 kbdse kbDSupport kbVBp kbNoKeyWord
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Keywords : kbGrpDSVBDB
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Issue type : kbhowto
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