Q130526: How to Use the ON ERROR Command and the Error Event

Article: Q130526
Product(s): Microsoft FoxPro
Version(s): 3.00
Operating System(s): 
Keyword(s): kbcode
Last Modified: 21-AUG-1999

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

In version 2.x of FoxPro, the ON ERROR command allowed the developer to code a
routine to handle errors that occurred while an application was running. This
functionality is still present in Visual FoxPro. In addition, Visual FoxPro has
a new error event. This article demonstrates how to use the ON ERROR command and
the error event in a Visual FoxPro application.

MORE INFORMATION
================

The error event allows error processing for a specific control, class, or form
to be encapsulated for that object. The error event overrides any ON ERROR
routine that is in effect.

An application can contain ON ERROR commands, error events, or a combination of
both.

Step-by-Step Example
--------------------

In the following example, there might be an application-wide ON ERROR routine, an
error event for the form itself, and an error event for the employee ID field
itself.

The application handles payroll processing. A form in the application allows you
to input new employees into a table.

1. At the beginning of the main program, add a command to handle generic errors
  that occur in the application:

     ON ERROR DO errhandler

2. In the form.error event, add code that will execute if the program is unable
  to open the Employee table in case the system administrator is packing or
  reindexing the file. The generic ON ERROR routine will be accessed if a
  different error occurs:

     LPARAMETERS nError, cMethod, nLine
     * File in use by another error
     IF nError = 108
        =MESSAGEBOX("System maintenance is in effect." + ;
                    "Please try again later")
     ELSE
        DO errhandler
     ENDIF

3. In the Employee ID field, add code to handle a duplicate value plus any other
  errors that may occur:

     LPARAMETERS nError, cMethod, nLine
     * Uniqueness of index has been violated
     IF nError = 1884
        =MESSAGEBOX("This employee ID already exists on file")
        =TABLEREVERT()
     ELSE
        DO errhandler
     ENDIF

As an alternative to using the ON ERROR routine, you can define an error event
within a class. Once the class is instantiated, the error event is available to
be called within the application. The syntax for calling the class error event
is:

  oMyObject.Error

Using an error event within a class allows you to encapsulate specific error
processing within the class. The error event is inherited by any subclasses
defined from the parent class.

The error event is not inherited or called automatically from any controls or
objects defined within the class or form. For example, if an error event is
defined for a form and no error events are coded for individual controls on that
form, the error event will not be called if an error is encountered in a text
box or command button on the form. To call the form's error event, code the
following in each control's error event:

     ThisForm.Error

In this case, an ON ERROR command might be preferable to placing error handling
code in the error event of the form itself. Then each control would not need to
include code to invoke error processing.

When an error event is invoked, Visual FoxPro automatically passes the number of
the error that occurred, the name of the method where the error occurred, and
the line number of code on which it occurred. This information should be passed
to an ON ERROR program as parameters by using the ERROR() and PROGRAM()
functions. As an alternative, the AERROR() function can be used within the
routine to obtain this information.

Additional query words: VFoxWin

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Keywords          : kbcode 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300
Version           : 3.00

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