visual-foxpro Getting started with visual-foxpro Add Global Error Handler

Example

A simple way to catch unhandled errors (exceptions) in a VFP application is to use the ON ERROR command near the beginning of your main program.

The following ON ERROR command calls a method in the current program called "errorHandler". The values returned by ERROR (the VFP Error Number), MESSAGE (the VFP Error Message), PROGRAM (name of the currently executing program) and LINENO (the line number of the error) are passed to the errorHandler method.

ON ERROR DO errorHandler WITH ERROR(), MESSAGE(), PROGRAM(), LINENO()

A simple errorHandler method may look like the following.

PROCEDURE errorHandler
    LPARAMETERS tnVFPErrorNumber, tcVFPErrorMessage, tcProcWithError, tnLineNumber

    STORE 'Error message: ' + tcVFPErrorMessage + CHR(13) + ;
        'Error number: ' + TRANSFORM(tnVFPErrorNumber) + CHR(13) + ;
        'Procedure with error: ' + tcProcWithError + CHR(13) + ;
        'Line number of error: ' + TRANSFORM(tnLineNumber) TO lcDetails

    MESSAGEBOX(lcDetails, 16, "Unhandled Exception")

    ON ERROR *
    ON SHUTDOWN
    CLEAR EVENTS

    QUIT
ENDPROC

You can also change and restore the error handler in between. For example, at one point you want to open all tables in a folder exclusively, and if you can't you don't want to continue:

procedure DoSomethingWithExclusiveLock(tcFolder)
local lcOldError, llInUse, ix && by default these variables have a value of .F.
lcError = on('error') && save current handler
on error llInUse = .T.  && new handler
local array laTables[1]  
for ix=1 to adir(laTables, addbs(m.tcFolder) + '*.dbf'))
   use (addbs(m.tcFolder)+laTables[m.ix,1]) in 0 exclusive
endfor
on error &lcError && restore old handler
if m.llInUse && couldn't get exclusive lock on all tables
   close databases all
   return
endif
* do whatever
endproc

Tip: Sometimes, especially during debugging, you would want to restore default error handler which allows you to break and step into the code where the error has occurred, then anywhere before where you got the error, temporarily adding:

on error

would do this.