Using the SystemError and Error events
Error event
If a runtime error occurs, an error structure that describes
the error is created. If the error occurs in the context of a connection
to a remote server (such as EAServer) then the Error event on the
Connection, JaguarORB, DataWindow, or OLE control object is triggered,
with the information in the error structure as arguments.
The error can be handled in this Error event by use of a special
reference argument that allows the error to be ignored. If the error
does not occur in the context described above, or if the error in
that context is not dealt with, then the error structure information
is used to populate the global error variable and the SystemError
event on the Application object is triggered.
SystemError event
In the SystemError event, unexpected error conditions can
be dealt with in a limited way. In general, it is not a good idea
to continue running the application after the SystemError event
is triggered. However, error-handling code can and should be added
to this event. Typically you could use the SystemError event to
save data before the application terminates and to perform last-minute cleanup
(such as closing files or database connections).
Precedence of exception handlers and events
If you write code in the Error event, then that code is executed
first in the event of a thrown exception.
If the exception is not thrown in any of the described contexts
or the object’s Error event does not handle the exception
or you do not code the Error event, then the exception is handled
by any active exception handlers (CATCH clauses)
that are applicable to that type of exception. Information from
the exception class is copied to the global error variable and the
SystemError event on the Application object is fired only if there
are no exception handlers to handle the exception.
Error handling for new applications
For new PowerBuilder applications, the recommended approach
for handling errors is to use a try-catch block instead of coding
the Error event on Connection, DataWindow, or OLE control objects.
You should still have a SystemError event coded in your Application
object to handle any uncaught exceptions. The SystemError event
essentially becomes a global exception handler for a PowerBuilder
application.