Errors in property and data expressions and the Error event – PB Docs 2019

A DataWindow control’s Error event is triggered whenever an error
occurs in a data or property expression at execution time. These
expressions that refer to data and properties of a DataWindow object
might be valid under some execution-time conditions but not others. The
Error event allows you to respond with error recovery logic when an
expression is not valid.

PowerBuilder syntax
checking

In PowerBuilder, when you use a data or property expression, the
PowerScript compiler checks the syntax only as far as the Object
property. Everything following the Object property is evaluated at
execution time. For example, in the following expression, the column
name emp_name and the property Visible are not checked until execution
time:

If the emp_name column did not exist in the DataWindow, or if you
had misspelled the property name, the compiler would not detect the
error. However, at execution time, PowerBuilder would trigger the
DataWindow control’s Error event.

Using a Try-Catch block

The Error event is triggered even if you have surrounded an
error-producing data or property expression in a Try-Catch block. The
catch statement is executed after the Error event is triggered, but only
if you do not code the Error event or do not change the default Error
event action from ExceptionFail!. The following example shows a property
expression in a Try-Catch block:

Determining the cause of the
error

The Error event has several arguments that provide information
about the error condition. You can check the values of the arguments to
determine the cause of the error. For example, you can obtain the
internal error number and error text, the name of the object whose
script caused the error, and the full text of the script where the error
occurred. The information provided by the Error event’s arguments can be
helpful in debugging expressions that are not checked by the
compiler.

If you catch a DWRuntimeError error, you can use the properties of
that class instead of the Error event arguments to provide information
about the error condition. The following table displays the
correspondences between the Error event arguments and the DWRuntimeError
properties.

Controlling the outcome of the
event

When the Error event is triggered, you can have the application
ignore the error and continue processing, substitute a different return
value, or escalate the error by triggering the SystemError event. In the
Error event, you can set two arguments passed by reference to control
the outcome of the event.

For a complete description of the arguments of the Error event,
see the section called “Error” in DataWindow Reference.