Handling errors from DataWindow property expressions in PowerBuilder

In PowerBuilder, an invalid DataWindow property expression causes a runtime error in your application. A runtime error causes the application to terminate unless you catch the error in a runtime error handler or unless there is a script for the Error event.

You can prevent the application from terminating by handling the error in the DataWindow control’s Error event or by catching the error in a try-catch block.

The Error event’s arguments give you several options for responding to the error. You choose a course of action and set the action argument to a value of the ExceptionAction enumerated datatype.

If you are trying to find out a property value and you know the expression might cause an error, you can include code that prepares for the error by storing a default value in an instance variable. Then the Error event script can return that value in place of the failed expression.

There are three elements to this technique: the declaration of an instance variable, the script that sets the variable’s default value and then accesses a DataWindow property, and the Error event script. These elements are shown in Example 2 below.

You can prevent the application from terminating by handling the DataWindow runtime error (DWRuntimeError) in a try-catch block. If you are trying to find out a property value and you know the expression might cause an error, you can include code that automatically assigns a valid default value that can be substituted for the failed expression, as in Example 2 below.

Example 1 This code displays complete information about the error in a multilineedit mle_1.

The error event script:

mle_1.text = &
   "error#: " + string(errornumber) + "~r~n" + &
   "text: " + errortext + "~r~n" + &
   "parent: " + errorwindowmenu + "~r~n" + &
   "object: " + errorobject + "~r~n" + &
   "line: " + string(errorline) + "~r~n"
action = ExceptionIgnore!

The try-catch block:

Try
   ... //DataWindow property expression
Catch (DWRuntimeError myExc)
   mle_1.text = &
   "error#: " + string(myExc.number) + "~r~n" +&
   "text: " + myExc.text + "~r~n" + &
   "script: " + myExc.routinename + "~r~n" + &
   "object: " + myExc.objectname + "~r~n" + &
   "line: " + string(myExc.line) + "~r~n"
End Try

If the correct evaluation of the expression is not critical to the application, the application continues without terminating.

Example 2 This example provides a return value that will become the expression’s value if evaluation of the expression causes an error.

There are three elements to code in the error event script. The instance variable is a string:

string is_dwvalue

This script for a button or other control stores a valid return value in an instance variable and then accesses a DataWindow property:

is_dwvalue = "5"
ls_border = dw_1.Object.id.Border

The Error event script uses the instance variable to provide a valid return value:

action = ExceptionSubstituteReturnValue!
returnvalue = is_dwvalue

The try-catch block:

try
   ls_border = dw_1.Object.id.Border
catch (DWRuntimeError myDWError)
   ls_border = "5"
end try

At runtime, if the id column does not exist or some other error occurs, then the expression returns a valid border value—here the string "5". If you are using the Error event instead of a try-catch block, you must first store the value in an instance variable.