Whether an exception is thrown by the runtime system or by a THROW statement in an application script, you handle the exception by catching it. This is done by surrounding the set of application logic that throws the exception with code that indicates how the exception is to be dealt with.
To handle an exception in PowerScript, you must include some set of your application logic inside a try-catch block. A try-catch block begins with a TRY clause and ends with the END TRY statement. It must also contain either a CATCH clause or a FINALLY clause. A try-catch block normally contains a FINALLY clause for error condition cleanup. In between the TRY and FINALLY clauses you can add any number of CATCH clauses.
CATCH clauses are not obligatory, but if you do include them you must follow each CATCH statement with a variable declaration. In addition to following all of the usual rules for local variable declarations inside a script, the variable being defined must derive from the Throwable system type.
You can add a TRY-CATCH-FINALLY, TRY-CATCH, or TRY-FINALLY block using the Script view Paste Special feature for PowerScript statements. If you select the Statement Templates check box on the AutoScript tab of the Design Options dialog box, you can also use the AutoScript feature to insert these block structures.
Example catching a system error This is an example of a TRY-CATCH-FINALLY block that catches a system error when an arccosine argument, entered by the application user (in a SingleLineEdit) is not in the required range. If you do not catch this error, the application goes to the system error event, and eventually terminates:
Double ld_num
ld_num = Double (sle_1.text)
TRY
sle_2.text = string (acos (ld_num))
CATCH (runtimeerror er)
MessageBox("Runtime Error", er.GetMessage())
FINALLY
// Add cleanup code here
of_cleanup()
Return
END TRY
MessageBox("After", "We are finished.")
The system runtime error message might be confusing to the end user, so for production purposes, it would be better to catch a user-defined exception—see the example in “Creating user-defined exception types”—and set the message to something more understandable.
The TRY reserved word signals the start of a block of statements to be executed and can include more than one CATCH clause. If the execution of code in the TRY block causes an exception to be thrown, then the exception is handled by the first CATCH clause whose variable can be assigned the value of the exception thrown. The variable declaration after a CATCH statement indicates the type of exception being handled (a system runtime error, in this case).
It is important to order your CATCH clauses in such a way that one clause does not hide another. This would occur if the first CATCH clause catches an exception of type Exception and a subsequent CATCH clause catches a descendant of Exception. Since they are processed in order, any exception thrown that is a descendant of Exception would be handled by the first CATCH clause and never by the second. The PowerScript compiler can detect this condition and signals an error if found.
If an exception is not dealt with in any of the CATCH clauses, it is thrown up the call stack for handling by other exception handlers (nested try-catch blocks) or by the system error event. But before the exception is thrown up the stack, the FINALLY clause is executed.
The FINALLY clause is generally used to clean up after execution of a TRY or CATCH clause. The code in the FINALLY clause is guaranteed to execute if any portion of the try-catch block is executed, regardless of how the code in the try-catch block completes.
If no exceptions occur, the TRY clause completes, followed by the execution of the statements contained in the FINALLY clause. Then execution continues on the line following the END TRY statement.
In cases where there are no CATCH clauses but only a FINALLY clause, the code in the FINALLY clause is executed even if a return is encountered or an exception is thrown in the TRY clause.
If an exception occurs within the context of the TRY clause and an applicable CATCH clause exists, the CATCH clause is executed, followed by the FINALLY clause. But even if no CATCH clause is applicable to the exception thrown, the FINALLY clause still executes before the exception is thrown up the call stack.
If an exception or a return is encountered within a CATCH clause, the FINALLY clause is executed before execution is transferred to the new location.
