DataWindow Programmers Guide
Using DataWindow Objects

Retrieve and update testing

When using the Retrieve or Update method in a DataWindow control, you should test the method's return code to see whether the activity succeeded.

Example

PowerBuilder

If you want to commit changes to the database only if an update succeeds, you can code:

IF dw_emp.Update() > 0 THEN
      COMMIT USING EmpSQL;
ELSE
      ROLLBACK USING EmpSQL;
END IF

Using the DBError event

The DataWindow control triggers its DBError event whenever there is an error following a retrieval or update; that is, if the Retrieve or Update methods return -1. For example, if you try to insert a row that does not have values for all columns that have been defined as not allowing NULL, the DBMS rejects the row and the DBError event is triggered.

By default, the DataWindow control displays a message box describing the error message from the DBMS, as shown here:

In many cases you might want to code your own processing in the DBError event and suppress the default message box. Here are some tips for doing this:

Example

PowerBuilder

Here is a sample script for the DBError event:

// Database error -195 means that some of the
// required values are missing
IF sqldbcode = -195 THEN
      MessageBox("Missing Information",  &   
         "You have not supplied values for all " &
         +"the required fields.")
END IF
// Return code suppresses default message box
RETURN 1

During execution, the user would see the following message box after the error:

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:

dw_1.Object.emp_name.Visible = "0"

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:

TRY
   dw_1.Object.emp_name.Visible = "0"
CATCH (dwruntimeerror dw_e)
   MessageBox ("DWRuntimeError", dw_e.text)
END TRY

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.