Mastering PowerBuilder

DataWindow Dot Notation - Error Handling

When you use the ".Object" convention, the compiler checks the syntax only till "Object". Everything following the Object property is evaluated at execution time. PowerBuilder triggers the DataWindow control's Error event whenever an error occurs in the expression at execution time. The Error event allows you to respond with error recovery logic when an expression is not valid. If you write a script for this event, you can catch an error before it triggers the "SystemError" event at the application object. For example:

dw_product.Object.Data[ dw_product.GetRow(),2] = & 'obsolete ' + & dw_product.Object.Data[ dw_product.GetRow(),2]

In the above expression, we are marking a product obsolete by prefixing "obsolete" to the product_description. The above code assumes the "product_description" would be the second column in the DataWindow. Just for testing, replace 2 with 1 and append this code to the ue_retrieve event of the w_product_master and run the application. You won't get the compilation error and will get run-time error. Since, we wrote no code for the "Error" event for the dw_product DataWindow control, this error will trigger "SystemError" event.

The "Error" event has several arguments that provide information about the error condition. The first five arguments in the following listing are the same as the attributes that are available at error object.

	errornumber
	errortext
	errorwindowmenu
	errorobject
	errorscript
	errorline
	action
	returnvalue

The last two arguments "action" and "ReturnValue" are passed by reference that allow you to affect the outcome of the event. You can specify one of the following four values for the action:

	ExceptionFail!
	ExceptionIgnore!
	ExceptionRetry!
	ExceptionSubstituteReturnValue!

The default value for action is "ExceptionFail!". That means, if you don't write any code for this event, action contains ExceptionFail!. This value triggers "SystemError" event. You can populate "ReturnValue" with a value whose data type matches the expected value that the DataWindow would have returned. This value is used when the value of "action" is ExceptionSubstituteReturnValue! Do not use ExceptionSubstituteReturnValue! to substitute a return value when an element in the middle of an expression causes an error. The ExceptionSubstituteReturnValue! is most useful for handling errors in data expressions.

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