Use OnException to change the default behavior that occurs when an exception is not handled by application code. If an exception passes through the try blocks in the application code, the application automatically calls the HandleException method. The HandleException method generates an OnException event when it encounters an exception that descends from Exception. If there is no OnException event handler, or if the Exception object does not descend from Exception, the applet displays a message box indicating an error occurred. 

HandleException