The UI (user interface) object provides a number of methods that can be used to display custom and built-in dialog boxes.
This member can be accessed only by forms running in the same domain as the currently open form, or by forms that have been granted cross-domain permissions.
In the following example, the UI property of the XDocument object is used to display a simple message box using the Alert method:
thisXDocument.UI.Alert("Here is the message text.");