An application needs a built-in capability to test conditions and take a different course of action depending on the outcome of the test. Visual Basic provides three control flow, or decision, structures :
Validation is a form of self protecting; it is better to reject bad data then spend hours trying to find out an error only to discover that the problem was caused by a “user error”.
A message box is special type of Visual Basic window in which you can display a message to the user. You can display message, an optional icon, a title bar caption, and command buttons in a message box.