In the previous two chapters, you saw how to create a Windows Forms application that uses a variety of controls for data entry. You created menus to make the application easier to use. You have learned how to trap events raised by menus, forms, and controls so that your application can actually do something besides just look pretty. While careful design of a form and the appropriate use of controls can help to ensure that the information entered by a user makes sense, there are often additional checks that you need to perform. In this chapter, you will learn how to validate the data entered by a user running an application to ensure that it matches any business rules specified by the application's requirements.