Validation controls are added to a Web Forms page like other server controls. There are controls for specific types of validation, such as range checking or pattern matching, plus a RequiredFieldValidator that ensures that a user does not skip an entry field. You can attach more than one validation control to an input control. For example, you might specify both that an entry is required and that it must contain a specific range of values.
Validation controls work with a limited subset of HTML and Web server controls.
For each control, a specific property contains the value to be validated. The following table lists the input controls that may be validated. Value DropDownList SelectedItem. Value RadioButtonList SelectedItem. The simplest form of validation is a required field.