How to: Add and Configure a RangeValidator Control
Use the RangeValidator control to check that the value of another control falls within a range that you specify.
If the user leaves the targeted control blank, the control passes the range validation. To force the user to enter a value, add a RequiredFieldValidator control as well.
To validate the value of a control against a range of values
Add a RangeValidator control to the form that contains the target control.
Set the control's ControlToValidate property to the ID of the control that you want to validate.
Select the type of the data being validated from one of the types in the drop-down list next to the Type property.
If the value in the targeted control is not valid according to the Type property, the validation fails.