Perform server-side validation of user input.

Use events belonging to server controls, for example, the TextChanged event of the TextBox control.

Perform client-side validation of user input.

Use a Validation control. Set the ControlToValidate property to the control to be validated and the ErrorMessage property to an error message to be displayed. Verify that the EnableClientScript property is set to True.

Force the user to enter a value in a text box.

Use a RequiredFieldValidator control.

Check the type and range of data values entered into a text box.

Use a RangeValidator control. Set the Type, MaximumValue, and MinimumValue properties as required.

Display a summary of validation error messages.

Use a ValidationSummary control. Verify that the ShowSummary property is set to True.