Yes, the RequiredFieldValidator should only service a single control. IMHO, you can either go for a customvalidator as this validator does not require you to specify the ControlToValidate property and you can set the your client side function in the ClientValidationFunction property, or you can register your client side validation when the update button is clicked. The clientid of the textbox should be in the format <datagrid_id>$_ctl<row_number>$_<control_id>, if the id of the textbox is not specified, the datagrid will automatically add the suffix to the place of the control_id. If you are not familiar with the clientid value, you can also emit the clientid value of the textbox in the web page to a client side variable so that you can use the variable later in validation at the client side.
