Validation of invisible textbox

I have a textbox which gets the value of the select option from a dropdownlist, the textbox is binded to a property of the ItemType which has the Required set to true.  So if no selection is made from the dropdownlist, it will trigger the validation for the required field upon save. However it works (textbox has the value selected, no validation message pops up) when the textbox is visible. But it doesn't seem to work if I change it to invisible, as ideally the textbox should be hidden, the required validation message always pops up even when a selection has been made from the dropdownlist. Please advise if this is possible or should I use other ways to achieve this?