How to dynamically set a MaskedEditExtender and MaskedEditValidator to a TextBox

I have a TextBox that is used for user input when searching. I have a DropDownList that is used to indicate what field to search. When the user selects Date from the DropDownList I would like to set a MaskedEditExtender and MaskedEditValidator to the TextBox for date entry.

Does anyone know how to do this? I tried setting the TargetControlID="" and ControlToValidate="" initially and then set these properties when the user selects Date from the DropDownList, but it did not like that.

Thanks in advance.