In asp.net Frameword 4.0 or less,
Textbox have Textmode property which is very helpful for the user.
Currently Textmode property can only be set to following 3 options.
This mode can be selected if you want to show the input in textbox as dot's (Password type).
Use to retain the textbox as single line field.
Use to make textbox as commentbox , i.e to have multiple lines...
HOpe this will help...
as error you mentioned above is only because of this reason coz you are trying to put textmode those values which are not supported.
Further if you want to do the validation on textboxes for Date or for Numbers,
You can use AjaxMaskExtender or FilteredTextBoxExtenders,