>I would like to assign a TextBox.Text = Null, but couldn't.
>As this TextBox is used to store Date value, and I would like
>to set it to Null when there isn't any date to be set.
>Any clues, or better ideas?

You can't assign NULL to a control but you can insert NULL into the database tables: INSERT INTO Table1(F1,F2,F3) Values('Something',NULL,12345)

So instead of putting NULL into controls, put it during table updates!