Can anyone tell me how to set up a validator on a control for each row of a datagrid that is dependent on the values of another control on the row.  For example, I want to ensure that a textbox on the datagrid row is blank if a particular item of a dropdown list on the row has been selected.

I have read articles that explain how to use a customvalidator to reference other controls on a page, but I don't know how to get this to work for each row on a datagrid.
I'm very new to all of this, so the answer may be simple.