Must declare the scalar variable @original_xxxx

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I'm trying to understand the full interaction between controls on a formview
that are bound to a SQLDataSource and the binding of the data using ASP.Net
2.0. When I set ConflictDetection to CompareAllValues, I get the error: Must
declare the scalar variable xxxx on fields that are not represented by
controls in the formview templates. If all fields are there, it works fine,
if not, I get the error, even though the fields are in all the commands
(select, update and delete). Since the OldValuesParameterFormatString is an
attribute of the SQLDataSource, shouldn't these parameters be persisted and
used regardless of whether or not they are bould to controls on the
formview's templates?

Tanks