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If you write your own DFX function, call IsValidOperation at the beginning of your function to determine whether the current operation can be performed on a particular field data member type (a CDaoFieldExchange::outputColumn or a CDaoFieldExchange::param).

BOOL IsValidOperation( );

Nonzero if the current operation is appropriate for the type of field being updated.

Some of the operations performed by the DFX mechanism apply only to one of the possible field types. Follow the model of the existing DFX functions.

For additional information on writing custom DFX routines, see Technical Note 53.