DataSetBroker1.DataSet.First;
while not DataSetBroker1.DataSet.Eof do
begin
DataSetBroker1.DataSet.Edit;
DataSetBroker1.DataSet.FieldByName('C').asFloat :=
DataSetBroker1.DataSet.FieldByName('A').asFloat + DataSetBroker1.DataSet.FieldByName('B').asFloat;
DataSetBroker1.DataSet.Next;
end;