Aspose::Cells::Pivot::PivotField::SetBaseItemPosition method. Represents the item in the base field for a custom calculation. Valid Only for data fields. Because PivotItemPosition.Custom is Only for read,if you need to set PivotItemPosition.Custom, please set PivotField.BaseItemIndex attribute in C++.... Valid only for data fields. Because PivotItemPosition...PivotItemPosition.Custom is only for read,if you need to set PivotItemPosition...