public override object Eval(DataRow row)
{
//TODO: implement a better caching strategy and a mechanism for cache invalidation.
//for now only aggregation over the table owning 'row' (e.g. 'sum(parts)'
//in constrast to 'sum(child.parts)') is cached.
if (cacheResults && result != null && column.ReferencedTable == ReferencedTable.Self)
{
return(result);
}
count = 0;
result = null;
object[] values;
if (rows == null)
{
values = column.GetValues(column.GetReferencedRows(row));
}
else
{
values = column.GetValues(rows);
}
foreach (object val in values)
{
if (val == null)
{
continue;
}
count++;
Aggregate((IConvertible)val);
}
switch (function)
{
case AggregationFunction.StDev:
case AggregationFunction.Var:
result = CalcStatisticalFunction(values);
break;
case AggregationFunction.Avg:
result = ((count == 0) ? DBNull.Value : Numeric.Divide(result, count));
break;
case AggregationFunction.Count:
result = count;
break;
}
if (result == null)
{
result = DBNull.Value;
}
if (cacheResults && column.ReferencedTable == ReferencedTable.Self)
{
table = row.Table;
row.Table.RowChanged += RowChangeHandler;
}
return(result);
}