I ended up working this out, and will post my solution here in case others find it useful.
If a weighted average consists of both a value and a weight per row, then column that contains the value should have the weight GridColumn object assigned to its Tag property. Then, this event handler will do the rest:
private static void gridView_CustomSummaryCalculate(object sender, CustomSummaryEventArgs e)
{
GridColumn weightColumn = ((GridSummaryItem)e.Item).Tag as GridColumn;
if (weightColumn == null)
return;
switch (e.SummaryProcess)
{
case CustomSummaryProcess.Start:
{
e.TotalValue = new WeightedAverageCalculator();
break;
}
case CustomSummaryProcess.Calculate:
{
double size = Convert.ToDouble(e.FieldValue);
double weight = Convert.ToDouble(((GridView)sender).GetRowCellValue(e.RowHandle, weightColumn));
((WeightedAverageCalculator)e.TotalValue).Add(weight, size);
break;
}
case CustomSummaryProcess.Finalize:
{
e.TotalValue = ((WeightedAverageCalculator)e.TotalValue).Value;
break;
}
}
}
private sealed class WeightedAverageCalculator
{
private double _sumOfProducts;
private double _totalWeight;
public void Add(double weight, double size)
{
_sumOfProducts += weight * size;
_totalWeight += weight;
}
public double Value
{
get { return _totalWeight==0 ? 0 : _sumOfProducts / _totalWeight; }
}
}
The code assumes that the underlying column values can be converted to doubles via Convert.ToDouble(object)
.