How to use aggregatefuncitions to filter some data(just like only make even row number to operate),my code seems wrong?

[PEIYANG]

At first I write like this,But it can't solve my problem, column.AggregateFunctions.Add(new AverageFunction

{

FunctionName = "DisplayAverageFunction",

Caption = "Average: ",

ResultFormatString = "{0:n}",

SourceField = FilterSourceField(column.Header.ToString())  where value.Row%2==0

});  And I write another code,it still can't  solve my problem

 var avg = new AggregateFunction<student, double>
                {
                    AggregationExpression = myItem =>myItem.Where(i => i.age>=20 && i.row%2==0)
                };
                ((GridViewDataColumn)this.RgvVerify.Columns[0]).AggregateFunctions.Add(avg);