After aggregate funtion,How can I get the function result?

[PEIYANG]

My code is：

          var avg= new AggregateFunction<student, decimal>
                {
                    AggregationExpression = myItem => Math.Round(myItem.Where(i => i.age!= 0).Average(c => c.age), 3)
                };
                ((GridViewDataColumn)this.RgvVerify.Columns[1]).AggregateFunctions.Add(avg);
What is the result,How can I get the function result??