Engine中如何统计要素类中某一类要素的某数值型字段的总和？

 IDataStatistics dataStatistics = new DataStatisticsClass();

            IQueryFilter queryFilter = new QueryFilterClass();

            queryFilter.WhereClause = Name= ''松花江''; //要统计的类别

            ICursor cursor = featureClass.Search(queryFilter, false) as ICursor;

            dataStatistics.Field = aa;//数值型字段名称

            dataStatistics.Cursor = cursor;

            IStatisticsResults results = dataStatistics.Statistics;

            double sum = results.Sum;