public enum FieldStatisticsFlag : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable  

//Note: Call within QueuedTask.Run()
//The fields in the datasource used for the report
//This uses a US Cities dataset
var listFields = new List<CIMReportField> {
          //Grouping should be the first field
          new CIMReportField{Name = "STATE_NAME", FieldOrder = 0, Group = true, SortInfo = FieldSortInfo.Desc}, //Group cities using STATES
          new CIMReportField{Name = "CITY_NAME", FieldOrder = 1},
          new CIMReportField{Name = "POP1990", FieldOrder = 2, },
      };
//Definition query to use for the data source
var defQuery = "STATE_NAME LIKE 'C%'";
//Define the Datasource
//pass true to use the selection set
var reportDataSource = new ReportDataSource(featureLayer, defQuery, false, listFields);
//The CIMPage defintion - page size, units, etc
var cimReportPage = new CIMPage
{
  Height = 11,
  StretchElements = false,
  Width = 6.5,
  ShowRulers = true,
  ShowGuides = true,
  Margin = new CIMMargin { Bottom = 1, Left = 1, Right = 1, Top = 1 },
  Units = LinearUnit.Inches
};

//Report template
var reportTemplates = await ReportTemplateManager.GetTemplatesAsync();
var reportTemplate = reportTemplates.Where(r => r.Name == "Attribute List with Grouping").First();

//Report Styling
var reportStyles = await ReportStylingManager.GetStylingsAsync();
var reportStyle = reportStyles.Where(s => s == "Cool Tones").First();

//Field Statistics
var fieldStatisticsList = new List<ReportFieldStatistic> {
          new ReportFieldStatistic{ Field = "POP1990", Statistic = FieldStatisticsFlag.Sum}
          //Note: NoStatistics option for FieldStatisticsFlag is not supported.
      };
var report = ReportFactory.Instance.CreateReport("USAReport", reportDataSource, cimReportPage, fieldStatisticsList, reportTemplate, reportStyle);