public IReadOnlyList<Field> GroupBy {get; set;}

{
  // Must be called within QueuedTask.Run
  // Calculate the Sum and Average of the Population_1990 and Population_2000 fields, grouped and ordered by Region
  void CalculateStatistics(FeatureClass countryFeatureClass)
  {
    using (FeatureClassDefinition featureClassDefinition = countryFeatureClass.GetDefinition())
    {
      // Get fields
      Field regionField = featureClassDefinition.GetFields()
        .First(x => x.Name.Equals("Region"));

      Field pop1990Field = featureClassDefinition.GetFields()
        .First(x => x.Name.Equals("Population_1990"));

      Field pop2000Field = featureClassDefinition.GetFields()
        .First(x => x.Name.Equals("Population_2000"));

      // Create StatisticsDescriptions
      StatisticsDescription pop1990StatisticsDescription = new StatisticsDescription(pop1990Field,
        new List<StatisticsFunction>()
        {
          StatisticsFunction.Sum,
          StatisticsFunction.Average
        });

      StatisticsDescription pop2000StatisticsDescription = new StatisticsDescription(pop2000Field,
        new List<StatisticsFunction>()
        {
          StatisticsFunction.Sum,
          StatisticsFunction.Average
        });

      // Create TableStatisticsDescription
      TableStatisticsDescription tableStatisticsDescription = new TableStatisticsDescription(
        new List<StatisticsDescription>()
        {
          pop1990StatisticsDescription, pop2000StatisticsDescription
        });
      tableStatisticsDescription.GroupBy = new List<Field>() { regionField };
      tableStatisticsDescription.OrderBy = new List<SortDescription>() { new SortDescription(regionField) };

      // Calculate Statistics
      IReadOnlyList<TableStatisticsResult> tableStatisticsResults =
        countryFeatureClass.CalculateStatistics(tableStatisticsDescription);

      foreach (TableStatisticsResult tableStatisticsResult in tableStatisticsResults)
      {
        // Get the Region name
        // If multiple fields had been passed into TableStatisticsDescription.GroupBy, there would be multiple values in TableStatisticsResult.GroupBy
        string regionName = tableStatisticsResult.GroupBy.First().Value.ToString();

        // Get the statistics results for the Population_1990 field
        StatisticsResult pop1990Statistics = tableStatisticsResult.StatisticsResults[0];
        double population1990Sum = pop1990Statistics.Sum;
        double population1990Average = pop1990Statistics.Average;

        // Get the statistics results for the Population_2000 field
        StatisticsResult pop2000Statistics = tableStatisticsResult.StatisticsResults[1];
        double population2000Sum = pop2000Statistics.Sum;
        double population2000Average = pop2000Statistics.Average;

        // Do something with the results here...
      }
    }
  }
}