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

// Calculate the Sum and Average of the Population_1990 and Population_2000 fields, grouped and ordered by Region
public 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...
    }
  }
}