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...
        }
    }
}