public string SortField {get; set;}

{
  //Modify the currently active map series and changes its sort field and page number field.
  //Note: Must be on QueuedTask.Run
  SpatialMapSeries SMS = layout.MapSeries as SpatialMapSeries; //cast as spatial map series for additional members
  SMS.SortField = "State_Name";
  SMS.SortAscending = true;
  SMS.PageNumberField = "PageNum";

  //Overwrite the current map series with these new settings
  layout.SetMapSeries(SMS);
}

{
  // This example create a new spatial map series and then applies it to the active layout. This will automatically 
  // overwrite an existing map series if one is already present.
  //Note: Must be on QueuedTask.Run;
  //SpatialMapSeries constructor - required parameters
  BasicFeatureLayer indexLyr = map.FindLayers("Countries").FirstOrDefault() as BasicFeatureLayer;
  SpatialMapSeries SMS = MapSeries.CreateSpatialMapSeries(layout, mapFrame, indexLyr, "Name");

  //Set optional, non-default values
  SMS.CategoryField = "Continent";
  SMS.SortField = "Population";
  SMS.ExtentOptions = ExtentFitType.BestFit;
  SMS.MarginType = ArcGIS.Core.CIM.UnitType.PageUnits;
  SMS.MarginUnits = ArcGIS.Core.Geometry.LinearUnit.Centimeters;
  SMS.Margin = 1;
  SMS.ScaleRounding = 1000;
  layout.SetMapSeries(SMS);  //Overwrite existing map series.
}