public string SortField {get; set;}

//Modify the currently active map series and changes its sort field and page number field.

//Perform on the worker thread
await 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.

//Reference map frame and index layer
MapFrame mf = layout.FindElement("Map Frame") as MapFrame;
Map m = mf.Map;
BasicFeatureLayer indexLyr = m.FindLayers("Countries").FirstOrDefault() as BasicFeatureLayer;

//Construct map series on worker thread
await QueuedTask.Run(() =>
{
  //SpatialMapSeries constructor - required parameters
  SpatialMapSeries SMS = MapSeries.CreateSpatialMapSeries(layout, mf, 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.
});