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

//Map Tool is used to perform Spatial selection.
//Graphic selection uses the selection geometry 
//to intersect the geometries of those elements (graphic or group) 
//that will be selected and then highlights them. 
protected override async Task<bool> OnSketchCompleteAsync(Geometry geometry)
{
  var selPoly = geometry as Polygon;
  return await QueuedTask.Run(() =>
  {     
    //note: the selected elements may belong to more than one layer...
    var elems = MapView.Active.SelectElements(selPoly, SelectionCombinationMethod.New);
    return true;
  });
}

//on the QueuedTask
//Create an extent to use for the spatial selection
var extent = MapView.Active.Extent;
var selectionExtent = extent.Expand(0.5, 0.5, true);
//Select elements in specified graphics layer using the selection extent.
var selectedElements = MapView.Active.SelectElements(graphicsLayer, selectionExtent, SelectionCombinationMethod.Add);

var graphicsLayer = MapView.Active.Map.GetLayersAsFlattenedList()
                    .OfType<ArcGIS.Desktop.Mapping.GraphicsLayer>().FirstOrDefault();
if (graphicsLayer == null)
  return;
var all_text = graphicsLayer.GetElementsAsFlattenedList()
              .Where(e => e.GetGraphic() is CIMTextGraphic);

var options = ApplicationOptions.SelectionOptions;

QueuedTask.Run(() =>
{
  var defaultColor = options.DefaultSelectionColor;

  var color = options.SelectionColor as CIMRGBColor;
  options.SetSelectionColor(ColorFactory.Instance.CreateRGBColor(255, 0, 0));


  var defaultFill = options.DefaultSelectionFillColor;
  var fill = options.SelectionFillColor;
  var isHatched = options.IsSelectionFillHatched;
  options.SetSelectionFillColor(ColorFactory.Instance.CreateRGBColor(100, 100, 0));
  if (!isHatched)
    options.SetSelectionFillIsHatched(true);

  var showSelectionChip = options.ShowSelectionChip;
  options.SetShowSelectionChip(!showSelectionChip);

  var showSelectionGraphic = options.ShowSelectionGraphic;
  options.SetShowSelectionGraphic(!showSelectionGraphic);

  var saveSelection = options.SaveSelection;
  options.SetSaveSelection(!saveSelection);

  var defaultTol = options.DefaultSelectionTolerance;
  var tol = options.SelectionTolerance;
  options.SetSelectionTolerance(2 * defaultTol);

  // extension methods available 
  var selMethod = options.SelectionMethod;
  options.SetSelectionMethod(SelectionMethod.Contains);

  var combMethod = options.CombinationMethod;
  options.SetCombinationMethod(SelectionCombinationMethod.Add);

  // note that the following SelectionCombinationMethod is not supported
  //options.SetCombinationMethod(SelectionCombinationMethod.XOR);
});

var options = ApplicationOptions.SelectionOptions;

QueuedTask.Run(() =>
{
  var defaultColor = options.DefaultSelectionColor;

  var color = options.SelectionColor as CIMRGBColor;
  options.SetSelectionColor(ColorFactory.Instance.CreateRGBColor(255, 0, 0));


  var defaultFill = options.DefaultSelectionFillColor;
  var fill = options.SelectionFillColor;
  var isHatched = options.IsSelectionFillHatched;
  options.SetSelectionFillColor(ColorFactory.Instance.CreateRGBColor(100, 100, 0));
  if (!isHatched)
    options.SetSelectionFillIsHatched(true);

  var showSelectionChip = options.ShowSelectionChip;
  options.SetShowSelectionChip(!showSelectionChip);

  var showSelectionGraphic = options.ShowSelectionGraphic;
  options.SetShowSelectionGraphic(!showSelectionGraphic);

  var saveSelection = options.SaveSelection;
  options.SetSaveSelection(!saveSelection);

  var defaultTol = options.DefaultSelectionTolerance;
  var tol = options.SelectionTolerance;
  options.SetSelectionTolerance(2 * defaultTol);

  // extension methods available 
  var selMethod = options.SelectionMethod;
  options.SetSelectionMethod(SelectionMethod.Contains);

  var combMethod = options.CombinationMethod;
  options.SetCombinationMethod(SelectionCombinationMethod.Add);

  // note that the following SelectionCombinationMethod is not supported
  //options.SetCombinationMethod(SelectionCombinationMethod.XOR);
});