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

//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);