public Geometry GetFeatureOutline( 
   MapView view,
   FeatureOutlineType outlineType
)

          // Note: Run within QueuedTask.Run
          // Set the spatial filter geometry

          //Get the geometry from the selected features in the feature layer
          var spatialClauseGeom = spatialDefnLayer.GetFeatureOutline(MapView.Active, FeatureOutlineType.Selected);

DefinitionQuery definitionQuery = new DefinitionQuery
{
  WhereClause = whereClause,
  Name = $"{layerToQuery.Name}"
};
//Setting the spatial filter to the Definition Query
if (definitionQuery.CanSetFilterGeometry(spatialClauseGeom))
{
  definitionQuery.SetFilterGeometry(spatialClauseGeom);
}

layerToQuery.InsertDefinitionQuery(definitionQuery);
layerToQuery.SetActiveDefinitionQuery(definitionQuery.Name);

    var greatLakes = map.GetLayersAsFlattenedList()
                .OfType<FeatureLayer>().First(l => l.Name == "Great Lakes");
    var usa_states = map.GetLayersAsFlattenedList()
    .OfType<FeatureLayer>().First(l => l.Name == "US_States");

    QueuedTask.Run(() =>
    {
//name must be unique
        var def_query = new DefinitionQuery("GreatLakes",
                                                    "NAME in ('Huron','Michigan','Erie')");

        //create a filter geometry - in this example we will use the outline geometry
        //of all visible features from a us states layer...the filter geometry will be
//intersected with the layer feature geometry when added to the def query
        var filter_geom = usa_states.GetFeatureOutline(mv, FeatureOutlineType.Visible);
        //other options...
        //var filter_geom = usa_states.GetFeatureOutline(mv, FeatureOutlineType.All);
        //var filter_geom = usa_states.GetFeatureOutline(mv, FeatureOutlineType.Selected);

        //Geometry must have a valid SR and be point, multi-point, line, or poly
        if (def_query.CanSetFilterGeometry(filter_geom))
{
  def_query.SetFilterGeometry(filter_geom);
}

        //Apply the def query
        greatLakes.InsertDefinitionQuery(def_query);
        //Set it active
        greatLakes.SetActiveDefinitionQuery(def_query.Name);

        //or....also - set it active when it is inserted
        //greatLakes.InsertDefinitionQuery(def_query, true);
    });

      var greatLakes = map.GetLayersAsFlattenedList()
                  .OfType<FeatureLayer>().First(l => l.Name == "Great Lakes");
var michigan = map.GetBookmarks().First(b => b.Name == "Michigan");

      QueuedTask.Run(() =>
      {

          //get all features - multiple feature geometries are always returned as a
  //single multi-part
          var all_features_outline = greatLakes.GetFeatureOutline(mv, FeatureOutlineType.All);

  //or get just the outline of selected features
  var qry = new QueryFilter()
  {
    SubFields = "*",
    WhereClause = "NAME in ('Huron','Michigan','Erie')"
  };
  greatLakes.Select(qry);
          var sel_features_outline = greatLakes.GetFeatureOutline(
      mv, FeatureOutlineType.Selected);
  greatLakes.ClearSelection();

          //or just the visible features
          mv.ZoomTo(michigan);
          var visible_features_outline = greatLakes.GetFeatureOutline(
                  mv, FeatureOutlineType.Visible);
      });