public Geometry FilterGeometry {get; set;}

public async Task SearchingAFeatureClass()
{
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
        using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
        using (FeatureClass schoolBoundaryFeatureClass = geodatabase.OpenDataset<FeatureClass>("LocalGovernment.GDB.SchoolBoundary"))
        {
            // Using a spatial query filter to find all features which have a certain district name and lying within a given Polygon.
            SpatialQueryFilter spatialQueryFilter = new SpatialQueryFilter
            {
                WhereClause = "DISTRCTNAME = 'Indian Prairie School District 204'",
                FilterGeometry = new PolygonBuilderEx(new List<Coordinate2D>
          {
      new Coordinate2D(1021880, 1867396),
      new Coordinate2D(1028223, 1870705),
      new Coordinate2D(1031165, 1866844),
      new Coordinate2D(1025373, 1860501),
      new Coordinate2D(1021788, 1863810)
          }).ToGeometry(),

                SpatialRelationship = SpatialRelationship.Within
            };

            using (RowCursor indianPrairieCursor = schoolBoundaryFeatureClass.Search(spatialQueryFilter, false))
            {
                while (indianPrairieCursor.MoveNext())
                {
                    using (Feature feature = (Feature)indianPrairieCursor.Current)
                    {
                        // Process the feature. For example...
                        Console.WriteLine(feature.GetObjectID());
                    }
                }
            }
        }
    });
}

public async Task SelectingFeaturesFromAFeatureClass()
{
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
        using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
        using (FeatureClass enterpriseFeatureClass = geodatabase.OpenDataset<FeatureClass>("LocalGovernment.GDB.FacilitySite"))
        {
            List<Coordinate2D> newCoordinates = new List<Coordinate2D>
        {
    new Coordinate2D(1021570, 1880583),
    new Coordinate2D(1028730, 1880994),
    new Coordinate2D(1029718, 1875644),
    new Coordinate2D(1021405, 1875397)
        };

            SpatialQueryFilter spatialFilter = new SpatialQueryFilter
            {
                WhereClause = "FCODE = 'Park'",
                FilterGeometry = new PolygonBuilderEx(newCoordinates).ToGeometry(),
                SpatialRelationship = SpatialRelationship.Crosses
            };

            // For Selecting all matching entries.
            using (Selection anotherSelection = enterpriseFeatureClass.Select(spatialFilter, SelectionType.ObjectID, SelectionOption.Normal))
            {
            }

            // This can be used to get one record which matches the criteria. No assumptions can be made about which record satisfying the 
            // criteria is selected.
            using (Selection onlyOneSelection = enterpriseFeatureClass.Select(spatialFilter, SelectionType.ObjectID, SelectionOption.OnlyOne))
            {
            }

            // This can be used to obtain a empty selction which can be used as a container to combine results from different selections.
            using (Selection emptySelection = enterpriseFeatureClass.Select(spatialFilter, SelectionType.ObjectID, SelectionOption.Empty))
            {
            }

            // If you want to select all the records in a table.
            using (Selection allRecordSelection = enterpriseFeatureClass.Select(null, SelectionType.ObjectID, SelectionOption.Normal))
            {
            }
        }
    });
}

//var featSceneLayer = ...;
//var sname = featSceneLayer.Name;
await QueuedTask.Run(() =>
{
  if (!featSceneLayer.HasAssociatedFeatureService)
    return;//no search or select

  //Select all features within the current map view
  var sz = MapView.Active.GetViewSize();
  var map_pt1 = MapView.Active.ClientToMap(new System.Windows.Point(0, sz.Height));
  var map_pt2 = MapView.Active.ClientToMap(new System.Windows.Point(sz.Width, 0));

  //Convert to an envelope
  var temp_env = EnvelopeBuilderEx.CreateEnvelope(map_pt1, map_pt2, MapView.Active.Map.SpatialReference);

  //Project if needed to the layer spatial ref
  SpatialReference sr = null;
  using (var fc = featSceneLayer.GetFeatureClass())
  using (var fdef = fc.GetDefinition())
    sr = fdef.GetSpatialReference();

  var env = GeometryEngine.Instance.Project(temp_env, sr) as Envelope;

  //Set up a query filter
  var sf = new SpatialQueryFilter()
  {
    FilterGeometry = env,
    SpatialRelationship = SpatialRelationship.Intersects,
    SubFields = "*"
  };

  //Select against the feature service
  var select = featSceneLayer.Select(sf);
  if (select.GetCount() > 0)
  {
    //enumerate over the selected features
    using (var rc = select.Search())
    {
      while (rc.MoveNext())
      {
        using (var feature = rc.Current as Feature)
        {
          var oid = feature.GetObjectID();
          //etc.
        }
      }
    }
  }

  MapView.Active.Map.ClearSelection();

});