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();

});