public class QueryFilter 

var count = await QueuedTask.Run(() =>
{
  QueryFilter qf = new QueryFilter()
  {
    WhereClause = "Class = 'city'"
  };

  //Getting the first selected feature layer of the map view
  var flyr = (FeatureLayer)MapView.Active.GetSelectedLayers()
                    .OfType<FeatureLayer>().FirstOrDefault();
  using (RowCursor rows = flyr.Search(qf)) //execute
  {
    //Looping through to count
    int i = 0;
    while (rows.MoveNext()) i++;

    return i;
  }
});
MessageBox.Show(String.Format(
   "Total features that matched the search criteria: {0}", count));