public abstract class CIMBasicFeatureLayer : CIMBaseLayer, ICIMBasicFeatureLayer, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // get the CIM definition from the layer
  var cimFeatureDefinition = featureLayer.GetDefinition() as ArcGIS.Core.CIM.CIMBasicFeatureLayer;
  // get the view of the source table underlying the layer
  var cimDisplayTable = cimFeatureDefinition.FeatureTable;
  // this field is used as the 'label' to represent the row
  var displayField = cimDisplayTable.DisplayField;
});