public enum MaplexLineFeatureType : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable  

{
  // Note: call within QueuedTask.Run()
  {
    //Get the layer's definition
    var lyrDefn = featureLayer.GetDefinition() as CIMFeatureLayer;
    //Get the label classes - we need the first one
    var listLabelClasses = lyrDefn.LabelClasses.ToList();
    var theLabelClass = listLabelClasses.FirstOrDefault();
    //Modify label Placement 
    //Check if the label engine is Maplex or standard.
    CIMGeneralPlacementProperties labelEngine =
        MapView.Active.Map.GetDefinition().GeneralPlacementProperties;
    if (labelEngine is CIMStandardGeneralPlacementProperties)
    {
      //Current labeling engine is Standard labeling engine
      var lineLablePosition = new CIMStandardLineLabelPosition
      {
        Perpendicular = true,
        Parallel = false,
        ProduceCurvedLabels = false,
        Horizontal = false,
        OnTop = true
      };
      theLabelClass.StandardLabelPlacementProperties.LineLabelPosition =
          lineLablePosition;
    }
    else //Current labeling engine is Maplex labeling engine
    {
      theLabelClass.MaplexLabelPlacementProperties.LinePlacementMethod =
                    MaplexLinePlacementMethod.CenteredPerpendicularOnLine;
      theLabelClass.MaplexLabelPlacementProperties.LineFeatureType =
                    MaplexLineFeatureType.General;
    }
    //theLabelClass.MaplexLabelPlacementProperties.LinePlacementMethod = MaplexLinePlacementMethod.CenteredPerpendicularOnLine;
    lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
    featureLayer.SetDefinition(lyrDefn); //set the layer's definition
  }
}