public void SetLabelVisibility( 
   bool isLabelVisible
)

{
    //Note: needs to be called on the MCT
    // toggle the label visibility
    featureLayer.SetLabelVisibility(!featureLayer.IsLabelVisible);
}

{
  var cimMap = MapView.Active.Map.GetDefinition();

  CIMTextSymbol simpleTextSymbol = SymbolFactory.Instance.ConstructTextSymbol(ColorFactory.Instance.GreenRGB, 8.5, "Corbel", "Regular");
  var lyrDefn = theLayerToLabel.GetDefinition() as CIMFeatureLayer;
  //Get the label classes - we need the first one
  var listLabelClasses = lyrDefn.LabelClasses.ToList();
  var theLabelClass = listLabelClasses.FirstOrDefault();
  //Place all labels horizontally
  theLabelClass.StandardLabelPlacementProperties.LineLabelPosition.Horizontal = true;
  //theLabelClass.MaplexLabelPlacementProperties.AlignLabelToLineDirection = true;
  //Set the label classes' symbol to the custom text symbol
  theLabelClass.TextSymbol.Symbol = simpleTextSymbol;
  lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
  theLayerToLabel.SetDefinition(lyrDefn);
  (theLayerToLabel as FeatureLayer).SetLabelVisibility(true);
}

/// <summary>
{
  // Note: call within QueuedTask.Run()
  {
    if (!featureLayer.IsLabelVisible)
      //set the label's visibility
      featureLayer.SetLabelVisibility(true);
  }
}