public class CIMLabelClass : CIMObject, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

//Consult https://github.com/Esri/arcade-expressions/ and
//https://developers.arcgis.com/arcade/ for more examples
//and arcade reference

var map = MapView.Active.Map;
QueuedTask.Run(() =>
{
  //Assume we a layer - Oregon County (poly) that has an arcade labelling
  //expression and we want to evaluate that interactively...
  var def = oregon_cnts.GetDefinition() as CIMFeatureLayer;
  //Get the label class
  var label_class = def.LabelClasses
                     .FirstOrDefault(lc => {
                       return lc.Name == "Arcade_Example_1" &&
                              lc.ExpressionEngine == LabelExpressionEngine.Arcade;
                     });
  if (label_class == null)
    return;

  //evaluate the label expression against the features
  var expr_info = new CIMExpressionInfo()
  {
    Expression = label_class.Expression,
    ReturnType = ExpressionReturnType.String
  };

  //https://developers.arcgis.com/arcade/profiles/labeling/
  using (var arcade = ArcadeScriptEngine.Instance.CreateEvaluator(
                                            expr_info, ArcadeProfile.Labeling))
  {
    //loop through the features
    using (var rc = oregon_cnts.Search())
    {
      while (rc.MoveNext())
      {
        var variables = new List<KeyValuePair<string, object>>() {
          new KeyValuePair<string, object>("$feature", rc.Current)
        };
        var result = arcade.Evaluate(variables).GetResult();
        //output
        System.Diagnostics.Debug.WriteLine(
           $"[{rc.Current.GetObjectID()}]: {result}");
      }
    }
  }
});

var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.ShapeType == esriGeometryType.esriGeometryPolygon);
if (lyr == null) return;
QueuedTask.Run(() =>
{
  //Get the layer's definition
  //community sample Data\Admin\AdminSample.aprx
  var lyrDefn = lyr.GetDefinition() as CIMFeatureLayer;
  if (lyrDefn == null) return;
  //Get the label classes - we need the first one
  var listLabelClasses = lyrDefn.LabelClasses.ToList();
  var theLabelClass = listLabelClasses.FirstOrDefault();
  //set the label class Expression to use the Arcade expression
  theLabelClass.Expression = "return $feature.STATE_NAME + TextFormatting.NewLine + $feature.POP2000;";
  //Set the label definition back to the layer.
  lyr.SetDefinition(lyrDefn);
});

public async Task CreatingAnAnnotationFeature(Geodatabase geodatabase)
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (AnnotationFeatureClass annotationFeatureClass = geodatabase.OpenDataset<AnnotationFeatureClass>("Annotation // feature // class // name"))
    using (AnnotationFeatureClassDefinition annotationFeatureClassDefinition = annotationFeatureClass.GetDefinition())
    using (RowBuffer rowBuffer = annotationFeatureClass.CreateRowBuffer())
    using (AnnotationFeature annotationFeature = annotationFeatureClass.CreateRow(rowBuffer))
    {
      annotationFeature.SetAnnotationClassID(0);
      annotationFeature.SetStatus(AnnotationStatus.Placed);

      // Get the annotation labels from the label collection
      IReadOnlyList<CIMLabelClass> labelClasses =
        annotationFeatureClassDefinition.GetLabelClassCollection();

      // Setup the symbol reference with the symbol id and the text symbol
      CIMSymbolReference cimSymbolReference = new CIMSymbolReference();
      cimSymbolReference.Symbol = labelClasses[0].TextSymbol.Symbol;
      cimSymbolReference.SymbolName = labelClasses[0].TextSymbol.SymbolName;

      // Setup the text graphic
      CIMTextGraphic cimTextGraphic = new CIMTextGraphic();
      cimTextGraphic.Text = "Charlotte, North Carolina";
      cimTextGraphic.Shape = new MapPointBuilderEx(new Coordinate2D(-80.843, 35.234), SpatialReferences.WGS84).ToGeometry();
      cimTextGraphic.Symbol = cimSymbolReference;

      // Set the symbol reference on the graphic and store
      annotationFeature.SetGraphic(cimTextGraphic);
      annotationFeature.Store();
    }
  });
}

public void CreateStandAloneAnnotationFeatureClass(Geodatabase geodatabase, SpatialReference spatialReference)
{
  // Creating a Cities annotation feature class
  // with following user defined fields
  // Name 
  // GlobalID

  // Annotation feature class name
  string annotationFeatureClassName = "CitiesAnnotation";

  // Create user defined attribute fields for annotation feature class 
  FieldDescription globalIDFieldDescription = FieldDescription.CreateGlobalIDField();
  FieldDescription nameFieldDescription = FieldDescription.CreateStringField("Name", 255);

  // Create a list of all field descriptions
  List<FieldDescription> fieldDescriptions = new List<FieldDescription> { globalIDFieldDescription, nameFieldDescription };

  // Create a ShapeDescription object
  ShapeDescription shapeDescription = new ShapeDescription(GeometryType.Polygon, spatialReference);

  // Create general placement properties for Maplex engine 
  CIMMaplexGeneralPlacementProperties generalPlacementProperties =
    new CIMMaplexGeneralPlacementProperties
    {
      AllowBorderOverlap = true,
      PlacementQuality = MaplexQualityType.High,
      DrawUnplacedLabels = true,
      InvertedLabelTolerance = 1.0,
      RotateLabelWithDisplay = true,
      UnplacedLabelColor = new CIMRGBColor
      {
        R = 0,
        G = 255,
        B = 0,
        Alpha = 0.5f // Green
      }
    };

  // Create general placement properties for Standard engine

  //CIMStandardGeneralPlacementProperties generalPlacementProperties =
  //  new CIMStandardGeneralPlacementProperties
  //  {
  //    DrawUnplacedLabels = true,
  //    InvertedLabelTolerance = 3.0,
  //    RotateLabelWithDisplay = true,
  //    UnplacedLabelColor = new CIMRGBColor
  //    {
  //      R = 255, G = 0, B = 0, Alpha = 0.5f // Red
  //    } 
  //   };


  // Create annotation  label classes
  // Green label
  CIMLabelClass greenLabelClass = new CIMLabelClass
  {
    Name = "Green",
    ExpressionTitle = "Expression-Green",
    ExpressionEngine = LabelExpressionEngine.Arcade,
    Expression = "$feature.OBJECTID",
    ID = 1,
    Priority = 0,
    Visibility = true,
    TextSymbol = new CIMSymbolReference
    {
      Symbol = new CIMTextSymbol()
      {
        Angle = 45,
        FontType = FontType.Type1,
        FontFamilyName = "Tahoma",
        FontEffects = FontEffects.Normal,
        HaloSize = 2.0,

        Symbol = new CIMPolygonSymbol
        {
          SymbolLayers = new CIMSymbolLayer[]
          {
            new CIMSolidFill
            {
              Color = CIMColor.CreateRGBColor(0, 255, 0)
            }
          },
          UseRealWorldSymbolSizes = true
        }
      },
      MaxScale = 0,
      MinScale = 0,
      SymbolName = "TextSymbol-Green"
    },
    StandardLabelPlacementProperties = new CIMStandardLabelPlacementProperties
    {
      AllowOverlappingLabels = true,
      LineOffset = 1.0
    },
    MaplexLabelPlacementProperties = new CIMMaplexLabelPlacementProperties
    {
      AlignLabelToLineDirection = true,
      AvoidPolygonHoles = true
    }
  };

  // Blue label
  CIMLabelClass blueLabelClass = new CIMLabelClass
  {
    Name = "Blue",
    ExpressionTitle = "Expression-Blue",
    ExpressionEngine = LabelExpressionEngine.Arcade,
    Expression = "$feature.OBJECTID",
    ID = 2,
    Priority = 0,
    Visibility = true,
    TextSymbol = new CIMSymbolReference
    {
      Symbol = new CIMTextSymbol()
      {
        Angle = 45,
        FontType = FontType.Type1,
        FontFamilyName = "Consolas",
        FontEffects = FontEffects.Normal,
        HaloSize = 2.0,

        Symbol = new CIMPolygonSymbol
        {
          SymbolLayers = new CIMSymbolLayer[]
          {
            new CIMSolidFill
            {
              Color = CIMColor.CreateRGBColor(0, 0, 255)
            }
          },
          UseRealWorldSymbolSizes = true
        }
      },
      MaxScale = 0,
      MinScale = 0,
      SymbolName = "TextSymbol-Blue"
    },
    StandardLabelPlacementProperties = new CIMStandardLabelPlacementProperties
    {
      AllowOverlappingLabels = true,
      LineOffset = 1.0
    },
    MaplexLabelPlacementProperties = new CIMMaplexLabelPlacementProperties
    {
      AlignLabelToLineDirection = true,
      AvoidPolygonHoles = true
    }
  };

  // Create a list of labels
  List<CIMLabelClass> labelClasses = new List<CIMLabelClass> { greenLabelClass, blueLabelClass };

  // Create an annotation feature class description object to describe the feature class to create
  AnnotationFeatureClassDescription annotationFeatureClassDescription =
    new AnnotationFeatureClassDescription(annotationFeatureClassName, fieldDescriptions, shapeDescription,
      generalPlacementProperties, labelClasses)
    {
      IsAutoCreate = true,
      IsSymbolIDRequired = false,
      IsUpdatedOnShapeChange = true
    };

  // Create a SchemaBuilder object
  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

  // Add the creation of the Cities annotation feature class to the list of DDL tasks
  schemaBuilder.Create(annotationFeatureClassDescription);

  // Execute the DDL
  bool success = schemaBuilder.Build();

  // Inspect error messages
  if (!success)
  {
    IReadOnlyList<string> errorMessages = schemaBuilder.ErrorMessages;
    //etc.
  }
}

public void CreateFeatureLinkedAnnotationFeatureClass(Geodatabase geodatabase, SpatialReference spatialReference)
{
  // Creating a feature-linked annotation feature class between water pipe and valve in water distribution network
  // with following user defined fields
  // PipeName 
  // GlobalID

  // Annotation feature class name
  string annotationFeatureClassName = "WaterPipeAnnotation";

  // Create user defined attribute fields for annotation feature class
  FieldDescription pipeGlobalID = FieldDescription.CreateGlobalIDField();
  FieldDescription nameFieldDescription = FieldDescription.CreateStringField("Name", 255);

  // Create a list of all field descriptions
  List<FieldDescription> fieldDescriptions = new List<FieldDescription> { pipeGlobalID, nameFieldDescription };

  // Create a ShapeDescription object
  ShapeDescription shapeDescription = new ShapeDescription(GeometryType.Polygon, spatialReference);

  // Create general placement properties for Maplex engine 
  CIMMaplexGeneralPlacementProperties generalPlacementProperties = new CIMMaplexGeneralPlacementProperties
  {
    AllowBorderOverlap = true,
    PlacementQuality = MaplexQualityType.High,
    DrawUnplacedLabels = true,
    InvertedLabelTolerance = 1.0,
    RotateLabelWithDisplay = true,
    UnplacedLabelColor = new CIMRGBColor
    {
      R = 255,
      G = 0,
      B = 0,
      Alpha = 0.5f
    }
  };

  // Create annotation  label classes
  // Green label
  CIMLabelClass greenLabelClass = new CIMLabelClass
  {
    Name = "Green",
    ExpressionTitle = "Expression-Green",
    ExpressionEngine = LabelExpressionEngine.Arcade,
    Expression = "$feature.OBJECTID",
    ID = 1,
    Priority = 0,
    Visibility = true,
    TextSymbol = new CIMSymbolReference
    {
      Symbol = new CIMTextSymbol()
      {
        Angle = 45,
        FontType = FontType.Type1,
        FontFamilyName = "Tahoma",
        FontEffects = FontEffects.Normal,
        HaloSize = 2.0,

        Symbol = new CIMPolygonSymbol
        {
          SymbolLayers = new CIMSymbolLayer[]
          {
            new CIMSolidFill
            {
              Color = CIMColor.CreateRGBColor(0, 255, 0)
            }
          },
          UseRealWorldSymbolSizes = true
        }
      },
      MaxScale = 0,
      MinScale = 0,
      SymbolName = "TextSymbol-Green"
    },
    StandardLabelPlacementProperties = new CIMStandardLabelPlacementProperties
    {
      AllowOverlappingLabels = true,
      LineOffset = 1.0
    },
    MaplexLabelPlacementProperties = new CIMMaplexLabelPlacementProperties
    {
      AlignLabelToLineDirection = true,
      AvoidPolygonHoles = true
    }
  };

  // Blue label
  CIMLabelClass blueLabelClass = new CIMLabelClass
  {
    Name = "Blue",
    ExpressionTitle = "Expression-Blue",
    ExpressionEngine = LabelExpressionEngine.Arcade,
    Expression = "$feature.OBJECTID",
    ID = 2,
    Priority = 0,
    Visibility = true,
    TextSymbol = new CIMSymbolReference
    {
      Symbol = new CIMTextSymbol()
      {
        Angle = 45,
        FontType = FontType.Type1,
        FontFamilyName = "Consolas",
        FontEffects = FontEffects.Normal,
        HaloSize = 2.0,

        Symbol = new CIMPolygonSymbol
        {
          SymbolLayers = new CIMSymbolLayer[]
          {
            new CIMSolidFill
            {
              Color = CIMColor.CreateRGBColor(0, 0, 255)
            }
          },
          UseRealWorldSymbolSizes = true
        }
      },
      MaxScale = 0,
      MinScale = 0,
      SymbolName = "TextSymbol-Blue"
    },
    StandardLabelPlacementProperties = new CIMStandardLabelPlacementProperties
    {
      AllowOverlappingLabels = true,
      LineOffset = 1.0
    },
    MaplexLabelPlacementProperties = new CIMMaplexLabelPlacementProperties
    {
      AlignLabelToLineDirection = true,
      AvoidPolygonHoles = true
    }
  };

  // Create a list of labels
  List<CIMLabelClass> labelClasses = new List<CIMLabelClass> { greenLabelClass, blueLabelClass };


  // Create linked feature description
  // Linked feature class name
  string linkedFeatureClassName = "WaterPipe";

  // Create fields for water pipe
  FieldDescription waterPipeGlobalID = FieldDescription.CreateGlobalIDField();
  FieldDescription pipeName = FieldDescription.CreateStringField("PipeName", 255);

  // Create a list of water pipe field descriptions
  List<FieldDescription> pipeFieldDescriptions = new List<FieldDescription> { waterPipeGlobalID, pipeName };

  // Create a linked feature class description
  FeatureClassDescription linkedFeatureClassDescription = new FeatureClassDescription(linkedFeatureClassName, pipeFieldDescriptions,
    new ShapeDescription(GeometryType.Polyline, spatialReference));

  // Create a SchemaBuilder object
  SchemaBuilder schemaBuilder = new SchemaBuilder(geodatabase);

  // Add the creation of the linked feature class to the list of DDL tasks
  FeatureClassToken linkedFeatureClassToken = schemaBuilder.Create(linkedFeatureClassDescription);

  // Create an annotation feature class description object to describe the feature class to create
  AnnotationFeatureClassDescription annotationFeatureClassDescription =
    new AnnotationFeatureClassDescription(annotationFeatureClassName, fieldDescriptions, shapeDescription,
      generalPlacementProperties, labelClasses, new FeatureClassDescription(linkedFeatureClassToken))
    {
      IsAutoCreate = true,
      IsSymbolIDRequired = false,
      IsUpdatedOnShapeChange = true
    };

  // Add the creation of the annotation feature class to the list of DDL tasks
  schemaBuilder.Create(annotationFeatureClassDescription);

  // Execute the DDL
  bool success = schemaBuilder.Build();

  // Inspect error messages
  if (!success)
  {
    IReadOnlyList<string> errorMessages = schemaBuilder.ErrorMessages;
    //etc.
  }
}

//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();
//Set the label classes' symbol to the custom text symbol
//Refer to the ProSnippets-TextSymbols wiki page for help with creating custom text symbols.
//Example: var textSymbol = await CreateTextSymbolWithHaloAsync();
theLabelClass.TextSymbol.Symbol = textSymbol;
lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition
                                     //set the label's visiblity
featureLayer.SetLabelVisibility(true);

//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               
  theLabelClass.StandardLabelPlacementProperties.PointPlacementMethod = 
           StandardPointPlacementMethod.OnTopPoint;
else    //Current labeling engine is Maplex labeling engine            
  theLabelClass.MaplexLabelPlacementProperties.PointPlacementMethod = 
          MaplexPointPlacementMethod.CenteredOnPoint;

lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition