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