public class CIMSimpleRenderer : CIMRenderer, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

{
    //Note: Needs QueuedTask to run
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = featureLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
        return;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbol.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbol.MakeSymbolReference();
    //Update the feature layer renderer
    featureLayer.SetRenderer(currentRenderer);
}

{
    //Note: Needs QueuedTask to run
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = featureLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
        return;
    //Get symbol from the SymbolStyleItem
    CIMSymbol symbolToApply = symbolStyleItem.Symbol;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbolToApply.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbolToApply.MakeSymbolReference();
    //Update the feature layer renderer
    featureLayer.SetRenderer(currentRenderer);
}

/// <summary>
/// Renders a Polygon feature layer using a single symbol.
/// ![Simple Renderer for Polygon features](https://ArcGIS.github.io/arcgis-pro-sdk/images/Renderers/simple-polygon.png)
/// </summary>
{
  //Check feature layer name
  //Code works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data
  var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.Name == "USDemographics");
  if (featureLayer == null)
  {
    MessageBox.Show("This renderer works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data", "Data missing");
  }
  //Creating a polygon with a red fill and blue outline.
  CIMStroke outline = SymbolFactory.Instance.ConstructStroke(
        ColorFactory.Instance.BlueRGB, 2.0, SimpleLineStyle.Solid);
  CIMPolygonSymbol fillWithOutline = SymbolFactory.Instance.ConstructPolygonSymbol(
        ColorFactory.Instance.CreateRGBColor(255, 190, 190), SimpleFillStyle.Solid, outline);
  //Get the layer's current renderer
  //Note: Run within QueuedTask
  CIMSimpleRenderer renderer = featureLayer.GetRenderer() as CIMSimpleRenderer;

  //Update the symbol of the current simple renderer
  renderer.Symbol = fillWithOutline.MakeSymbolReference();

  //Update the feature layer renderer
  featureLayer.SetRenderer(renderer);
}

/// <summary>
/// Renders a Point feature layer using a single symbol.
/// ![Simple Renderer for Point features](https://ArcGIS.github.io/arcgis-pro-sdk/images/Renderers/simple-point.png)
/// </summary>
{
  //Check feature layer name
  //Code works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data
  var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.ShapeType == esriGeometryType.esriGeometryPoint);
  if (featureLayer == null)
  {
    MessageBox.Show("This renderer works with a point feature layer", "Data missing");
  }
  //Create a circle marker
  var pointSymbol = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.RedRGB, 8, SimpleMarkerStyle.Circle);

  //Get the layer's current renderer
  //Note: Run within QueuedTask
  CIMSimpleRenderer renderer = featureLayer.GetRenderer() as CIMSimpleRenderer;

  //Update the symbol of the current simple renderer
  renderer.Symbol = pointSymbol.MakeSymbolReference();

  //Update the feature layer renderer
  featureLayer.SetRenderer(renderer);
}

/// <summary>
/// Renders a Line feature layer using a single symbol.
/// ![Simple Renderer for Line features](https://ArcGIS.github.io/arcgis-pro-sdk/images/Renderers/simple-line.png)
/// </summary>
{
  //Check feature layer name
  //Code works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data
  var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.Name == "U.S. National Transportation Atlas Interstate Highways");
  if (featureLayer == null)
  {
    MessageBox.Show("This renderer works with the U.S. National Transportation Atlas Interstate Highways feature layer available with the ArcGIS Pro SDK Sample data", "Data missing");
  }
  //Create a circle marker
  var lineSymbol = SymbolFactory.Instance.ConstructLineSymbol(ColorFactory.Instance.RedRGB, 2, SimpleLineStyle.DashDotDot);

  //Get the layer's current renderer
  //Note: Run within QueuedTask
  CIMSimpleRenderer renderer = featureLayer.GetRenderer() as CIMSimpleRenderer;

  //Update the symbol of the current simple renderer
  renderer.Symbol = lineSymbol.MakeSymbolReference();

  //Update the feature layer renderer
  featureLayer.SetRenderer(renderer);
}

/// <summary>
/// Renders a Line feature layer using a style from a StyleProjectItem.
/// ![Simple Renderer Style item](https://ArcGIS.github.io/arcgis-pro-sdk/images/Renderers/simple-line-style-item.png)
/// </summary>
{
  //Check feature layer name
  //Code works with the USDemographics feature layer available with the ArcGIS Pro SDK Sample data
  var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.Name == "U.S. National Transportation Atlas Interstate Highways");
  if (featureLayer == null)
  {
    MessageBox.Show("This renderer works with the U.S. National Transportation Atlas Interstate Highways feature layer available with the ArcGIS Pro SDK Sample data", "Data missing");
  }
  //Get all styles in the project
  var styleProjectItem2D = Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 2D");

  //Get a specific style in the project by name
  var arrowLineSymbol = styleProjectItem2D.SearchSymbols(StyleItemType.LineSymbol, "Arrow Line 2 (Mid)")[0];
  if (arrowLineSymbol == null) return;
  //Note: Run within QueuedTask
  //Get the layer's current renderer
  var renderer = featureLayer?.GetRenderer() as CIMSimpleRenderer;

  //Update the symbol of the current simple renderer
  renderer.Symbol = arrowLineSymbol.Symbol.MakeSymbolReference();

  //Update the feature layer renderer
  featureLayer.SetRenderer(renderer);
}