public CIMExpressionInfo ValueExpressionInfo {get; set;}

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

var mv = MapView.Active;
var map = mv.Map;

QueuedTask.Run(() =>
{
  //Assume we a layer - Oregon County (poly) that is using Visual Variable
  //expressions that we want to evaluate interactively...
  var def = oregon_cnts.GetDefinition() as CIMFeatureLayer;

  //Most all feature renderers have a VisualVariable collection
  var renderer = def.Renderer as CIMUniqueValueRenderer;
  var vis_variables = renderer.VisualVariables?.ToList() ??
                        new List<CIMVisualVariable>();
  if (vis_variables.Count == 0)
    return;//there are none
  var vis_var_with_expr = new Dictionary<string, string>();
  //see if any are using expressions
  foreach (var vv in vis_variables)
  {
    if (vv is CIMColorVisualVariable cvv)
    {
      if (!string.IsNullOrEmpty(cvv.ValueExpressionInfo?.Expression))
        vis_var_with_expr.Add("Color", cvv.ValueExpressionInfo?.Expression);
    }
    else if (vv is CIMTransparencyVisualVariable tvv)
    {
      if (!string.IsNullOrEmpty(tvv.ValueExpressionInfo?.Expression))
        vis_var_with_expr.Add("Transparency", tvv.ValueExpressionInfo?.Expression);
    }
    else if (vv is CIMSizeVisualVariable svv)
    {
      if (!string.IsNullOrEmpty(svv.ValueExpressionInfo?.Expression))
        vis_var_with_expr.Add("Outline", svv.ValueExpressionInfo?.Expression);
    }
  }
  if (vis_var_with_expr.Count == 0)
    return;//there arent any with expressions

  //loop through the features (outer)
  //per feature evaluate each visual variable.... (inner)
  //....
  //the converse is to loop through the expressions (outer)
  //then per feature evaluate the expression (inner)
  using (var rc = oregon_cnts.Search())
  {
    while (rc.MoveNext())
    {
      foreach (var kvp in vis_var_with_expr)
      {
        var expr_info = new CIMExpressionInfo()
        {
          Expression = kvp.Value,
          ReturnType = ExpressionReturnType.Default
        };
        //per feature eval each expression...
        using (var arcade = ArcadeScriptEngine.Instance.CreateEvaluator(
                                    expr_info, ArcadeProfile.Visualization))
        {

          var variables = new List<KeyValuePair<string, object>>() {
            new KeyValuePair<string, object>("$feature", rc.Current)
          };
          //note 2D maps can also have view scale...
          //...if necessary...
          if (mv.ViewingMode == MapViewingMode.Map)
          {
            variables.Add(new KeyValuePair<string, object>(
              "$view.scale", mv.Camera.Scale));
          }
          var result = arcade.Evaluate(variables).GetResult().ToString();
          //output
          System.Diagnostics.Debug.WriteLine(
             $"[{rc.Current.GetObjectID()}] '{kvp.Key}': {result}");
        }
      }
    }
  }

  ////foreach (var kvp in vis_var_with_expr)
  ////{
  ////  var expr_info = new CIMExpressionInfo()
  ////  {
  ////    Expression = kvp.Value,
  ////    ReturnType = ExpressionReturnType.Default
  ////  };

  ////  using (var arcade = ArcadeScriptEngine.Instance.CreateEvaluator(
  ////                                  expr_info, ArcadeProfile.Visualization))
  ////  {
  ////    //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
  ////        //...
  ////      }
  ////    }
  ////  }
  ////}
});

var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.ShapeType == esriGeometryType.esriGeometryPolygon);
if (lyr == null) return;
QueuedTask.Run(() =>
{
  // GetRenderer from Layer (assumes it is a unique value renderer)
  var uvRenderer = lyr.GetRenderer() as CIMUniqueValueRenderer;
  if (uvRenderer == null) return;
  //layer has STATE_NAME field
  //community sample Data\Admin\AdminSample.aprx
  string expression = "if ($view.scale > 21000000) { return $feature.STATE_NAME } else { return 'All' }";
  CIMExpressionInfo updatedExpressionInfo = new CIMExpressionInfo
  {
    Expression = expression,
    Title = "Custom" // can be any string used for UI purpose.
  };
  //set the renderer's expression
  uvRenderer.ValueExpressionInfo = updatedExpressionInfo;

  //SetRenderer on Layer
  lyr.SetRenderer(uvRenderer);
});