public string Expression {get; set;}

{
  //Consult https://github.com/Esri/arcade-expressions/ and
  //https://developers.arcgis.com/arcade/ for more examples
  //and arcade reference
  // Note: the following should be embedded in a QueuedTask.Run() statement
  {
    //construct an expression
    var query = @"Count($layer)";//count of features in "layer"

    //construct a CIMExpressionInfo
    var arcade_expr = new CIMExpressionInfo()
    {
      Expression = query.ToString(),
      //Return type can be string, numeric, or default
      //When set to default, add-in is responsible for determining
      //the return type
      ReturnType = ExpressionReturnType.Default
    };

    //Construct an evaluator
    //select the relevant profile - it must support Pro and it must
    //contain any profile variables you are using in your expression.
    //Consult: https://developers.arcgis.com/arcade/profiles/
    using var arcade = ArcadeScriptEngine.Instance.CreateEvaluator(
                            arcade_expr, ArcadeProfile.Popups);
    //Provision  values for any profile variables referenced...
    //in our case '$layer'
    var variables = new List<KeyValuePair<string, object>>() {
        new("$layer", featureLayer)
      };
    //evaluate the expression
    try
    {
      var result = arcade.Evaluate(variables).GetResult();
      System.Diagnostics.Debug.WriteLine($"Result: {result}");
    }
    //handle any exceptions
    catch (InvalidProfileVariableException)
    {
      //something wrong with the profile variable specified
      //TODO...
    }
    catch (EvaluationException)
    {
      //something wrong with the query evaluation
      //TODO...
    }
  }
}

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

  // Note: the following should be embedded in a QueuedTask.Run() statement
  {
    //construct an expression
    var query = @"$feature.AreaInAcres * 43560.0";//convert acres to ft 2

    //construct a CIMExpressionInfo
    var arcade_expr = new CIMExpressionInfo()
    {
      Expression = query.ToString(),
      //Return type can be string, numeric, or default
      //When set to default, add-in is responsible for determining
      //the return type
      ReturnType = ExpressionReturnType.Default
    };

    //Construct an evaluator
    //select the relevant profile - it must support Pro and it must
    //contain any profile variables you are using in your expression.
    //Consult: https://developers.arcgis.com/arcade/profiles/
    using var arcade = ArcadeScriptEngine.Instance.CreateEvaluator(
                            arcade_expr, ArcadeProfile.Popups);
    //we are evaluating the expression against all features
    using var rc = featureLayer.Search();

    while (rc.MoveNext())
    {
      //Provision  values for any profile variables referenced...
      //in our case '$feature'
      var variables = new List<KeyValuePair<string, object>>() {
          new("$feature", rc.Current)
        };
      //evaluate the expression (per feature in this case)
      try
      {
        var result = arcade.Evaluate(variables).GetResult();
        var val = ((double)result).ToString("0.0#");
        System.Diagnostics.Debug.WriteLine(
          $"{rc.Current.GetObjectID()} area: {val} ft2");
      }
      //handle any exceptions
      catch (InvalidProfileVariableException)
      {
        //something wrong with the profile variable specified
        //TODO...
      }
      catch (EvaluationException)
      {
        //something wrong with the query evaluation
        //TODO...
      }
    }
  }
}

{
  // Note: the following should be embedded in a QueuedTask.Run() statement
  {
    // GetRenderer from Layer (assumes it is a unique value renderer)
    if (featureLayer.GetRenderer() is not CIMUniqueValueRenderer uvRenderer)
    {
      // not a unique value renderer, leave
      return;
    }
    //layer has STATE_NAME field if using the community sample Data\Admin\AdminSample.aprx
    string expression = "if ($view.scale > 21000000) { return $feature.STATE_NAME } else { return 'All' }";
    CIMExpressionInfo updatedExpressionInfo = new()
    {
      Expression = expression,
      Title = "Custom" // can be any string used for UI purpose.
    };
    //set the renderer's expression
    uvRenderer.ValueExpressionInfo = updatedExpressionInfo;

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