public CIMExpressionInfo ValueExpressionInfo {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
  {
    //Assume we have a layer that is using Visual Variable
    //expressions that we want to evaluate interactively...
    var def = featureLayer.GetDefinition() as CIMFeatureLayer;

    //Most all feature renderers have a VisualVariable collection
    var renderer = def.Renderer as CIMUniqueValueRenderer;
    List<CIMVisualVariable> vis_variables = renderer.VisualVariables?.ToList() ??
                          [];
    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 aren't 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 RowCursor rc = featureLayer.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 ArcadeEvaluator arcade = ArcadeScriptEngine.Instance.CreateEvaluator(
                                    expr_info, ArcadeProfile.Visualization);

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

{
  // 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);
  }
}