public string ArcadeExpression {get; set;}

{
    var usaStatesLayer = map.GetLayersAsFlattenedList().OfType<FeatureLayer>()
                   .FirstOrDefault(fl => fl.Name == "USA States");
    if (usaStatesLayer == null) return;

    //Note: Run within a QueuedTask
    //Create a color ramp from red to blue
    var colorRampForPopulation = ColorFactory.Instance.ConstructColorRamp(
          ColorRampAlgorithm.LinearContinuous,
          ColorFactory.Instance.RedRGB, ColorFactory.Instance.BlueRGB);

    //This Arcade expression categorizes the POP2000 field in the usaStates layer
    //into Low, Medium, and High values
    var arcade = @"if ($feature.POP2000 < 1000000)  
             {return ""Low"";}" +
                 @"else if ($feature.POP2000 >= 100000 && $feature.POP2000 < 5000000) 
            {return ""Medium""; }" +
                  @"else if ($feature.POP2000 >= 5000000) 
            {return ""High"";}" +
                  @"else{return ""NA"";}";
    //Create the Arcade expression info
    CIMExpressionInfo arcadeExp = new CIMExpressionInfo();
    arcadeExp.Expression = arcade;
    arcadeExp.ReturnType = ExpressionReturnType.Default;
    arcadeExp.Title = "State Population Categorized";
    arcadeExp.Name = "USA States Population 2000";

    //Each population category is assigned a color from the color ramp and symbolized on the map
    // using a unique value renderer
    var uvr_desc = new UniqueValueRendererDefinition()
    {
        //ValueFields = new List<string> { "STATION" },
        ColorRamp = colorRampForPopulation,
        UseDefaultSymbol = true,
        ValuesLimit = 100,
        ArcadeExpression = arcadeExp.Expression
    };
    //Create the renderer and assign it to the layer
    var renderer = usaStatesLayer.CreateRenderer(uvr_desc);
    usaStatesLayer.SetRenderer(renderer);
}