public class UnclassedColorsRendererDefinition : RendererDefinition 

string colorBrewerSchemesName = "ArcGIS Colors";
StyleProjectItem style = Project.Current.GetItems<StyleProjectItem>().First(s => s.Name == colorBrewerSchemesName);
string colorRampName = "Heat Map 4 - Semitransparent";
IList<ColorRampStyleItem> colorRampList = await QueuedTask.Run(() =>
{
    return style.SearchColorRamps(colorRampName);
});
ColorRampStyleItem colorRamp = colorRampList[0];

await QueuedTask.Run(() =>
{
    CIMPointSymbol pointSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.GreenRGB, 16.0, SimpleMarkerStyle.Diamond);
    CIMSymbolReference symbolPointTemplate = pointSym.MakeSymbolReference();

    //defining an unclassed renderer with custom upper and lower stops
    //all features with value >= 5,000,000 will be drawn with the upper color from the color ramp
    //all features with value <= 50,000 will be drawn with the lower color from the color ramp
    UnclassedColorsRendererDefinition unclassRndrDef = new UnclassedColorsRendererDefinition
                                ("Population", symbolPointTemplate, colorRamp.ColorRamp, "Highest", "Lowest", 5000000, 50000)
    {

        //drawing features with null values with a different symbol
        ShowNullValues = true,
        NullValueLabel = "Unknown"
    };
    CIMPointSymbol nullSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.RedRGB, 16.0, SimpleMarkerStyle.Circle);
    unclassRndrDef.NullValueSymbol = nullSym.MakeSymbolReference();
    FeatureLayer flyr = MapView.Active.Map.Layers[0] as FeatureLayer;
    CIMClassBreaksRenderer cbRndr = (CIMClassBreaksRenderer)flyr.CreateRenderer(unclassRndrDef);
    flyr.SetRenderer(cbRndr);
});