public sealed class Legend : MapSurround, ArcGIS.Desktop.Mapping.IElement, System.ComponentModel.INotifyPropertyChanged, System.IComparable, System.IDisposable, System.IEquatable<Element>  

//Create a legend for an associated map frame.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2D envelope geometry
  Coordinate2D leg_ll = new Coordinate2D(6, 2.5);
  Coordinate2D leg_ur = new Coordinate2D(8, 4.5);
  //At 2.x - Envelope leg_env = EnvelopeBuilder.CreateEnvelope(leg_ll, leg_ur);
  Envelope leg_env = EnvelopeBuilderEx.CreateEnvelope(leg_ll, leg_ur);

  //Reference MF, create legend and add to layout
  MapFrame mapFrame = layout.FindElement("New Map Frame") as MapFrame;
  if (mapFrame == null)
  {
    //TODO handle null map frame
    return;
  }
  //At 2.x - Legend legendElm = LayoutElementFactory.Instance.CreateLegend(
  //                                               layout, leg_env, mapFrame);
  //         legendElm.SetName("New Legend"); 
  var legendInfo = new LegendInfo()
  {
    MapFrameName = mapFrame.Name
  };
  Legend legendElm = ElementFactory.Instance.CreateMapSurroundElement(
                      layout, leg_env, legendInfo, "New Legend") as Legend;
});

//Must be on QueuedTask.Run(() => { ...

//Build geometry
Coordinate2D ll = new Coordinate2D(6, 2.5);
Coordinate2D ur = new Coordinate2D(8, 4.5);
Envelope env = EnvelopeBuilderEx.CreateEnvelope(ll, ur);

//Reference MF, create legend and add to layout
MapFrame mf = layout.FindElement(mapFrameName) as MapFrame;
var surroundInfo = new LegendInfo()
{
  MapFrameName = mf.Name
};

var legendElm = ElementFactory.Instance.CreateMapSurroundElement(
  layout, env.Center, surroundInfo) as Legend;
legendElm.SetName("New Legend");