public IList<DisplayUnitFormat> GetAvailableElevationUnitFormats()

//var map = MapView.Active.Map;
//Must be on the QueuedTask.Run()

//If the map is not a scene, the list of current
//Project distance units will be returned
var elev_units = map.GetAvailableElevationUnitFormats();

var map = MapView.Active.Map;

QueuedTask.Run(() =>
{
  //Trying to set the elevation unit on a map other than
  //a scene will throw an InvalidOperationException
  if (map.IsScene)
  {
    //Get the list of available elevation unit formats
    //for the current map
    var loc_units = map.GetAvailableElevationUnitFormats();
    //arbitrarily use the last unit in the list
    map.SetElevationUnitFormat(loc_units.Last());
  }

});