public Envelope GetDefaultExtent()

//Selection tolerance for the map in pixels
var selectionTolerance = SelectionEnvironment.SelectionTolerance;
QueuedTask.Run(() =>
{
  //Get the map center
  var mapExtent = MapView.Active.Map.GetDefaultExtent();
  var mapPoint = mapExtent.Center;
  //Map center as screen point
  var screenPoint = MapView.Active.MapToScreen(mapPoint);
  //Add selection tolerance pixels to get a "radius".
  var radiusScreenPoint = new System.Windows.Point((screenPoint.X + selectionTolerance), screenPoint.Y);
  var radiusMapPoint = MapView.Active.ScreenToMap(radiusScreenPoint);
  //Calculate the selection tolerance distance in map uints.
  var searchRadius = GeometryEngine.Instance.Distance(mapPoint, radiusMapPoint);
});