public Envelope GetDefaultExtent()

{
  //Selection tolerance for the map in pixels
  var selectionTolerance = SelectionEnvironment.SelectionTolerance;
  // Note: Needs QueuedTask to 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 units.
    var searchRadius = GeometryEngine.Instance.Distance(mapPoint, radiusMapPoint);
  }
}