public override double Length {get;}

{
  // use the convenience builders
  Envelope env1 = EnvelopeBuilderEx.CreateEnvelope(0, 0, 1, 1, SpatialReferences.WGS84);
  Envelope env2 = EnvelopeBuilderEx.CreateEnvelope(0.5, 0.5, 1.5, 1.5, SpatialReferences.WGS84);

  Envelope env3 = env1.Union(env2);

  double area = env3.Area;
  double depth = env3.Depth;
  double height = env3.Height;
  double width = env3.Width;
  double len = env3.Length;

  MapPoint centerPt = env3.Center;
  Coordinate2D coord = env3.CenterCoordinate;

  bool isEmpty = env3.IsEmpty;
  int pointCount = env3.PointCount;

  // coordinates
  //env3.XMin, env3.XMax, env3.YMin, env3.YMax
  //env3.ZMin, env3.ZMax, env3.MMin, env3.MMax

  bool isEqual = env1.IsEqual(env2);    // false

  // or use the builderEx constructors which don't need to run on the MCT.
  EnvelopeBuilderEx builderEx = new EnvelopeBuilderEx(0, 0, 1, 1, SpatialReferences.WGS84);
  builderEx.Union(env2);      // builder is updated to the result

  depth = builderEx.Depth;
  height = builderEx.Height;
  width = builderEx.Width;

  centerPt = builderEx.Center;
  coord = builderEx.CenterCoordinate;

  isEmpty = builderEx.IsEmpty;

  env3 = builderEx.ToGeometry() as Envelope;
}