public Envelope Union( 
   Envelope envelope
)

// 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;