public static Envelope CreateEnvelope( 
   Envelope envelope,
   SpatialReference spatialReference
)

// Use a builder convenience method or use a builder constructor.

MapPoint minPt = MapPointBuilder.CreateMapPoint(1.0, 1.0);
MapPoint maxPt = MapPointBuilder.CreateMapPoint(2.0, 2.0);

// Create an envelope
Envelope env = EnvelopeBuilder.CreateEnvelope(minPt, maxPt);

// Builder convenience methods don't need to run on the MCT.
Polygon polygonFromEnv = PolygonBuilder.CreatePolygon(env);

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (PolygonBuilder polygonBuilder = new PolygonBuilder(env))
  {
    polygonBuilder.SpatialReference = SpatialReferences.WGS84;
    polygonFromEnv = polygonBuilder.ToGeometry();
  }
});

// builderEx constructors don't need to run on the MCt
PolygonBuilderEx polygonBuilderEx = new PolygonBuilderEx(env);
polygonBuilderEx.SpatialReference = SpatialReferences.WGS84;
polygonFromEnv = polygonBuilderEx.ToGeometry() as Polygon;

// builderEx convenience methods don't need to run on MCT
polygonFromEnv = PolygonBuilderEx.CreatePolygon(env);