public override Geometry ToGeometry()

// 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);

// Builder convenience methods don't need to run on the MCT.
Envelope envelope = EnvelopeBuilder.CreateEnvelope(minPt, maxPt);

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (EnvelopeBuilder builder = new EnvelopeBuilder(minPt, maxPt))
  {
    // do something with the builder

    envelope = builder.ToGeometry();
  }
});

// builderEx constructors don't need to run on the MCT.
EnvelopeBuilderEx builderEx = new EnvelopeBuilderEx(minPt, maxPt);
envelope = builderEx.ToGeometry() as Envelope;