public SpatialReference ToSpatialReference()

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

// Builder convenience methods don't need to run on the MCT.
SpatialReference sr3857 = SpatialReferenceBuilder.CreateSpatialReference(3857);

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (SpatialReferenceBuilder srBuilder = new SpatialReferenceBuilder(3857))
  {
    // do something with the builder

    sr3857 = srBuilder.ToSpatialReference();
  }
});