public SpatialReference ToSpatialReference()

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

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

  // SpatialReferenceBuilder 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();
    }
  });
}

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

  string wkt = "GEOGCS[\"MyGCS84\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137.0,298.257223563]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Radian\",1.0]]";

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

  // SpatialReferenceBuilder constructors need to run on the MCT.
  ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (SpatialReferenceBuilder builder = new SpatialReferenceBuilder(wkt))
    {
      // do something with the builder
      SpatialReference anotherSR = builder.ToSpatialReference();
    }
  });
}