public int GcsWkid {get;}

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

  // Custom PCS, Predefined GCS
  string customWkt = "PROJCS[\"WebMercatorMile\",GEOGCS[\"GCS_WGS_1984\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137.0,298.257223563]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]],PROJECTION[\"Mercator_Auxiliary_Sphere\"],PARAMETER[\"False_Easting\",0.0],PARAMETER[\"False_Northing\",0.0],PARAMETER[\"Central_Meridian\",0.0],PARAMETER[\"Standard_Parallel_1\",0.0],PARAMETER[\"Auxiliary_Sphere_Type\",0.0],UNIT[\"Mile\",1609.344000614692]]";

  // Builder convenience methods don't need to run on the MCT.
  SpatialReference spatialReference = SpatialReferenceBuilder.CreateSpatialReference(customWkt);
  // SpatialReferenceBuilder properties
  //   spatialReference.Wkt == customWkt
  //   spatialReference.Wkid == 0
  //   spatialReference.VcsWkid == 0
  //   spatialReference.GcsWkid == 4326

  SpatialReference gcs = spatialReference.Gcs;
  // gcs.Wkid == 4326
  // gcs.IsGeographic == true
  // spatialReference.GcsWkt == gcs.Wkt

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

      spatialReference = sb.ToSpatialReference();
    }
  });
}