public MapPointBuilderEx( 
   double x,
   double y,
   double z
)

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

// Builder convenience methods don't need to run on the MCT.
// create a 3d point with M
MapPoint pt = MapPointBuilder.CreateMapPoint(1.0, 2.0, 3.0, 4.0);

MapPoint ptWithM = null;

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (MapPointBuilder mb = new MapPointBuilder(1.0, 2.0, 3.0, 4.0))
  {
    // do something with the builder

    ptWithM = mb.ToGeometry();
  }
});

MapPoint clone = ptWithM.Clone() as MapPoint;
MapPoint anotherM = MapPointBuilder.CreateMapPoint(ptWithM);


// builderEx constructors don't need to run on the MCT.
MapPointBuilderEx builderEx = new MapPointBuilderEx(1.0, 2.0, 3.0);
builderEx.HasM = true;
builderEx.M = 4.0;

pt = builderEx.ToGeometry() as MapPoint;


// or another alternative with builderEx constructor
builderEx = new MapPointBuilderEx(1.0, 2.0, true, 3.0, true, 4.0, false, 0);
pt = builderEx.ToGeometry() as MapPoint;