public int PointCount {get;}

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

  List<MapPoint> list = new List<MapPoint>();
  list.Add(MapPointBuilderEx.CreateMapPoint(1.0, 1.0));
  list.Add(MapPointBuilderEx.CreateMapPoint(1.0, 2.0));
  list.Add(MapPointBuilderEx.CreateMapPoint(2.0, 2.0));
  list.Add(MapPointBuilderEx.CreateMapPoint(2.0, 1.0));

  // use the builderEx constructors which don't need to run on the MCT.
  // use AttributeFlags.NoAttributes since we have 2d points in the list
  MultipointBuilderEx builderEx = new MultipointBuilderEx(list, AttributeFlags.None);
  Multipoint multiPoint = builderEx.ToGeometry() as Multipoint;
  int ptCount = builderEx.PointCount;

  // builderEx convenience methods don't need to run on the MCT
  multiPoint = MultipointBuilderEx.CreateMultipoint(list);
  // multiPoint.HasZ, HasM, HasID will be false - the attributes are determined 
  //    based on the attribute state of the points in the list

  // or specifically set the state
  multiPoint = MultipointBuilderEx.CreateMultipoint(list, AttributeFlags.None);
  // multiPoint.HasM = false

  multiPoint = MultipointBuilderEx.CreateMultipoint(list, AttributeFlags.HasM);
  // multiPoint.HasM = true

  ptCount = multiPoint.PointCount;
}