public IReadOnlyList<Coordinate3D> Copy3DCoordinatesToList()

{
  // assume a multiPoint has been built from 4 points
  // the modified multiPoint will have the first point removed and the last point moved

  // use the builderEx constructors which don't need to run on the MCT.
  MultipointBuilderEx builderEx = new MultipointBuilderEx(multipoint);
  // remove the first point
  builderEx.RemovePoint(0);
  // modify the coordinates of the last point
  var ptEx = builderEx.GetPoint(builderEx.PointCount - 1);
  builderEx.RemovePoint(builderEx.PointCount - 1);

  var newPtEx = MapPointBuilderEx.CreateMapPoint(ptEx.X + 1.0, ptEx.Y + 2.0);
  builderEx.AddPoint(newPtEx);
  Multipoint modifiedMultiPointEx = builderEx.ToGeometry() as Multipoint;
}

{
  ReadOnlyPointCollection pointsReadOnly = multipoint.Points;
  IReadOnlyList<Coordinate2D> coords2d = multipoint.Copy2DCoordinatesToList();
  IReadOnlyList<Coordinate3D> coords3d = multipoint.Copy3DCoordinatesToList();
}