public override int PointCount {get;}

MapPoint point = MapPointBuilderEx.CreateMapPoint(1, 2, SpatialReferences.WebMercator);

List<Coordinate2D> coords2D = new List<Coordinate2D>()
{
  new Coordinate2D(0, 0),
  new Coordinate2D(0, 1),
  new Coordinate2D(1, 1),
  new Coordinate2D(1, 0)
};

Multipoint multipoint = MultipointBuilderEx.CreateMultipoint(coords2D, SpatialReferences.WGS84);
Polyline polyline = PolylineBuilderEx.CreatePolyline(coords2D, SpatialReferences.WebMercator);

GeometryBagBuilderEx builder = new GeometryBagBuilderEx(SpatialReferences.WGS84);

GeometryBag emptyBag = builder.ToGeometry();
// emptyBag.IsEmpty = true

builder.AddGeometry(point);
// builder.GeometryCount = 1

GeometryBag geometryBag = builder.ToGeometry();
// geometryBag.PartCount = 1
// geometryBag.PointCount = 1
// geometryBag.IsEmpty = false

IReadOnlyList<Geometry> geometries = geometryBag.Geometries;
// geometries.Count = 1
// geometries[0] is MapPoint with a sr of WGS84

bool isEqual = geometryBag.IsEqual(emptyBag);   // isEqual = false

builder.InsertGeometry(0, multipoint);
geometryBag = builder.ToGeometry();
// geometryBag.PartCount = 2

geometries = geometryBag.Geometries;
// geometries.Count = 2
// geometries[0] is Multipoint
// geometries[1] is MapPoint

builder.AddGeometry(polyline);
builder.RemoveGeometry(1);
geometryBag = builder.ToGeometry();
// geometryBag.PartCount = 2

geometries = geometryBag.Geometries;
// geometries.Count = 2
// geometries[0] is Multipoint
// geometries[1] is Polyline