public string ToXml()

{
  const string jsonString = "{\"geometries\":[{\"x\":1,\"y\":2},{\"rings\":[[[0,0],[0,4],[3,4],[3,0],[0,0]]]}],\"spatialReference\":{\"wkid\":4326,\"latestWkid\":4326}}";
  var geometryBag = GeometryBagBuilderEx.FromJson(jsonString);

  string xml = geometryBag.ToXml();
  var xmlString = GeometryBagBuilderEx.FromXml(xml);
}

{
  MapPoint minPoint = MapPointBuilderEx.CreateMapPoint(1, 1, 1, 1, 3);
  MapPoint maxPoint = MapPointBuilderEx.CreateMapPoint(5, 5, 5);

  // 
  //  MapPoint
  // 
  string xml = minPoint.ToXml();
  MapPoint minPointImport = MapPointBuilderEx.FromXml(xml);
  // minPointImport = minPoint

  //
  // Envelope
  //
  Envelope envelopeWithID = EnvelopeBuilderEx.CreateEnvelope(minPoint, maxPoint);

  // Envelopes don't have IDs
  // envelopeWithID.HasID = false
  // envelopeWithID.HasM = true
  // envelopeWithID.HasZ = true

  xml = envelopeWithID.ToXml();
  Envelope envelopeImport = EnvelopeBuilderEx.FromXml(xml);

  //
  // Multipoint
  //
  List<MapPoint> list = new List<MapPoint>();
  list.Add(minPoint);
  list.Add(maxPoint);

  Multipoint multiPoint = MultipointBuilderEx.CreateMultipoint(list);

  xml = multiPoint.ToXml();
  Multipoint multipointImport = MultipointBuilderEx.FromXml(xml);
  // multipointImport.PointCount == 2
  // multipointImport.HasID = true
  // multipointImport.HasM = true
  // multipointImport.HasZ= true
}