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}}";
//At 2.x - GeometryBag geometryBag = GeometryBagBuilder.FromJson(jsonString);
var geometryBag = GeometryBagBuilderEx.FromJson(jsonString);

string xml = geometryBag.ToXml();
//At 2.x - GeometryBag xmlString = GeometryBagBuilder.FromXML(xml);
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