.NET (Core and Framework)

// Contained class.
    public class Circle
    {
    }

    // Container class.
    [XmlType(Namespace="http://mbshapes.com")] // Why does the Shapes element not use this namespace?
    public class Shapes
    {
        private Circle _circle = new Circle();

        public Circle Circle
        {
            get
            {
                return _circle;
            }
            set
            {
                _circle = value;
            }
        }
    }

    // Serialize Shapes object.
    public class SerializeTest
    {
        public static void Serialize()
        {
            // Create Shapes object.
            Shapes shapes = new Shapes();

            // Open file.
            using (TextWriter textWriter = new StreamWriter("Shapes.xml"))
            {                
                // Serialize.
                XmlSerializer xmlSerializer = new XmlSerializer(typeof(Shapes));
                xmlSerializer.Serialize(textWriter, shapes);
            }
        }
    }

<?xml version="1.0" encoding="utf-8"?>
<Shapes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Circle xmlns="http://mbshapes.com" />
</Shapes>

<?xml version="1.0" encoding="utf-8"?>
<Shapes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://mbshapes.com">
  <Circle />
</Shapes>