XmlSerializer Namespaces

We can also have the XmlSerializer create prefix declarations and qualified names instead of repeating namespace declarations on every element or attribute. The first approach puts the control over prefix declarations inside the serialized object. The XmlSerializer checks objects it serializes for a field of type XmlSerializerNamespaces adorned with an XmlNamespaceDeclarations attribute. If it finds such a field, it writes the prefix declarations contained in the XmlNamespaceDeclarations object with the top element of the object graph. Whenever a field or child object references a namespace declared in the XmlNamespaceDeclarations, the XmlSerializer generates the declared prefix instead of writing out another local namespace declaration.

The code in Listing 10.10 demonstrates the use of an XmlSerializerNamespaces field to declare prefixes for the namespaces of the Make, Model and Year fields.

14        Listing 10.10 A class with a field for namespace prefix declarations.

public class CarWithQualifiedNames

{

  [XmlAttribute(Namespace="urn:cars-makes")]

  public string Make;  

  [XmlElement(Namespace="urn:cars-models")]

  public string Model;

  public int Year;

  [XmlNamespaceDeclarations] 

  public XmlSerializerNamespaces Namespaces;

}   

public static void SerializeCar(CarWithQualifiedNames car,

  XmlTextWriter writer )

{

  car.Namespaces = new XmlSerializerNamespaces();

  car.Namespaces.Add( "mk", "urn:cars-makes" );

  car.Namespaces.Add( "md", "urn:cars-models" );     

  XmlSerializer serializer = new XmlSerializer(

    typeof(CarWithQualifiedNames) );

  serializer.Serialize( writer, car );

}

When we serialize an object of this class, the XmlSerializer declares the prefixes we added to the Namespaces map before we called Serialize() on the start tag of the object. While these declarations are in scope, i.e. for all fields or the CarWithQualifiedNames class and its children, these namespaces are referenced through these prefixes (listing 10.11).

15        Listing 10.11 A serialized Car objects with namespace prefixes

<CarWithQualifiedNames

  xmlns:mk="urn:cars-makes"
  xmlns:md="urn:cars-models"

  mk:Make="Ford" …>

  <md:Model>Explorer</md:Model>

  <Year>1997</Year>

</CarWithQualifiedNames>

The single prefix declaration in this example does not illustrate the benefit of prefix declarations well. However, when you serialize large hierarchies with types from many different namespaces, the resulting documents quickly become cluttered with local namespace declarations.

WARNING: Declaring prefixes is a good and widely-used approach to tidy up XML documents, but declaring prefixes in the class code is very risky. You should always define namespace prefixes in the scope of the document in which they are valid to maintain flexibility into which documents you serialize you classes. Always leave prefix declarations up to the application, not to the serialized objects.

The Namespaces collection also serves a second purpose. The XmlSerializer stores the prefixes found the in the XML document when it deserializes an object. Nevertheless, you should not care about the namespace prefixes defined throughout a particular document instance.