Runtime Customization

Modifying the default behavior of the XmlSerializer at runtime goes further than providing type information for undeclared types. We can perform (almost) all the same customizations we do statically through attributes dynamically as well. The XmlSerializer class defines two constructor overloads to override the default behavior or hard-coded attributes:

XmlSerializer(Type types, XmlAttributeOverrides overrides);

XmlSerializer(Type type, XmlAttributeOverrides overrides,

  Type[] extraTypes, XmlRootAttribute root, string defaultNamespace);

XmlAttributeOverrides Class

These constructors are helpful when we need to adjust static attributes only in certain cases, or if we have to support an extended XML format but we cannot change the attributes in the source code. The constructors accept the overriding attributes inside a special container object of the XmlAttributeOverrides class. This class exposes two overloaded Add() methods to apply attributes either on a class or an individual class member:

public void Add(Type type, XmlAttributes overrides);

public void Add(Type type, string elementName,

XmlAttributes overrides);

The type parameter identifies the class to which we want to add serialization attributes, the elementName parameter identifies the class member. For fields, we attached attributes for each type we expected the field to reference at runtime. In the same vein we can assign an XmlAttributes object for each type a field might reference.

NOTE: You need to make sure you supply the same set of attributes if you use different instances of the XmlSerializer to serialize and deserialize a type. This happens automatically when the attributes are attached in source code, but not when you supply them at runtime. If you do not supply the same set of attributes the type mappings are different and your deserializing the XML stream might fail.

The XmlAttributes Class

The attributes themselves are stored in a yet another container object of the XmlAttributes class. This class exposes properties for each metadata attribute defined in the System.Xml.Serialization namespace, e.g. an XmlElements collection for XmlElementAttributes, an XmlRoot property for an XmlRootAttributes and so on. The only attribute we can not attach at runtime is the XmlIncludeAttribute, i.e. we cannot declare type substitutions at runtime. We can, however, declare types globally as we have seen in section 10.2.1.1 or declare substitute types for individual fields by dynamically attaching XmlElement or XmlArrayItem attributes.

1.4            Table 10.2 The properties of the XmlAttributes class store serialization attributes to customize the XmlSerializer’s class-to-type mapping at runtime.

The constructor of the XmlSerializer throws an InvalidOperationException if the overriding attributes are incompatible with the type. For example, the XmlSerializer does not support dynamically adding an XmlArrayItem attribute to arrays or collections at the root of the serialized object graph.