Event notifications

The second way to gain access to unmapped XML nodes in an XML stream that is deserialized is to register event handlers with the XmlSerializer. Attaching the XmlAnyAttribute or XmlAnyElement attributes works great where we anticipate the content we process to contain unmapped nodes on a regular basis. Registering event handlers for unmapped nodes on the other hand works better for cases where we want an application to handle them as an exception rather than as the rule. The XmlSerializer raises four different events for unmapped nodes when it is deserializing a class without XmlAnyAttribute or XmlAnyElement attributes. We can register event sinks for the four events listed in table 10.2 to receive notifications for unexpected XML in the stream.

1.5            Table 10.2 Events defined by the XmlSerializer

The XmlSerializer fires one of these events for each unmapped node it encounters while deserializing an object, but object’s class does not designate fields with the XmlAnyAttribute or the XmlAnyElement. Each event provides details about the unmapped node in the form of a event specific arguments class passed to the event handler. For example, when the XmlSerializer fires and for an unmapped attribute in the XML stream, it passes a reference to itself and an XmlAttributeEventArgs object to the registered event handler. The arguments object contains the line number and position of the attribute within the deserialized XML document, as well as the attribute itself.

The following example shows how to set up event handlers to log the event details about nodes the XmlSerializer could not map to any class members to the console.

10        Listing 10.5 Logging nodes the XmlSerializer can not map to class fields during deserialization

public void BindSerializerEvents( XmlSerializer ser )
{

  ser.UnknownAttribute +=                              | #1

    new XmlAttributeEventHandler(OnUnknownAttribute);  | #2

  ser.UnknownElement +=

    new XmlElementEventHandler(OnUnknownElement);

  ser.UnknownNode +=

    new XmlNodeEventHandler(OnUnknownNode);

}

public static void OnUnknownAttribute(Object sender,

      XmlAttributeEventArgs args)

{

  string typeName = "N/A";

  if( args.ObjectBeingDeserialized != null )

  {

    typeName = 
      args.ObjectBeingDeserialized.GetType().FullName;

  }

  Console.WriteLine(                                   |#3

    @"Unknown attribute {0}:{1}='{2}' at line {3},      |
    position {4}, type: {5})",                         |
    args.Attr.NamespaceURI,   args.Attr.LocalName,     |
    args.Attr.Value, args.LineNumber,                  |

    args.LinePosition, typeName );

}

public static void OnUnknownElement(Object sender,    XmlElementEventArgs args)

{

  string typeName = "N/A";

  if( args.ObjectBeingDeserialized != null )

  {

    typeName = 
      args.ObjectBeingDeserialized.GetType().FullName;

  }

  Console.WriteLine(                                  |#4

    "Unknown element {0}:{1}='{2}' at line {3},       |
    position {4}, type: {5})",                        |

    args.Element.NamespaceURI,                        |
    args.Element.LocalName, args.Element.InnerXml,    |
    args.LineNumber, args.LinePosition, typeName);    |

}

public static void OnUnknownNode(Object sender,
      XmlNodeEventArgs args)

{

  string typeName = "N/A";

  if( args.ObjectBeingDeserialized != null )

  {

    typeName =
      args.ObjectBeingDeserialized.GetType().FullName;

  }

  Console.WriteLine(                                   |#5

    "Unkown Node type {6} {0}:{1}='{2}' at line {3},   |
    position {4}, type: {5})",                         |
    args.NamespaceURI, args.LocalName, args.Text,      |
    args.LineNumber, args.LinePosition, typeName,      |

    args.NodeType.ToString() );                        |

}

(annotation) <#1 The += operator is used to register an event sink >

(annotation) <#2 We have to instantiate a delegate object for the event sink method>

(annotation) <#3 The XmlAttributeEventArgs object contains detailed information about the unmapped attribute.>

(annotation) <#4 The XmlElementEventArgs object contains detailed information about the unmapped element.>

(annotation) <#5 The XmlNodeEventArgs object contains detailed information about the unmapped node.>