XmlElement Attribute Type

Attaching attributes to individual class members is another way to declare additional types. This can be useful when it is impossible to attach an XmlInclude attribute to a class to declare derived classes, but we can declare the derived classes on the field that is referencing the base class.

In the next example we create a new class VerySmallParkingLot with a field of the Vehicle base class from the previous section. We attach two XmlElement attributes to the field, one for each type we expect the member to reference at runtime. The class and its XML counterpart are shown above. Besides declaring the Car type for the field, the XmlElement attribute also causes the XmlSerializer to identify the object type through the element name instead of an xsi:type attribute.

public class VerySmallParkingLot {   public VerySmallParkingLot (){}   [XmlElement(     Type=typeof(Car))]   [XmlElement(     Type=typeof(Motorcycle))]   public Vehicle ParkedVehicle; }

<VerySmallParkingLot …>   <Car>    <Make>Ford</Make>     <Model>Explorer</Model>     <Year>1997</Year>     <VIN>1234</VIN>   </Car> </VerySmallParkingLot>

The more prevalent use-case where we need to define types through multiple XmlElement attributes is when we develop a class mapping to an XML type containing which can contain different child elements. We express this flexible setup with the <choice> model group in an XSD schema. Generally, a model group defines usage rules for a group of elements, or in XML terminology: Particles. A model group defines which particles can occur in a group, in which order they occur in and how many times. The <choice> model group in particular defines a group of mutually exclusive particles. The next XML schema snippet shows a type description for an XML type roughly corresponding to the VerySmallParkingLot class from above. Each instance of the XML type VerySmallParkingLot, can either contain a ParkedMotorcycle element of type Motorcycle or a ParkedCar element of type Car. The difference to the class above is that the element names vary with the element type because the choice model group requires unique names for each particle.

Listing 9.5 An XML type definition with the choice model group

<xs:complexType name="VerySmallParkingLot">

  <xs:sequence>

    <xs:choice minOccurs="1" maxOccurs="1">

      <xs:element minOccurs="0" maxOccurs="1" name="ParkedMotorcycle"

        type="Motorcycle" />

      <xs:element minOccurs="0" maxOccurs="1" name="ParkedCar"

        type="Car" />

    </xs:choice>

  </xs:sequence>

</xs:complexType>

But specifying the element names for each particle is no big deal to XML serialization, as long as the each particle’s type is unique. You simply specify the element name for each type by setting the

ElementName property of the XmlElement attribute as shown below.