Control XML serialization using attributes
Attributes can be used to control the XML serialization of an object or to create an alternate XML stream from the same set of classes. For more information about creating an alternate XML stream, see How to: Specify an Alternate Element Name for an XML Stream.
Note
If the XML generated must conform to section 5 of the World Wide Web Consortium (W3C) document titled Simple Object Access Protocol (SOAP) 1.1, use the attributes listed in Attributes That Control Encoded SOAP Serialization.
By default, an XML element name is determined by the class or member name. In a class named Book, a field named ISBN will produce an XML element tag <ISBN>, as shown in the following example:
Public Class Book
Public ISBN As String
End Class
' When an instance of the Book class is serialized, it might
' produce this XML:
' <ISBN>1234567890</ISBN>.
public class Book
{
public string ISBN;
}
// When an instance of the Book class is serialized, it might
// produce this XML:
// <ISBN>1234567890</ISBN>.
The default behavior can be changed if you want to give the element a new name. The following code shows how an attribute enables this functionality by setting the ElementName property of an XmlElementAttribute:
Public Class TaxRates
< XmlElement(ElementName = "TaxRate")> _
Public ReturnTaxRate As Decimal
End Class
public class TaxRates {
[XmlElement(ElementName = "TaxRate")]
public decimal ReturnTaxRate;
}
For more information about attributes, see Attributes. For a list of attributes that control XML serialization, see Attributes That Control XML Serialization.
Controlling Array Serialization
The XmlArrayAttribute and the XmlArrayItemAttribute attributes control the serialization of arrays. Using these attributes, you can control the element name, namespace, and XML Schema (XSD) data type as defined in the W3C document titled XML Schema Part 2: Datatypes. You can also specify the types that can be included in an array.
The XmlArrayAttribute will determine the properties of the enclosing XML element that results when an array is serialized. For example, by default, serializing the array below will result in an XML element named Employees. The Employees element will contain a series of elements named after the array type Employee.
Public Class Group
Public Employees() As Employee
End Class
Public Class Employee
Public Name As String
End Class
public class Group {
public Employee[] Employees;
}
public class Employee {
public string Name;
}
A serialized instance might resemble the following code:
<Group>
<Employees>
<Employee>
<Name>Haley</Name>
</Employee>
</Employees>
</Group>
By applying a XmlArrayAttribute, you can change the name of the XML element, as follows:
Public Class Group
<XmlArray("TeamMembers")> _
Public Employees() As Employee
End Class
public class Group {
[XmlArray("TeamMembers")]
public Employee[] Employees;
}
The resulting XML might resemble the following code:
<Group>
<TeamMembers>
<Employee>
<Name>Haley</Name>
</Employee>
</TeamMembers>
</Group>
The XmlArrayItemAttribute, on the other hand, controls how the items contained in the array are serialized.
Note
The attribute is applied to the field returning the array.
Public Class Group
<XmlArrayItem("MemberName")> _
Public Employee() As Employees
End Class
public class Group {
[XmlArrayItem("MemberName")]
public Employee[] Employees;
}
The resulting XML might resemble the following code:
<Group>
<Employees>
<MemberName>Haley</MemberName>
</Employees>
</Group>
Serializing Derived Classes
Another use of the XmlArrayItemAttribute is to allow the serialization of derived classes. For example, another class named Manager that derives from Employee can be added to the previous example. If you don't apply the XmlArrayItemAttribute, the code will fail at run time because the derived class type won't be recognized. To remedy this outcome, apply the attribute twice, each time setting the Type property for each acceptable type (base and derived).
Public Class Group
<XmlArrayItem(Type:=GetType(Employee)), _
XmlArrayItem(Type:=GetType(Manager))> _
Public Employees() As Employee
End Class
Public Class Employee
Public Name As String
End Class
Public Class Manager
Inherits Employee
Public Level As Integer
End Class
public class Group {
[XmlArrayItem(Type = typeof(Employee)),
XmlArrayItem(Type = typeof(Manager))]
public Employee[] Employees;
}
public class Employee {
public string Name;
}
public class Manager:Employee {
public int Level;
}
A serialized instance might resemble the following code:
<Group>
<Employees>
<Employee>
<Name>Haley</Name>
</Employee>
<Employee xsi:type = "Manager">
<Name>Ann</Name>
<Level>3</Level>
</Employee>
</Employees>
</Group>
Serializing an Array as a Sequence of Elements
You can also serialize an array as a flat sequence of XML elements by applying a XmlElementAttribute to the field returning the array as follows:
Public Class Group
<XmlElement> _
Public Employees() As Employee
End Class
public class Group {
[XmlElement]
public Employee[] Employees;
}
A serialized instance might resemble the following code:
<Group>
<Employees>
<Name>Haley</Name>
</Employees>
<Employees>
<Name>Noriko</Name>
</Employees>
<Employees>
<Name>Marco</Name>
</Employees>
</Group>
Another way to differentiate the two XML streams is to use the XML Schema Definition tool to generate the XML Schema (XSD) document files from the compiled code. For more information on using the tool, see The XML Schema Definition Tool and XML Serialization. When no attribute is applied to the field, the schema describes the element in the following manner:
<xs:element minOccurs="0" maxOccurs ="1" name="Employees" type="ArrayOfEmployee" />
When the XmlElementAttribute is applied to the field, the resulting schema describes the element as follows:
<xs:element minOccurs="0" maxOccurs="unbounded" name="Employees" type="Employee" />
Serializing an ArrayList
The ArrayList class can contain a collection of diverse objects. You can therefore use an ArrayList much as you use an array. Instead of creating a field that returns an array of typed objects, however, you can create a field that returns a single ArrayList. However, as with arrays, you must inform the XmlSerializer of the types of objects the ArrayList contains. To accomplish this, assign multiple instances of the XmlElementAttribute to the field, as shown in the following example.
Public Class Group
<XmlElement(Type:=GetType(Employee)), _
XmlElement(Type:=GetType(Manager))> _
Public Info As ArrayList
End Class
public class Group {
[XmlElement(Type = typeof(Employee)),
XmlElement(Type = typeof(Manager))]
public ArrayList Info;
}
Controlling Serialization of Classes Using XmlRootAttribute and XmlTypeAttribute
You can apply two attributes to a class only: XmlRootAttribute and XmlTypeAttribute. These attributes are similar. The XmlRootAttribute can be applied to only one class: the class that, when serialized, represents the XML document's opening and closing element—in other words, the root element. The XmlTypeAttribute, on the other hand, can be applied to any class, including the root class.
For example, in the previous examples, the Group class is the root class, and all its public fields and properties become the XML elements found in the XML document. Therefore, you can have only one root class. By applying the XmlRootAttribute, you can control the XML stream generated by the XmlSerializer. For example, you can change the element name and namespace.
The XmlTypeAttribute allows you to control the schema of the generated XML. This capability is useful when you need to publish the schema through an XML Web service. The following example applies both the XmlTypeAttribute and the XmlRootAttribute to the same class:
<XmlRoot("NewGroupName"), _
XmlType("NewTypeName")> _
Public Class Group
Public Employees() As Employee
End Class
[XmlRoot("NewGroupName")]
[XmlType("NewTypeName")]
public class Group {
public Employee[] Employees;
}
If this class is compiled, and the XML Schema Definition tool is used to generate its schema, you would find the following XML describing Group:
<xs:element name="NewGroupName" type="NewTypeName" />
In contrast, if you were to serialize an instance of the class, only NewGroupName would be found in the XML document:
<NewGroupName>
. . .
</NewGroupName>
Preventing Serialization with the XmlIgnoreAttribute
You might come across a situation where a public property or field doesn't need to be serialized. For example, a field or property could be used to contain metadata. In such cases, apply the XmlIgnoreAttribute to the field or property and the XmlSerializer will skip over it.
