XML serialization is the process of converting an object's public properties and fields to a serial format (in this case, XML) for storage or transport. Deserialization re-creates the object in its original state from the XML output. You can think of serialization as a way of saving the state of an object into a stream or buffer. For example, ASP.NET uses the System.Xml.Serialization.XmlSerializer class to encode XML Web service messages.

The data in your objects is described using programming language constructs like classes, fields, properties, primitive types, arrays, and even embedded XML in the form of System.Xml.XmlElement or System.Xml.XmlAttribute objects. You have the option of creating your own classes, annotated with attributes, or using the XML Schema Definition Tool (Xsd.exe) to generate the classes based on an existing XML Schema definition (XSD) document. If you have an XML Schema, you can run the Xsd.exe to produce a set of classes that are strongly typed to the schema and annotated with attributes to adhere to the schema when serialized.

To transfer data between objects and XML requires a mapping from the programming language constructs to XML schema and from the XML schema to the programming language constructs. The System.Xml.Serialization.XmlSerializer and related tools like Xsd.exe provide the bridge between these two technologies at both design time and runtime. At design time, use the Xsd.exe to produce an XML schema document (.xsd) from your custom classes or to produce classes from a given schema. In either case, the classes are annotated with custom attributes to instruct the System.Xml.Serialization.XmlSerializer how to map between the XML schema system and the common language runtime. At runtime, instances of the classes can be serialized into XML documents that follow the given schema. Likewise, these XML documents can be deserialized into runtime objects. Note that the XML schema is optional, and not required at design time or runtime.

Controlling Generated XML

To control the generated XML, you can apply special attributes to classes and members. For example, to specify a different XML element name, apply an System.Xml.Serialization.XmlElementAttribute to a public field or property, and set the XmlElementAttribute.ElementName property. For a complete list of similar attributes, see Attributes That Control XML Serialization. You can also implement the System.Xml.Serialization.IXmlSerializable interface to control the XML output.

If the XML generated must conform to section 5 of the World Wide Consortium (www.w3.org) document, "Simple Object Access Protocol (SOAP) 1.1", you must construct the System.Xml.Serialization.XmlSerializer with an System.Xml.Serialization.XmlTypeMapping. To further control the encoded SOAP XML, use the attributes listed in Attributes That Control Encoded SOAP Serialization.

With the System.Xml.Serialization.XmlSerializer you can take advantage of working with strongly typed classes and still have the flexibility of XML. Using fields or properties of type System.Xml.XmlElement, System.Xml.XmlAttribute or System.Xml.XmlNode in your strongly typed classes, you can read parts of the XML document directly into XML objects.

If you work with extensible XML schemas, you can also use the System.Xml.Serialization.XmlAnyElementAttribute and System.Xml.Serialization.XmlAnyAttributeAttribute attributes to serialize and deserialize elements or attributes that are not found in the original schema. To use the objects, apply an System.Xml.Serialization.XmlAnyElementAttribute to a field that returns an array of System.Xml.XmlElement objects, or apply an System.Xml.Serialization.XmlAnyAttributeAttribute to a field that returns an array of System.Xml.XmlAttribute objects.

If a property or field returns a complex object (such as an array or a class instance), the System.Xml.Serialization.XmlSerializer converts it to an element nested within the main XML document. For example, the first class in the following code returns an instance of the second class.

Public Class MyClass
    Public MyObjectProperty As MyObject
End Class

Public Class MyObject
    Public ObjectName As String
End Class

public class MyClass
{
    public MyObject MyObjectProperty;
}
public class MyObject
{
    public string ObjectName;
}

The serialized, XML output looks like this:

<MyClass>
  <MyObjectProperty>
  <ObjectName>My String</ObjectName>
  </MyObjectProperty>
</MyClass>

If a schema includes an element that is optional (minOccurs = '0'), or if the schema includes a default value, you have two options. One option is to use System.ComponentModel.DefaultValueAttribute to specify the default value, as shown in the following code.

Public Class PurchaseOrder
    <System.ComponentModel.DefaultValueAttribute ("2002")> _
    Public Year As String
End Class

public class PurchaseOrder
{
    [System.ComponentModel.DefaultValueAttribute ("2002")]
    public string Year;
}

Another option is to use a special pattern to create a Boolean field recognized by the System.Xml.Serialization.XmlSerializer, and to apply the System.Xml.Serialization.XmlIgnoreAttribute to the field. The pattern is created in the form of propertyNameSpecified. For example, if there is a field named "MyFirstName" you would also create a field named "MyFirstNameSpecified" that instructs the System.Xml.Serialization.XmlSerializer whether to generate the XML element named "MyFirstName". This is shown in the following example.

Public Class OptionalOrder
    ' This field's value should not be serialized 
    ' if it is uninitialized.
    Public FirstOrder As String

    ' Use the XmlIgnoreAttribute to ignore the 
    ' special field named "FirstOrderSpecified".
    <System.Xml.Serialization.XmlIgnoreAttribute> _
    Public FirstOrderSpecified As Boolean
End Class

public class OptionalOrder
{
    // This field should not be serialized 
    // if it is uninitialized.
    public string FirstOrder;

    // Use the XmlIgnoreAttribute to ignore the 
    // special field named "FirstOrderSpecified".
    [System.Xml.Serialization.XmlIgnoreAttribute]
    public bool FirstOrderSpecified;
}

Overriding Default Serialization

You can also override the serialization of any set of objects and their fields and properties by creating one of the appropriate attributes, and adding it to an instance of the System.Xml.Serialization.XmlAttributes class. Overriding serialization in this way has two uses: first, you can control and augment the serialization of objects found in a DLL, even if you do not have access to the source; second, you can create one set of serializable classes, but serialize the objects in multiple ways. For more details, see the System.Xml.Serialization.XmlAttributeOverrides class and How to: Control Serialization of Derived Classes.

To serialize an object, call the XmlSerializer.Serialize(System.IO.TextWriter, object) method. To deserialize an object, call the XmlSerializer.Deserialize(System.IO.Stream) method.

To add XML namespaces to an XML document, see System.Xml.Serialization.XmlSerializerNamespaces.

You must have permission to write to the temporary directory (as defined by the TEMP environment variable) to deserialize an object.

Dynamically Generated Assemblies

To increase performance, the XML serialization infrastructure dynamically generates assemblies to serialize and deserialize specified types. The infrastructure finds and reuses those assemblies. This behavior occurs only when using the following constructors:

XmlSerializer.#ctor(Type)

XmlSerializer.#ctor(Type, string)

If you use any of the other constructors, multiple versions of the same assembly are generated and never unloaded, which results in a memory leak and poor performance. The easiest solution is to use one of the previously mentioned two constructors. Otherwise, you must cache the assemblies in a Hashtable, as shown in the following example.

Hashtable serializers = new Hashtable();

// Use the constructor that takes a type and XmlRootAttribute.
XmlSerializer s = new XmlSerializer(typeof(MyClass), myRoot);

// Implement a method named GenerateKey that creates unique keys 
// for each instance of the XmlSerializer. The code should take 
// into account all parameters passed to the XmlSerializer 
// constructor.
object key = GenerateKey(typeof(MyClass), myRoot);

// Check the local cache for a matching serializer.
XmlSerializer ser = (XmlSerializer)serializers[key];
if (ser == null) 
{
    ser = new XmlSerializer(typeof(MyClass), myRoot);
    // Cache the serializer.
    serializers[key] = ser;
}
else
{
    // Use the serializer to serialize, or deserialize.
}

Dim serializers As New Hashtable()

' Use the constructor that takes a type and XmlRootAttribute.
Dim s As New XmlSerializer(GetType([MyClass]), myRoot)

' Implement a method named GenerateKey that creates unique keys 
' for each instance of the XmlSerializer. The code should take 
' into account all parameters passed to the XmlSerializer 
' constructor.
Dim key As Object = GenerateKey(GetType([MyClass]), myRoot)

' Check the local cache for a matching serializer.
Dim ser As XmlSerializer = CType(serializers(key), XmlSerializer)

If ser Is Nothing Then
    ser = New XmlSerializer(GetType([MyClass]), myRoot)
    ' Cache the serializer.
    serializers(key) = ser
Else 
    ' Use the serializer to serialize, or deserialize.
End If

Serialization of ArrayList and Generic List

The System.Xml.Serialization.XmlSerializer cannot serialize or deserialize the following:

• Arrays of ArrayList

• Arrays of List`1

Serialization of Enumerations of Unsigned Long

The System.Xml.Serialization.XmlSerializer cannot be instantiated to serialize an enumeration if the following conditions are true: The enumeration is of type unsigned long (ulong in C#) and the enumeration contains any member with a value larger than 9,223,372,036,854,775,807. For example, the following cannot be serialized.

public enum LargeNumbers: ulong
{
    a = 9223372036854775808
}
// At runtime, the following code will fail.
xmlSerializer mySerializer=new XmlSerializer(typeof(LargeNumbers));

Objects marked with the Obsolete Attribute no longer serialized

In the netfx35_short the System.Xml.Serialization.XmlSerializer class no longer serializes objects that are marked as [Obsolete].