Introducing XML Serialization

아티클
10/17/2014

Serialization is the process of converting an object into a form that can be readily transported. For example, you can serialize an object and transport it over the Internet using HTTP between a client and a server. On the other end, deserialization reconstructs the object from the stream.

XML serialization serializes only the public fields and property values of an object into an XML stream. XML serialization does not include type information. For example, if you have a Book object that exists in the Library namespace, there is no guarantee that it will be deserialized into an object of the same type.

Note XML serialization does not convert methods, indexers, private fields, or read-only properties (except read-only collections). To serialize all an object's fields and properties, both public and private, use the BinaryFormatter instead of XML serialization.

The central class in XML serialization is the XmlSerializer class, and its most important methods are the Serialize and Deserialize methods. The XML stream generated by the XmlSerializer is compliant with the World Wide Web Consortium (www.w3.org) XML Schema definition language (XSD) 1.0 recommendation. Furthermore, the data types generated are compliant with the document titled "XML Schema Part 2: Datatypes."

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 XmlElement or XmlAttribute objects. You have the option of creating your own classes, annotated with attributes, or using the XML Schema Definition tool to generate the classes based on an existing XML Schema.

If you have an XML Schema, you can run the XML Schema Definition tool to produce a set of classes that are strongly typed to the schema and annotated with attributes. When an instance of such a class is serialized, the generated XML adheres to the XML Schema. Provided with such a class, you can program against an easily manipulated object model while being assured that the generated XML conforms to the XML schema. This is an alternative to using other classes in the .NET Framework, such as the XmlReader and XmlWriter classes, to parse and write an XML stream. (For more information about using those classes, see Employing XML in the .NET Framework.) Those classes allow you to parse any XML stream. In contrast, use the XmlSerializer when the XML stream is expected to conform to a known XML Schema.

Attributes control the XML stream generated by the XmlSerializer class, allowing you to set the XML namespace, element name, attribute name, and so on, of the XML stream. For more information about these attributes and how they control XML serialization, see Controlling XML Serialization Using Attributes. For a table of those attributes that are used to control the generated XML, see Attributes That Control XML Serialization.

The XmlSerializer class can further serialize an object and generate an encoded SOAP XML stream. The generated XML adheres to section 5 of the World Wide Web Consortium document titled "Simple Object Access Protocol (SOAP) 1.1." For more information about this process, see Generating SOAP Messages with XML Serialization. For a table of the attributes that control the generated XML, see Attributes That Control Encoded SOAP Serialization.

The XmlSerializer class generates the SOAP messages created by, and passed to, XML Web services. To control the SOAP messages, you can apply attributes to the classes, return values, parameters, and fields found in an XML Web service file (.asmx). You can use both the attributes listed in "Attributes That Control XML Serialization" and "Attributes That Control Encoded SOAP Serialization" because an XML Web service can use either the literal or encoded SOAP style. For more information about using attributes to control the XML generated by an XML Web service, see XML Serialization with XML Web Services. For more information about SOAP and XML Web services, see Customizing SOAP Messages.

Securing XmlSerializer Applications

When creating an application that uses the XmlSerializer, you should be aware of the following items and their implications:

The XmlSerializer creates C# files (.cs files) and compiles them into .dll files in the directory named by the TEMP environment variable; serialization occurs with those DLLs .

The code and the DLLs are vulnerable to a malicious process at the time of creation and compilation. When using a computer running Microsoft Windows NT 4.0 or later, it might be possible for two or more users to share the temp directory. Sharing a temp directory is dangerous if (1) the two accounts have different security privileges, and (2) the higher-privilege account runs an application using the XmlSerializer. In this case, one user can breach the computer's security by replacing either the .cs or .dll file that is compiled. To eliminate this concern, always be sure that each account on the computer has its own profile. If so, by default, the TEMP environment variable will point to a different directory for each account.
If a malicious user sends a continuous stream of XML data to a Web server (a denial of service attack), then the XmlSerializer will continue to process the data until the computer runs low on resources.

This kind of attack is eliminated if you are using a computer running Internet Information Services (IIS), and your application is running within IIS. IIS features a gate that will not process streams longer than a set amount (the default is 4 KB). If you create an application that does not use IIS, and that deserializes with the XmlSerializer, you should implement a similar gate that prevents a denial of service attack.
The XmlSerializer will serialize data and run any code using any type given to it.

There are two ways in which a malicious object presents a threat. It could run malicious code, or it could inject malicious code into the C# file created by the XmlSerializer. In the first case, if a malicious object tries to run a destructive procedure, code access security will help prevent any damage from being done. In the second case, there is a theoretical possibility that a malicious object will somehow inject code into the C# file created by the XmlSerializer. Although this issue has been examined thoroughly, and such an attack is considered unlikely, you should take the precaution of never serializing data with an unknown and untrusted type.
Serialized sensitive data might be vulnerable.

After the XmlSerializer has serialized data, it can be stored as an XML file or other data store. If your data store is available to other processes, or is visible on an intranet or the Internet, the data can be stolen and used maliciously. For example, if you create an application that serializes orders that include credit card numbers, the data is highly sensitive. To help prevent this, always protect the store for your data and take steps to keep it private.

Serialization of a Simple Class

The following example shows a simple class with a public field.

Public Class OrderForm
    Public OrderDate As DateTime
End Class
[C#]
public class OrderForm{
    public DateTime OrderDate;
}

When an instance of this class is serialized, it might resemble the following.

<OrderForm>
    <OrderDate>12/12/01</OrderDate>
</OrderForm>

For more examples of serialization, see Examples of XML Serialization.

Items That Can Be Serialized

The following items can be serialized using the XmLSerializer class.

Public read/write properties and fields of public classes.
Classes that implement ICollection or IEnumerable. (Note that only collections are serialized, not public properties.)
XmlElement objects.
XmlNode objects.
DataSet objects.

Serializing and Deserializing Objects

To serialize an object, first create the object that is to be serialized and set its public properties and fields. To do this, you must determine the transport format in which the XML stream is to be stored — either as a stream or as a file. For example, if the XML stream must be saved in a permanent form, create a FileStream object. When you deserialize an object, the transport format determines whether you will create a stream or file object. After the transport format is determined, you can call the Serialize or Deserialize methods, as required.

To serialize an object

Create the object and set its public fields and properties.
Construct an XmlSerializer using the type of the object. For more information, see the XmlSerializer class constructors.

Call the Serialize method to generate either an XML stream or a file representation of the object's public properties and fields. The following example creates a file.

Dim myObject As MySerializableClass = New MySerializableClass()
' Insert code to set properties and fields of the object.
Dim mySerializer As XmlSerializer = New XmlSerializer(GetType(MySerializableClass))
' To write to a file, create a StreamWriter object.
Dim myWriter As StreamWriter = New StreamWriter("myFileName.xml")
mySerializer.Serialize(myWriter, myObject)

[C#]
MySerializableClass myObject = new MySerializableClass();
// Insert code to set properties and fields of the object.
XmlSerializer mySerializer = new 
XmlSerializer(typeof(MySerializableClass));
// To write to a file, create a StreamWriter object.
StreamWriter myWriter = new StreamWriter("myFileName.xml");
mySerializer.Serialize(myWriter, myObject);

To deserialize an object

Construct an XmlSerializer using the type of the object to deserialize.

Call the Deserialize method to produce a replica of the object. When deserializing, you must cast the returned object to the type of the original, as shown in the following example. The following example deserializes the object into a file, although it could also be deserialized into a stream.

Dim myObject As MySerializableClass
' Constructs an instance of the XmlSerializer with the type
' of object that is being deserialized.
Dim mySerializer As XmlSerializer = New XmlSerializer(GetType(MySerializableClass))
' To read the file, creates a FileStream.
Dim myFileStream As FileStream = _
New FileStream("myFileName.xml", FileMode.Open)
' Calls the Deserialize method and casts to the object type.
myObject = CType( _
mySerializer.Deserialize(myFileStream), MySerializableClass)
[C#]
MySerializableClass myObject;
// Constructs an instance of the XmlSerializer with the type
// of object that is being deserialized.
XmlSerializer mySerializer = 
new XmlSerializer(typeof(MySerializableClass));
// To read the file, creates a FileStream.
FileStream myFileStream = 
new FileStream("myFileName.xml", FileMode.Open);
// Calls the Deserialize method and casts to the object type.
myObject = (MySerializableClass) 
mySerializer.Deserialize(myFileStream)

For more examples of XML serialization, see Examples of XML Serialization.

Advantages of Using XML Serialization

The XmlSerializer class gives you complete and flexible control when you serialize an object as XML. If you are creating an XML Web service, you can apply attributes that control serialization to classes and members to ensure that the XML output conforms to a specific schema.

For example, XmlSerializer enables you to:

Specify whether a field or property should be encoded as an attribute or an element.
Specify an XML namespace to use.
Specify the name of an element or attribute if a field or property name is inappropriate.

Another advantage of XML serialization is that you have no constraints on the applications you develop, as long as the XML stream that is generated conforms to a given schema. Imagine a schema that is used to describe books — it features a title, author, publisher, and ISBN number element. You can develop an application that processes the XML data in any way you want — for example, as a book order, or as an inventory of books. In either case, the only requirement is that the XML stream conforms to the specified XML Schema definition language (XSD) schema.

XML Serialization Considerations

The following should be considered when using the XmlSerializer class:

The serialized data contains only the data itself and the structure of your classes. Type identity and assembly information are not included.
Only public properties and fields can be serialized. If you need to serialize non-public data, use the BinaryFormatter class rather than XML serialization.
A class must have a default constructor to be serialized by XmlSerializer.
Methods cannot be serialized.
XmlSerializer can process classes that implement IEnumerable or ICollection differently if they meet certain requirements. A class that implements IEnumerable must implement a public Add method that takes a single parameter. The Add method's parameter must be consistent (polymorphic) with the type returned from the IEnumerator.Current property returned from the GetEnumerator method. A class that implements ICollection in addition to IEnumerable (such as CollectionBase) must have a public Item indexed property (an indexer in C#) that takes an integer, and it must have a public Count property of type integer. The parameter passed to the Add method must be the same type as that returned from the Item property, or one of that type's bases. For classes implementing ICollection, values to be serialized will be retrieved from the indexed Item property rather than by calling GetEnumerator. Also note that public fields and properties will not be serialized, with the exception of public fields that return another collection class (one that implements ICollection). For an example, see Examples of XML Serialization.

XSD Data Type Mapping

The World Wide Web Consortium (www.w3.org) document titled "XML Schema Part 2: Datatypes" specifies the simple data types that are allowed in an XML Schema definition language (XSD) schema. For many of these (for example, int and decimal), there is a corresponding data type in the .NET Framework. However, some XML data types do not have a corresponding data type in the .NET Framework (for example, the NMTOKEN data type). In such cases, if you use the XML Schema Definition tool (Xsd.exe) to generate classes from a schema, an appropriate attribute is applied to a member of type string, and its DataType property is set to the XML data type name. For example, if a schema contains an element named "MyToken" with the XML data type NMTOKEN, the generated class might contain a member as shown in the following example.

<XmlElement(DataType:="NMTOKEN")>
Public MyToken As String
[C#]
[XmlElement(DataType = "NMTOKEN")]
public string MyToken;

Similarly, if you are creating a class that must conform to a specific XML Schema (XSD), you should apply the appropriate attribute and set its DataType property to the desired XML data type name.

For a complete list of type mappings, see the DataType property for any of the following attribute classes: SoapAttributeAttribute, SoapElementAttribute, XmlArrayItemAttribute, XmlAttributeAttribute, XmlElementAttribute, or XmlRootAttribute.

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