XML Serialization in .NET

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Introduction

My previous article, Hash Table and Serialization in .NET, describes how the content of a hash table is serialized to and deserialized from a binary file. This article continues the serialization topic, and describes XML serialization in .NET, where objects are serialized and deserialized into and from XML documents.

XML serialization implemented by XMLSerializer class converts an object's public classes, properties and fields to a serial XML format. The following example shows how an object of the Person class is serialized into a XML document.

class Person
{
   public Person( string firstName, string lastName )
   {
      this.firstName = firstName;
      this.lastName = lastName;
   }

   public string firstName;
   public string lastName;
}   

static void Main()
{
   Person person = new Person( "John", "Doe" );

   XmlSerializer x = new XmlSerializer( typeof(Person) );
   TextWriter writer = new StreamWriter( "person.xml" );
   x.Serialize( writer, person );
}

If you run the above example, you will get a runtime error mesage, System.InvalidOperationException. The error is caused by the line XmlSerializer x = new XmlSerializer( typeof(Person) );. As it turns out the class Person requires a modifier public and a default constructor. These are common mistakes that I made while working with XmlSerializer class. After you add the public modifier and the default constructor to the Person class, you will get the following XML elements.

<?xml version="1.0 encoding="utf-8"?>
<Person xmlns:xsd="http:www.w3.org/2001/XMLScheme"
        xmlns:xsi="http:www.w3.org/2001/XMLScheme-instance">
    <firstName>John</firstName>
    <lastName>Doe</lastName>
</Person>

As you can see, the element names in the above XML document reflect the public member variable names. You may ask what if I want to make the member variables private. Can I still serialize this class?. XmlSerializer cannot serialize private members, however, you can use public properties as the access methods to the private members as shown below.

   public string FirstName
   {
      get { return firstName; }
      set { firstName = value; }
   }

   public string LastName
   {
      get { return lastName; }
      set { lastName = value; }
   }

   private string firstName;
   private string lastName;

In this case, the XML elements would be FirstName and LastName instead of firstname and lastname respectively to reflect the property names. You can change the XML element names, so that they are different from the property names by using C# attributes. For example, if you want to change the FirstName element to MyFirstName, you can add the following XML element attribute to the FirstName property.

[XmlElement(ElementName = "MyFirstName")]
public string FirstName
{
   get { return firstName; }
   set { firstName = value; }
}

Now, let's expand on the above concept to a more complex application such as a phone book application as described in my previous article, Hash Table and Serialization in .NET. The following UML class diagram shows the relationships between classes that are part of the application. The Contacts class contains one or more Contact, and Contact class contains a Person class and a PhoneNumber class.

The Person, PhoneNumber, and Contact classes are straight forward and self explanatory, so I will not go into details of these classes. The Contacts class has a private hash table member variable table that takes the Person object as the key and the Contact object as the value. Since this hash table member variable contains all of the contacts, it will be nice if it can be serialized directly to a XML document. However, as it turns out, XmlSerializer does not work with the Hashtable class because the Hashtable indexer takes a non-integer parameter (object parameter), and XmlSerializer expects indexers with only an integer parameter. So in order to serialize all of the contacts to the XML document, the hash table is converted to a Contact array, and this array is then serialized to a XML document as shown below.

// Create an instance of Contact array, and copy the content of 
// the hash table to this array
Contact[] aContact = new Contact[table.Count];
table.Values.CopyTo( aContact, 0 );

// Deserialize the content of the Contact array to a XML file
XmlSerializer x = new XmlSerializer( typeof(Contact[]) );
TextWriter writer = new StreamWriter( "phonebook.xml" );
x.Serialize( writer, aContact );

To deserialize the XML elements from the XML document to the hash table, the above steps will be reversed. The XML elements are first deserialize to a Contact array, and then each member of the array is added to the hash table as follows.

// Create an instance of the XmlSerializer class of type Contact array 
XmlSerializer x = new XmlSerializer( typeof(Contact[]) );

// Read the XML file if it exists ---
FileStream fs = null;
try
{
   fs = new FileStream( "phonebook.xml", FileMode.Open );

   // Deserialize the content of the XML file to a Contact array 
   // utilizing XMLReader
   XmlReader reader = new XmlTextReader(fs);         
   Contact[] contacts = (Contact[]) x.Deserialize( reader );

   // Add each member of the Contact array to the hash table
   for ( int i = 0; i < contacts.Length; i++ )
   {
      Contact contact = (Contact) contacts[i];
      table.Add( contact.GetPerson, contact );
   }
}
catch( FileNotFoundException )
{
   // Do nothing if the file does not exists
}
finally
{
   if ( fs != null ) fs.Close();
}

That is all for now. I hope you enjoy this brief introduction on XML serialization in .NET.