The XDocument class contains the information necessary for a valid XML document. This includes an XML declaration, processing instructions, and comments.

Note that you only have to create XDocument objects if you require the specific functionality provided by the XDocument class. In many circumstances, you can work directly with XElement. Working directly with XElement is a simpler programming model.

XDocument derives from XContainer. Therefore, it can contain child nodes. However, XDocument objects can have only one child XElement node. This reflects the XML standard that there can be only one root element in an XML document.

An XDocument can contain the following elements:

• One XDeclaration object. XDeclaration enables you to specify the pertinent parts of an XML declaration: the XML version, the encoding of the document, and whether the XML document is stand-alone.

• One XElement object. This is the root node of the XML document.

• Any number of XProcessingInstruction objects. A processing instruction communicates information to an application that processes the XML.

• Any number of XComment objects. The comments will be siblings to the root element. The XComment object cannot be the first argument in the list, because it is not valid for an XML document to start with a comment.

• One XDocumentType for the DTD.

When you serialize an XDocument, even if XDocument.Declaration is null, the output will have an XML declaration if the writer has Writer.Settings.OmitXmlDeclaration set to false (the default).

By default, LINQ to XML sets the version to "1.0", and sets the encoding to "utf-8".

As previously mentioned, the XElement class is the main class in the LINQ to XML programming interface. In many cases, your application will not require that you create a document. By using the XElement class, you can create an XML tree, add other XML trees to it, modify the XML tree, and save it.

To construct an XDocument, use functional construction, just like you do to construct XElement objects.

The following code creates an XDocument object and its associated contained objects.

XDocument d = new XDocument(
    new XComment("This is a comment."),
    new XProcessingInstruction("xml-stylesheet",
        "href='mystyle.css' title='Compact' type='text/css'"),
    new XElement("Pubs",
        new XElement("Book",
            new XElement("Title", "Artifacts of Roman Civilization"),
            new XElement("Author", "Moreno, Jordao")
        ),
        new XElement("Book",
            new XElement("Title", "Midieval Tools and Implements"),
            new XElement("Author", "Gazit, Inbar")
        )
    ),
    new XComment("This is another comment.")
);
d.Declaration = new XDeclaration("1.0", "utf-8", "true");
Console.WriteLine(d);

d.Save("test.xml");

Dim doc As XDocument = <?xml version="1.0" encoding="utf-8"?>
                       <!--This is a comment.-->
                       <?xml-stylesheet href='mystyle.css' title='Compact' type='text/css'?>
                       <Pubs>
                           <Book>
                               <Title>Artifacts of Roman Civilization</Title>
                               <Author>Moreno, Jordao</Author>
                           </Book>
                           <Book>
                               <Title>Midieval Tools and Implements</Title>
                               <Author>Gazit, Inbar</Author>
                           </Book>
                       </Pubs>
                       <!--This is another comment.-->
doc.Save("test.xml")

When you examine the file test.xml, you get the following output:

<?xml version="1.0" encoding="utf-8"?>
<!--This is a comment.-->
<?xml-stylesheet href='mystyle.css' title='Compact' type='text/css'?>
<Pubs>
  <Book>
    <Title>Artifacts of Roman Civilization</Title>
    <Author>Moreno, Jordao</Author>
  </Book>
  <Book>
    <Title>Midieval Tools and Implements</Title>
    <Author>Gazit, Inbar</Author>
  </Book>
</Pubs>
<!--This is another comment.-->

Concepts