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Creating XML Trees in C# (LINQ to XML)

This section provides information about creating XML trees in C#.

For information about using the results of LINQ queries as the content for an XElement, see Functional Construction (LINQ to XML).

The signatures of the XElement and XAttribute constructors let you pass the contents of the element or attribute as arguments to the constructor. Because one of the constructors takes a variable number of arguments, you can pass any number of child elements. Of course, each of those child elements can contain their own child elements. For any element, you can add any number of attributes.

When adding XNode (including XElement) or XAttribute objects, if the new content has no parent, the objects are simply attached to the XML tree. If the new content already is parented, and is part of another XML tree, the new content is cloned, and the newly cloned content is attached to the XML tree. The last example in this topic demonstrates this.

To create a contacts XElement, you could use the following code:

XElement contacts =
    new XElement("Contacts",
        new XElement("Contact",
            new XElement("Name", "Patrick Hines"), 
            new XElement("Phone", "206-555-0144"),
            new XElement("Address",
                new XElement("Street1", "123 Main St"),
                new XElement("City", "Mercer Island"),
                new XElement("State", "WA"),
                new XElement("Postal", "68042")
            )
        )
    );

If indented properly, the code to construct XElement objects closely resembles the structure of the underlying XML.

Visual Basic has another approach to creating XML trees: You can directly embed XML in your Visual Basic program as an XML literal. For more information, see Introduction to XML Literals in Visual Basic.

The XElement class uses the following constructors for functional construction. Note that there are some other constructors for XElement, but because they are not used for functional construction they are not listed here.

Constructor

Description

XElement(XName name, object content)

Creates an XElement. The name parameter specifies the name of the element; content specifies the content of the element.

XElement(XName name)

Creates an XElement with its XName initialized to the specified name.

XElement(XName name, params object[] content)

Creates an XElement with its XName initialized to the specified name. The attributes and/or child elements are created from the contents of the parameter list.

The content parameter is extremely flexible. It supports any type of object that is a valid child of an XElement. The following rules apply to different types of objects passed in this parameter:

  • A string is added as text content.

  • An XElement is added as a child element.

  • An XAttribute is added as an attribute.

  • An XProcessingInstruction, XComment, or XText is added as child content.

  • An IEnumerable is enumerated, and these rules are applied recursively to the results.

  • For any other type, its ToString method is called and the result is added as text content.

Creating an XElement with Content

You can create an XElement that contains simple content with a single method call. To do this, specify the content as the second parameter, as follows:

XElement n = new XElement("Customer", "Adventure Works");
Console.WriteLine(n);

This example produces the following output:

<Customer>Adventure Works</Customer>

You can pass any type of object as the content. For example, the following code creates an element that contains a floating point number as content:

XElement n = new XElement("Cost", 324.50);
Console.WriteLine(n);

This example produces the following output:

<Cost>324.5</Cost>

The floating point number is boxed and passed in to the constructor. The boxed number is converted to a string and used as the content of the element.

Creating an XElement with a Child Element

If you pass an instance of the XElement class for the content argument, the constructor creates an element with a child element:

XElement shippingUnit = new XElement("ShippingUnit",
    new XElement("Cost", 324.50)
);
Console.WriteLine(shippingUnit);

This example produces the following output:

<ShippingUnit>
  <Cost>324.5</Cost>
</ShippingUnit>

Creating an XElement with Multiple Child Elements

You can pass in a number of XElement objects for the content. Each of the XElement objects is included as a child element.

XElement address = new XElement("Address",
    new XElement("Street1", "123 Main St"),
    new XElement("City", "Mercer Island"),
    new XElement("State", "WA"),
    new XElement("Postal", "68042")
);
Console.WriteLine(address);

This example produces the following output:

<Address>
  <Street1>123 Main St</Street1>
  <City>Mercer Island</City>
  <State>WA</State>
  <Postal>68042</Postal>
</Address>

By extending the above example, you can create an entire XML tree, as follows:

XElement contacts =
    new XElement("Contacts",
        new XElement("Contact",
            new XElement("Name", "Patrick Hines"),                                                 
            new XElement("Phone", "206-555-0144"),
            new XElement("Address",
                new XElement("Street1", "123 Main St"),
                new XElement("City", "Mercer Island"),
                new XElement("State", "WA"),
                new XElement("Postal", "68042")
            )
        )
    );
Console.WriteLine(contacts);

This example produces the following output:

<Contacts>
  <Contact>
    <Name>Patrick Hines</Name>
    <Phone>206-555-0144</Phone>
    <Address>
      <Street1>123 Main St</Street1>
      <City>Mercer Island</City>
      <State>WA</State>
      <Postal>68042</Postal>
    </Address>
  </Contact>
</Contacts>

Creating an Empty Element

To create an empty XElement, you do not pass any content to the constructor. The following example creates an empty element:

XElement n = new XElement("Customer");
Console.WriteLine(n);

This example produces the following output:

<Customer />

Attaching vs. Cloning

As mentioned previously, when adding XNode (including XElement) or XAttribute objects, if the new content has no parent, the objects are simply attached to the XML tree. If the new content already is parented and is part of another XML tree, the new content is cloned, and the newly cloned content is attached to the XML tree.

// Create a tree with a child element.
XElement xmlTree1 = new XElement("Root",
    new XElement("Child1", 1)
);

// Create an element that is not parented.
XElement child2 = new XElement("Child2", 2);

// Create a tree and add Child1 and Child2 to it.
XElement xmlTree2 = new XElement("Root",
    xmlTree1.Element("Child1"),
    child2
);

// Compare Child1 identity.
Console.WriteLine("Child1 was {0}",
    xmlTree1.Element("Child1") == xmlTree2.Element("Child1") ?
    "attached" : "cloned");

// Compare Child2 identity.
Console.WriteLine("Child2 was {0}",
    child2 == xmlTree2.Element("Child2") ?
    "attached" : "cloned");

This example produces the following output:

Child1 was cloned
Child2 was attached
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