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Scope of default namespaces (LINQ to XML)

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Default namespaces as represented in the XML tree aren't in scope for queries. If you have XML that's in a default namespace, you must combine a namespace with the local name to make a qualified name to be used in the query.

A common mistake in querying an XML tree that has a default namespace is to write the query as if the XML weren't in a namespace. The first example shows a typical improper query of a default namespace. The second shows a proper query.

Example: Improper query of XML in a namespace

This example shows the creation of XML in a namespace, and an improper query that returns an empty result set.

XElement root = XElement.Parse(
@"<Root xmlns='http://www.adventure-works.com'>
    <Child>1</Child>
    <Child>2</Child>
    <Child>3</Child>
    <AnotherChild>4</AnotherChild>
    <AnotherChild>5</AnotherChild>
    <AnotherChild>6</AnotherChild>
</Root>");
IEnumerable<XElement> c1 =
    from el in root.Elements("Child")
    select el;
Console.WriteLine("Result set follows:");
foreach (XElement el in c1)
    Console.WriteLine((int)el);
Console.WriteLine("End of result set");

Module Module1
    Sub Main()
        Dim root As XElement = _
            <Root xmlns='http://www.adventure-works.com'>
                <Child>1</Child>
                <Child>2</Child>
                <Child>3</Child>
                <AnotherChild>4</AnotherChild>
                <AnotherChild>5</AnotherChild>
                <AnotherChild>6</AnotherChild>
            </Root>
        Dim c1 As IEnumerable(Of XElement) = _
                From el In root.<Child> _
                Select el
        Console.WriteLine("Result set follows:")
        For Each el As XElement In c1
            Console.WriteLine(CInt(el))
        Next
        Console.WriteLine("End of result set")
    End Sub
End Module

This example produces the following output:

Result set follows:
End of result set

Example: Proper query of XML in a namespace

This example shows the creation of XML in a namespace, and a proper query.

In C#, the correct approach is to declare and initialize an XNamespace object, and to use it when specifying XName objects. In this case, the argument to the Elements method is an XName object.

The correct approach when using Visual Basic is to declare and initialize a global default namespace. This places all XML properties in the default namespace.

Here is the code:

XElement root = XElement.Parse(
@"<Root xmlns='http://www.adventure-works.com'>
    <Child>1</Child>
    <Child>2</Child>
    <Child>3</Child>
    <AnotherChild>4</AnotherChild>
    <AnotherChild>5</AnotherChild>
    <AnotherChild>6</AnotherChild>
</Root>");
XNamespace aw = "http://www.adventure-works.com";
IEnumerable<XElement> c1 =
    from el in root.Elements(aw + "Child")
    select el;
Console.WriteLine("Result set follows:");
foreach (XElement el in c1)
    Console.WriteLine((int)el);
Console.WriteLine("End of result set");

Imports <xmlns="http://www.adventure-works.com">

Module Module1
    Sub Main()
        Dim root As XElement = _
            <Root xmlns='http://www.adventure-works.com'>
                <Child>1</Child>
                <Child>2</Child>
                <Child>3</Child>
                <AnotherChild>4</AnotherChild>
                <AnotherChild>5</AnotherChild>
                <AnotherChild>6</AnotherChild>
            </Root>
        Dim c1 As IEnumerable(Of XElement) = _
                From el In root.<Child> _
                Select el
        Console.WriteLine("Result set follows:")
        For Each el As XElement In c1
            Console.WriteLine(el.Value)
        Next
        Console.WriteLine("End of result set")
    End Sub
End Module

This example produces the following output:

Result set follows:
1
2
3
End of result set

See also