How do I parse an xml fragment into nodes and append them to a node with a specified default namespace so that they are a part of that namespace?

Question

I have the following input

<row test="1" />

and want to generate the following output when using XmlTextWriter.

<?xml version="1.0"?>
<root xmlns="urn:default">
  <row test="1" />
</root>

According to the documentation for InnerXml (MSDN), the following code should work correctly.

var outputdoc = new XmlDocument();
outputdoc.AppendChild(outputdoc.CreateXmlDeclaration("1.0", string.Empty, string.Empty));
outputdoc.AppendChild(outputdoc.CreateElement("root", "urn:default"));
outputdoc.DocumentElement.InnerXml = "<row test=\"1\" />";

var writer = new XmlTextWriter(filename, Encoding.UTF8) { Formatting = Formatting.Indented, Indentation = 1 };
outputdoc.WriteTo(writer);
writer.Close();

Instead, I get the following output:

<?xml version="1.0"?>
<root xmlns="urn:default">
  <row test="1" xmlns="" />
</root>

What do I need to do?

EDIT:

I didn't make the possible inputs clear enough. It was supposed to be an Xml fragment so it could be one element, more than one element with any number of children in either case. For example:

<row test="1" />

or

<row test="1" />
<row test="2" />

or

<row><test>1</test></row>

or

<row><test>1</test></row>
<row test="2" />

Answer 1

Must you use System.Xml? If you were to use XElement, it would be trivial:

XElement root = new XElement(XName.Get("root", "urn:default"));
XElement child = XElement.Parse("<row test=\"1\" />");
root.Add(child);
child.Name = XName.Get("row", "urn:default");
Console.WriteLine(root.ToString());

Prints out:

<root xmlns="urn:default">
    <row test="1" />
</root>

Answer 2

You could write it all out using the XmlTextWriter, although I would advise caution unless you are sure it is doing what you want with the default namespace.

using(var writer = new XmlTextWriter(filename, Encoding.UTF8) { Formatting = Formatting.Indented, Indentation = 1 }) {
    writer.WriteStartDocument();
    writer.WriteStartElement("row","urn:default");
    writer.WriteRaw("<row test=\"1\" />");
    writer.WriteEndElement();
    writer.WriteEndDocument();
}

Answer 3

This is because you are manipulating XML inconsistently: first, you are building an element properly with an API that handles namespaces, but then you build an element with InnerXML...

<row test="2"/> 

How could any application know that this element is in the default namespace or in empty namespace? Like in XPath, empty namespace is assumed.

Then, you have a parent in some default namespace and a child in empty namespace. So, the namespace fixup mechanism adds a default namespace reset xmlns="".

How to avoid this ussing InnerXML?

outputdoc.DocumentElement.InnerXml = "<row test=\"1\" xmlns=\"urn:default\" />"; 

Edit: About @somori comment, from this http://msdn.microsoft.com/en-us/library/system.xml.xmlelement.innerxml.aspx

Setting this property replaces the children of the node with the parsed contents of the given string. The parsing is done in the current namespace context.

This property is a Microsoft extension to the Document Object Model (DOM).

Extension of DOM means non standar...