LINQ2XML Moving all namespace declarations to root element

Question

This sounds like a query that many would have but after hours of searching I can't even find a question addressing this problem.

I'm using LINQ2Xml classes to generate an XML document. I'm using a number of namespaces and the nodes are created dynamically so can't be declared using the nice new XElement("abc",new XElement...)) format.

Here's an example:

XNamespace ns1 = "scheme://ns1";
XNamespace ns2 = "scheme://ns2";
XNamespace ns3 = "scheme://ns3";
XNamespace ns4 = "scheme://ns4";

XElement el1 = new XElement(ns1 + "el1");
XElement el2 = new XElement(ns2 + "el2");
XElement el3 = new XElement(ns3 + "el3");
XElement el4 = new XElement(ns4 + "el4");

XElement el5 = new XElement(ns1 + "el5");
XElement el6 = new XElement(ns1 + "el5");
XElement el7 = new XElement(ns1 + "el5");

el4.Add(el7);
el3.Add(el6);
el2.Add(el5);

el1.Add(el2);
el1.Add(el3);
el1.Add(el4);

XDocument doc = new XDocument(el1);
Debug.Write(doc.ToString());

The above code produces the following output:

<el1 xmlns="scheme://ns1">
  <el2 xmlns="scheme://ns2">
    <el5 xmlns="scheme://ns1" />
  </el2>
  <el3 xmlns="scheme://ns3">
    <el5 xmlns="scheme://ns1" />
  </el3>
  <el4 xmlns="scheme://ns4">
    <el5 xmlns="scheme://ns1" />
  </el4>
</el1>

Whereas, I would like to have output more akin to the following (E&OE) where all namesapce declarations sit at the top and not on every element.

<el1 xmlns="scheme://ns1" xmlns:ns2="scheme://ns2" xmlns:ns3="scheme://ns3" xmlns:ns4="scheme://ns4">
    <ns2:el2 >
        <ns1:el5 />
    </ns2:el2>
    ....
</el1>

I have already tried the following which makes absolutely no difference:

IEnumerable<XAttribute> allNs = doc.Root.Descendants().Attributes().Where(a => a.IsNamespaceDeclaration);
int i = 0;
foreach (string uri in allNs.Select(a => a.Value).Distinct()) {
    string prefix = "ns" + ++i;
    doc.Root.Add(new XAttribute(XNamespace.Xmlns + prefix, uri));
}
Debug.Write(doc.ToString(SaveOptions.OmitDuplicateNamespaces));

Reasoning: I expect a file to have maybe up to a few dozen namespaces and several hundred elements from each. The uri for each namespace will be maybe 50 characters long as such, I will be saving thousands of characters by only declaring the namespaces once. Additionally, it just looks nicer!

Anyone know how this could be done?

Answer 1

Add other namespaces as attributes to the root element:

XElement el1 = new XElement(ns1 + "el1", 
    new XAttribute(XNamespace.Xmlns + "ns2", ns2),
    new XAttribute(XNamespace.Xmlns + "ns3", ns3),
    new XAttribute(XNamespace.Xmlns + "ns4", ns4));

Output:

<el1 xmlns:ns2="scheme://ns2" xmlns:ns3="scheme://ns3" xmlns:ns4="scheme://ns4" xmlns="scheme://ns1">
  <ns2:el2>
    <el5 />
  </ns2:el2>
  <ns3:el3>
    <el5 />
  </ns3:el3>
  <ns4:el4>
    <el5 />
  </ns4:el4>
</el1>