How to remove xmlns attribute of a node other than root in an XDocument?

I think the code below is what you want. You need to put each element into the right namespace, and remove any xmlns='' attributes for the affected elements. The latter part is required as otherwise LINQ to XML basically tries to leave you with an element of

<!-- This would be invalid -->
<Firstelement xmlns="" xmlns="http://my.namespace">

Here's the code:

using System;
using System.Xml.Linq;

class Test
{
    static void Main()
    {
        XDocument doc = XDocument.Load("test.xml");
        // All elements with an empty namespace...
        foreach (var node in doc.Root.Descendants()
                                .Where(n => n.Name.NamespaceName == ""))
        {
             // Remove the xmlns='' attribute. Note the use of
             // Attributes rather than Attribute, in case the
             // attribute doesn't exist (which it might not if we'd
             // created the document "manually" instead of loading
             // it from a file.)
             node.Attributes("xmlns").Remove();
             // Inherit the parent namespace instead
             node.Name = node.Parent.Name.Namespace + node.Name.LocalName;
        }
        Console.WriteLine(doc); // Or doc.Save(...)
    }
}

There is no need to 'remove' the empty xmlns attribute. The whole reason that the empty xmlns attrib is added is because the namespace of your childnodes is empty (= '') and therefore differ from your root node. Adding the same namespace to your childs as well will solve this 'side-effect'.

XNamespace xmlns = XNamespace.Get("http://my.namespace");

// wrong
var doc = new XElement(xmlns + "Root", new XElement("Firstelement"));

// gives:
<Root xmlns="http://my.namespace">
    <Firstelement xmlns="" />
</Root>

// right
var doc = new XElement(xmlns + "Root", new XElement(xmlns + "Firstelement"));

// gives:
<Root xmlns="http://my.namespace">
    <Firstelement />
</Root>