Well, here is the final answer. I have used great Jimmy idea (which unfortunately is not complete itself) and complete recursion function to work properly.

Based on interface:

string RemoveAllNamespaces(string xmlDocument);

I represent here final clean and universal C# solution for removing XML namespaces:

//Implemented based on interface, not part of algorithm
public static string RemoveAllNamespaces(string xmlDocument)
{
    XElement xmlDocumentWithoutNs = RemoveAllNamespaces(XElement.Parse(xmlDocument));

    return xmlDocumentWithoutNs.ToString();
}

//Core recursion function
 private static XElement RemoveAllNamespaces(XElement xmlDocument)
    {
        if (!xmlDocument.HasElements)
        {
            XElement xElement = new XElement(xmlDocument.Name.LocalName);
            xElement.Value = xmlDocument.Value;

            foreach (XAttribute attribute in xmlDocument.Attributes())
                xElement.Add(attribute);

            return xElement;
        }
        return new XElement(xmlDocument.Name.LocalName, xmlDocument.Elements().Select(el => RemoveAllNamespaces(el)));
    }

It's working 100%, but I have not tested it much so it may not cover some special cases... But it is good base to start.

Many thanks to Ali Shah, this thread solved my problem perfectly! here's a C# conversion:

var dom = new XmlDocument();
        dom.Load("C:/ExampleFITrade.xml));
        var loaded = new XDocument();
        if (dom.DocumentElement != null)
            if( dom.DocumentElement.NamespaceURI != String.Empty)
            {
                dom.LoadXml(dom.OuterXml.Replace(dom.DocumentElement.NamespaceURI, ""));
                dom.DocumentElement.RemoveAllAttributes();
                loaded = XDocument.Parse(dom.OuterXml);
            }

@Markus Freitag To override the behaviour of XmlSerializer you need XmlWriterSettings for override or remove XML declaration and XmlSerializerNamespaces for override namespace:

public static string ToXML(this T obj)  
{  
 // Remove Declaration  
 var settings = new XmlWriterSettings  
        {  
             Indent = true,  
             OmitXmlDeclaration = true  
        };  
  
 // Remove Namespace  
    var ns = new XmlSerializerNamespaces(new[] { XmlQualifiedName.Empty });  
  
    using (var stream = new StringWriter())  
    using (var writer = XmlWriter.Create(stream, settings))  
    {  
        var serializer = new XmlSerializer(typeof(T));  
        serializer.Serialize(writer, obj, ns);  
        return stream.ToString();  
    }  
}  

See full example here.

here is how i had to fix the problem. xelement has local name under name so xelement.name.localname i can get the name without the name space prefix. the query looks like this

 header = From nm In xmlDoc.Elements Where nm.Name.LocalName = "Header"

i can now get around namespace issues. The trick was trying to find access to the localname, in the older method it was under the root, it is now under name.

It seems (although you didn't share it) that the movies XML file uses namespaces. So I assume that somewhere in that file (probably on the root element) you will have something like xmlns="mymovieurl". The important thing to realize is that each XML element and attribute is identified by a pair of strings. The local name (Movie, DVD, VHS, ...) and the namespace URI (empty, mymovieurl, ...). In your code above, since you didn't specify a default namespace all your elements are in the empty namespace (their namespace URI is an empty string). But your XML file to which you're adding these have its elements in some non-empty namespace (mymovieurl). In order to preserve the namespace for the element you're adding the code has to inject the xmlns="" attribute which marks that element and all its children to be in the empty namespace (just like you specified it in your code).

The solution depends on what you want to achieve. I assume that you want to add the elements into the namespace the rest of the file uses.

One simple way to do that is to add Imports

That means that all elements in your code without a prefix should belong to the namespace "mymovieurl". (Just change that to whatever namespace URI you movies file is using).

You need to register the prefix 'sd' using XmlNamespaceManager before you can use it in XPath query. Make sure you map the prefix to the default namespace http://g.h.i :

....
nsmgr = New XmlNamespaceManager(New XmlNameTable())
nsmgr.AddNamespace("sd", "http://g.h.i");

nodeList = xmlnode.DocumentElement.SelectNodes("/sd:ArrayOfAssessmentItemTimed/sd:AssessmentItemTimed", nsmgr)
....

XPath always treat element without prefix as an element with empty namespace, it doesn't have a notion of default namespace (only XML has), that's why you need to made up the prefix 'sd' here.