XLinq extension methods to select elements using the default namespace of the container.

XmlDefaultNamespaceExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace System.Xml.Linq
{
    public static class XmlDefaultNamespaceExtensions
    {
        public static IEnumerable<XElement> DescendantsNs(this XElement container, string name) 
		    => container.Descendants(container.Name.Namespace + name);
			
        public static IEnumerable<XElement> ElementsNs(this XElement container, string name) 
		    => container.Elements(container.Name.Namespace + name);
			
        public static XElement ElementNs(this XElement container, string name) 
		    => container.Element(container.Name.Namespace + name);
        
        public static IEnumerable<XElement> DescendantsNs(this IEnumerable<XElement> source, string name) 
		    => source.SelectMany(el => DescendantsNs(el, name));
			
        public static IEnumerable<XElement> ElementsNs(this IEnumerable<XElement> source, string name) 
		    => source.SelectMany(el => ElementsNs(el, name));
    }
}