How do I use XPath with a default namespace with no prefix?

Question

What is the XPath (in C# API to XDocument.XPathSelectElements(xpath, nsman) if it matters) to query all MyNodes from this document?

Answer 1

I like @mads-hansen, his answer, so well that I wrote these general-purpose utility-class members:

    /// 

    /// Gets the  into a local-name(), XPath-predicate query.
    /// 
    /// Name of the child element.
    /// 
    public static string GetLocalNameXPathQuery(string childElementName)
    {
        return GetLocalNameXPathQuery(namespacePrefixOrUri: null, childElementName: childElementName, childAttributeName: null);
    }

    /// 

    /// Gets the  into a local-name(), XPath-predicate query.
    /// 
    /// The namespace prefix or URI.
    /// Name of the child element.
    /// 
    public static string GetLocalNameXPathQuery(string namespacePrefixOrUri, string childElementName)
    {
        return GetLocalNameXPathQuery(namespacePrefixOrUri, childElementName, childAttributeName: null);
    }

    /// 

    /// Gets the  into a local-name(), XPath-predicate query.
    /// 
    /// The namespace prefix or URI.
    /// Name of the child element.
    /// Name of the child attribute.
    /// 
    /// 
    /// This routine is useful when namespace-resolving is not desirable or available.
    /// 
    public static string GetLocalNameXPathQuery(string namespacePrefixOrUri, string childElementName, string childAttributeName)
    {
        if (string.IsNullOrEmpty(childElementName)) return null;

        if (string.IsNullOrEmpty(childAttributeName))
        {
            return string.IsNullOrEmpty(namespacePrefixOrUri) ?
                string.Format("./*[local-name()='{0}']", childElementName)
                :
                string.Format("./*[namespace-uri()='{0}' and local-name()='{1}']", namespacePrefixOrUri, childElementName);
        }
        else
        {
            return string.IsNullOrEmpty(namespacePrefixOrUri) ?
                string.Format("./*[local-name()='{0}']/@{1}", childElementName, childAttributeName)
                :
                string.Format("./*[namespace-uri()='{0}' and local-name()='{1}']/@{2}", namespacePrefixOrUri, childElementName, childAttributeName);
        }
    }