using System;

using System.Xml;

using System.Xml.XPath;

using System.Xml.Linq;

using System.Linq.Expressions;

using System.Linq;

using System.Collections.Generic;

using System.Collections;

using System.Text;

using System.IO;

​

public class Program

{

    public static void Main()

    {

        String xml = @"<?xml version='1.0' encoding='Shift_JIS'?>

<CM_meta_data xsi:schemaLocation='http://www.nab.or.jp/TVCMMeta/schema/v100 TVCMMeta_v100.xsd' xmlns='http://www.nab.or.jp/TVCMMeta/schema/v100' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>

  <cm_code_advertiser_id>0187</cm_code_advertiser_id>

</CM_meta_data>";

        var reader =    XmlReader.Create(new StringReader(xml));

        XDocument d = XDocument.Load(reader);

        var xNamespace = d.Root.GetDefaultNamespace();

        Console.WriteLine(xNamespace.NamespaceName);

        var nameTable = reader.NameTable;

        var namespaceManager = new XmlNamespaceManager(nameTable);

        namespaceManager.AddNamespace("x", xNamespace.NamespaceName);

        var result = (IEnumerable) d.XPathEvaluate("/x:CM_meta_data/x:cm_code_advertiser_id", namespaceManager); 

        var test = result.Cast<XElement>().FirstOrDefault();

        Console.WriteLine(test.Value);

    }

}