初学者诚心请教XML的问题.
以下这个XML文件里,   第二行 <bk:Books   xmlns:bk=...>   究竟有和意义?

<?xml   version= '1.0 '   encoding= 'utf-8 '?>
<bk:Books   xmlns:bk= 'http://myserver/myschemas/Books '>
    <bk:Book>
        <bk:Title> Just   XML </bk:Title>
    </bk:Book>
    <bk:Book>
        <bk:Title> Professional   XML </bk:Title>
    </bk:Book>
    <bk:Book>
        <bk:Title> XML   Step   by   Step </bk:Title>
    </bk:Book>
    <bk:Book>
        <bk:Title> XML   By   Example </bk:Title>
    </bk:Book>
</bk:Books>

下面这个code中,   其它的都好理解,   就是那个XmlNamespaceManager不知道是怎么个机制,   为什么非要它不可?

        protected   void   Button1_Click(object   sender,   EventArgs   e)
        {
                XPathDocument   doc   =   new   XPathDocument(Server.MapPath( "Demo.xml "));
                XPathNavigator   nav   =   doc.CreateNavigator();

                XPathExpression   xpathExpress   =   nav.Compile( "//bk:Book[position() "+this.ddownlist.SelectedValue.Trim()+this.txtNum.Text.Trim()+ "] ");
                //use   AddNamespace
                XmlNamespaceManager   xmlManager   =   new   XmlNamespaceManager(nav.NameTable);
                xmlManager.AddNamespace( "bk ",   "http://myserver/myschemas/Books ");
                xpathExpress.SetContext(xmlManager);

                XPathNodeIterator   xIterator   =   nav.Select(xpathExpress);
                this.TextBox1.Text   =   " ";
                this.TextBox1.ForeColor   =   System.Drawing.Color.Empty;
                while   (xIterator.MoveNext())
                        this.TextBox1.Text   =   this.TextBox1.Text   +   "\r\n "   +   xIterator.Current.Value;

                if   (xIterator.Count.Equals(0))
                {
                        this.TextBox1.Text   =   "There   is   no   xml   data   in   the   condition. ";
                        this.TextBox1.ForeColor   =   System.Drawing.Color.Red;
                }
                xmlManager   =   null;