XML解析，奇怪的行为.(前3个日期有效，其余为空白)

所以这是我的代码：

static void Main(string[] args)
    {
        string Xml;
        Xml = System.IO.File.ReadAllText (@"C:'Users'user'Desktop'Practice Projects'C#'XML.txt");
        //Console.WriteLine(Xml);
        string created = Xml.Substring(Xml.IndexOf("<created>")+30,15);
        string modified = Xml.Substring(Xml.IndexOf("<modified>") + 31, 15);
        string revised = Xml.Substring(Xml.IndexOf("<revised>") + 30, 15);
        string lastAccessed = Xml.Substring(Xml.IndexOf("<lastaccessed>") + 35, 15);
        Console.WriteLine (lastAccessed);
        string updatedBy = Xml.Substring(Xml.IndexOf("<updatedby>") + 17, 9);
        Console.WriteLine (updatedBy);
        string mailBody = Xml.Substring(Xml.IndexOf(""));
        //string status;
        Mail mail = new Mail();
        mail.Created = DateTime.ParseExact (created, "yyyyMMddTHHmmss", CultureInfo.InvariantCulture);
        Console.WriteLine ("The e-Mail was created on: " + mail.Created);
        mail.Modified   = DateTime.ParseExact (modified,"yyyyMMddTHHmmss", CultureInfo.InvariantCulture);
        Console.WriteLine ("The e-MAil was last modified on: " + mail.Modified);
        mail.Revised = DateTime.ParseExact (revised, "yyyyMMddTHHmmss", CultureInfo.InvariantCulture);
        Console.WriteLine ("The e-Mail was revised on: " + mail.Revised);
        mail.LastAccessed = DateTime.ParseExact (lastAccessed, "yyyyMMddTHHmmss", CultureInfo.InvariantCulture);
        Console.WriteLine ("The e-Mail was last accessed on:", mail.LastAccessed);
        mail.UpdatedBy = updatedBy;
        Console.WriteLine ("The e-Mail was last updated by: ", mail.UpdatedBy);
        mail.MailBody = mailBody;
        Console.WriteLine ("The e-Mail body is: ", mail.MailBody);
Console.ReadLine();
    }
    public static string PraseDateTime (string Text, string StartTag, string EndTag)
    {
        int StartPosition = Text.IndexOf(StartTag) + StartTag.Length;
        int Length = Text.IndexOf(EndTag) - StartPosition;
        return Text.Substring(StartPosition, Length - 6);
    }
public class Mail
{
    public DateTime Created { get; set; }
    public DateTime Modified { get; set; }
    public DateTime Revised { get; set; }
    public DateTime LastAccessed { get; set; }
    public string UpdatedBy { get; set; }
    public Requester Caller { get; set; }
    public String MailBody { get; set; }
    public Mail ()
    {
        Caller = new Requester();
    }
}

所以问题是，我的代码适用于前3个，但当涉及到最后2个时，它什么都没有显示：lastAccessed和updatedBy，即使当我检查这2个变量时，文本仍然是空白的，它们显然包含了一些东西。（第一个包含日期，比如：20120522T230123，第二个（名称），比如：WGAMaster）我可以补充一下，lastAccessed和modified是一样的，但lastAccessed不起作用。

如果有任何帮助，或者有更好的方法从XML中解析文本，我将不胜感激。非常感谢。