public override string GetUserNameByEmail(string email)
{
if (email == null)
{
throw new ArgumentNullException("email");
}
var doc = new XmlDocument();
doc.Load(XmlFullyQualifiedPath);
return doc.GetElementsByTagName("User").Cast<XmlNode>().Where(
node => node.ChildNodes[2].InnerText.Equals(email.Trim(), StringComparison.OrdinalIgnoreCase)).Select(
node => node.ChildNodes[0].InnerText).FirstOrDefault();
}