在HttpModule中使用 System.Thread.Timer
 可以在一定程度上实现 定时任务/后台任务
 
 只是在 Timer的TimerCallback的代理方法，将在一个新线程中运行
 
若在方法中/或者该方法调用的一些方法中， 调用了 System.Web.HttpContext.Current 都将返回null
 ***
 如现在给上海贝尔作的一个eKap项目中，是用IBatisNet组件， 该组件中
  namespace IBatisNet.DataMapper.SessionContainer
{
 /// <summary>
 /// Build a session container for a Windows or Web context.
 /// </summary>
 public class SessionContainerFactory
 {
  #region Methods
  /// <summary>
  /// Get a session container for a Windows or Web context.
  /// </summary>
  /// <returns></returns>
  static public ISessionContainer GetSessionContainer()
  {
   if (System.Web.HttpContext.Current == null)
   {
    return new WindowSessionContainer();
   }
   else
   {
    return new WebSessionContainer();
   }
  }
  #endregion
 }
}
 其中 WebSessionContainer 类:
/// <summary>
 /// Session container for web application.
 /// </summary>
 public class WebSessionContainer : ISessionContainer
 {

  #region Constants

  /// <summary>
  /// Token for SqlMapConfig xml root.
  /// </summary>
  private const string LOCAL_SESSION = "_IBATIS_LOCAL_SQLMAPSESSION_";

  #endregion

  #region Constructor (s) / Destructor

  /// <summary>
  /// Constructor
  /// </summary>
  public WebSessionContainer()
  {
   System.Web.HttpContext.Current.Items[LOCAL_SESSION] = null;
  }
  #endregion

  #region ISessionContainer Members

  #region Properties
  /// <summary>
  /// Get the local session
  /// </summary>
  public SqlMapSession LocalSession
  {
   get
   {
    // add by sharper
     if(System.Web.HttpContext.Current != null)
    {
     return System.Web.HttpContext.Current.Items[LOCAL_SESSION] as SqlMapSession;

    }else
    {
     return null;
    }

   }
  }
  #endregion

  #region Methods
  /// <summary>
  /// Store the local session on the container.
  /// Ensure that the session is unique for each thread.
  /// </summary>
  /// <param name="session">The session to store</param>
  public void Store(SqlMapSession session)
  {
   System.Web.HttpContext.Current.Items[LOCAL_SESSION] = session;
  }

  /// <summary>
  /// Remove the local session from the container.
  /// </summary>
  public void Dispose()
  {
   System.Web.HttpContext.Current.Items[LOCAL_SESSION] = null;
  }

  #endregion

  #endregion
 }
 其中原先 LocalSession 属性没有判断
 System.Web.HttpContext.Current 是否为null
 所以会引发一个 NullReferenceException