sessionCodeProject的这篇文章确实对我有所启迪，
  http://www.codeproject.com/useritems/SessionWrapper.asp#xx1208856xx。
  
  诚如作者所说，我们经常在ASP.NET用许多类似于下面的代码来检测Session中存储的对象，来防止Session过期后存储的变量丢失问题：
  Int32 nUserID = -1;
  if ( null != Session["userID"] ) {
   if ( Session["userID"] is Int32 ) {
   if ( 0 < Session["userID"] ) {
   nUserID = (Int32) Session["userID"]
   }
   }
  }
  if ( -1 == nUserID )
  {
   throw new ApplicationException ( "Unexpected situation: userID invalid." );
  }
  
  this.doSomething( nUserID );
  这样的代码会遍布各处。
  
  那么，利用他的这个封装方案来做重构，确实让代码简洁干净了不少！
  经过他的封装，上面的代码用这么一句话就行了：
  
  this.doSomething( CCurrentSession.UserID )
  
  他的类其实也很简单，如下所示：
  
  
  using System;
  using System.Web;
  
  /**////--------------------------------------------------------------------
  /// Developed by M. van Eijkel - aug 2005
  /// [e]: marcelvaneijkelgmail.com
  /// [w]: www.vaneijkel.com
  
  namespace VanEijkel.Web
  {
   /**//// <summary>
   /// Wrapper class for the session object.
   /// It centralizes the logic for retrieving and validation of session information.
   /// By using an approach like this you improve the protection and encapsulation of existing code.
   /// It offers a simple, low-risk, easy manageable way to improve existing WebApplication.
   /// Therfore, I call it webRefactoring.
   /// </summary>
   public class CurrentSession
   {
   Constants#region Constants
   private const String sMANDATORY_SESSION_KEY_NOT_FOUND_MSG = "Session variable excepted but does not exist. Key={0}";
   private const String sMANDATORY_SESSION_VALUE_INVALID_NULL = "None null session value excepted. Key={0}";
  
   private const Int32 nUSERID_UNKOWN = -1;
   private const Int32 nUSERID_MINIMUM = 1;
   private const String sUSERID_INVALID = "Invalid UserID:{0}. UserID should be larger than:{1}";
   #endregion
  
   UserID#region UserID
   /**//// <summary>
   /// Returns the userID as a Int32 instead of an object.
   /// This way you will get the compiler protection and intelligence support you need.
   /// </summary>
   public static Int32 UserID
   {
   get
   {
   return (Int32) GetValueOrDefault( eKeys.UserID, nUSERID_UNKOWN );
   }
   set
   {
   if ( nUSERID_MINIMUM >= value )
   {
   throw new ApplicationException ( String.Format(sUSERID_INVALID, value, nUSERID_MINIMUM ));
   }
   SetValue( eKeys.UserID, value );
   }
   }
   #endregion
  
   private: GetValueOrDefault( eKeys eKey, Object oDefaultValue )#region private: GetValueOrDefault( eKeys eKey, Object oDefaultValue )
   /**//// <summary>
   /// Gets the value from the session object.
   /// </summary>
   /// <param name="eKey"> The session key to get the value for.</param>
   /// <param name="oDefaultValue">The default value to use if no valid value stored.</param>
   /// <returns>When the value is null or the key does not exist,
   /// the specified default value is returned.
   /// Otherwise, the value is returned</returns>
   private static object GetValueOrDefault( eKeys eKey, Object oDefaultValue )
   {
   //get the value
   object oValue = GetValue( eKey );
  
   //value not found or null?
   if (null == oValue)
   {
   //return default value
   return oDefaultValue;
   }
  
   //everything oke: return session value
   return oValue;
   }
   #endregion
   private: GetMandatoryValue( eKeys eKey )#region private: GetMandatoryValue( eKeys eKey )
   /**//// <summary>
   /// Returns the session value for a session-key that must exist.
   /// If the key does not exist an applicationException is thrown.
   /// </summary>
   /// <param name="eKey"> The session-key to return the session-value for. </param>
   /// <returns> A none-null value.</returns>
   private static object GetMandatoryValue( eKeys eKey )
   {
   //get the value
   object oValue = GetValue( eKey );
  
   //key not found or value null?
   if ( null == oValue )
   {
   //throw applicationException because its application logic error (none CLR)
   throw new ApplicationException ( String.Format( sMANDATORY_SESSION_KEY_NOT_FOUND_MSG, eKey.ToString() ));
   }
  
   //everything oke: return value
   return oValue;
   }
   #endregion
   private: GetValue( eKeys eKey )#region private: GetValue( eKeys eKey )
   /**//// <summary>
   /// Gets the session value from the specified key.
   /// </summary>
   /// <param name="eKey">The key to get the value from</param>
   /// <returns>The session value for the specified session key.
   /// If the key does not exist, null is returned.
   /// </returns>
   private static object GetValue( eKeys eKey )
   {
   return HttpContext.Current.Items[ eKey.ToString() ];
   }
   #endregion
  
   private SetMandatoryValue( eKeys eKey, Object oValue )#region private SetMandatoryValue( eKeys eKey, Object oValue )
   private static void SetMandatoryValue( eKeys eKey, Object oValue )
   {
   if ( null == oValue )
   {
   throw new ApplicationException( String.Format(sMANDATORY_SESSION_VALUE_INVALID_NULL, eKey.ToString() ) );
   }
   }
   #endregion
   private SetValue( eKeys eKey, Object oValue)#region private SetValue( eKeys eKey, Object oValue)
   /**//// <summary>
   /// Stores the specified session-value to the specified session-key.
   /// </summary>
   /// <param name="eKey">The key for the value to store in the session.</param>
   /// <param name="oValue">The value to store in the session</param>
   private static void SetValue ( eKeys eKey, Object oValue)
   {
   HttpContext.Current.Items[eKey.ToString()] = oValue;
   }
   #endregion
  
   /**//// <summary>
   /// An enum for the
   /// </summary>
   private enum eKeys
   {
   UserID
   }
   }
  }