CodePRoject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]: [email protected]
/// [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
    }
    }
}