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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]: marcelvaneijkel@gmail.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
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