if (Application["Users"] == null)
            {
                ht.Add(Convert.ToInt32(sdr["UserId"]), users);
                Application.Lock();
                Application["Users"] = ht; 
                Application.UnLock();
            }
            else
            {
                ht = Application["Users"] as Hashtable;
                ArrayList a = new ArrayList();

                if (ht[Convert.ToInt32(sdr["UserId"])] == null)
                {
                    ht.Add(Convert.ToInt32(sdr["UserId"]), users);
                    Application.Lock();
                    Application["Users"] = ht;
                    Application.UnLock();
                }
            }

------解决方案--------------------
楼上的看着有些晕，
public class OnlineManage
{
   private static OnlineManage _instance = new OnlineManage();
   //private Dictionary<int, OnlineUserInfo> _onlineUserList = new Dictionary<int, OnlineUserInfo>();
   private Dictionary<string, OnlineUserInfo> _onlineSession = new Dictionary<string, OnlineUserInfo>();

   public const int Timeout_Minutes = 20;  //非登录会员在线用户过期时间
   public const int Timeout_Minutes_Member = 40; //登录会员在线过期时间

   private OnlineManage()
   {
   }

   public void Clear()
   {
       lock(this)
       {
           //_onlineUserList.Clear();
           _onlineSession.Clear();
       }
   }

   /// <summary>
   /// 获取当前在线用户总数(包含登录和未登录的)
   /// </summary>
   public int Count
   {
       get { return _onlineSession.Count; }
   }

   /// <summary>
   /// 在线会员数
   /// </summary>
   public int MemberCount
   {
       get
       {
           int iRet = 0;
           lock(this)
           {
               foreach (string _sKey in _onlineSession.Keys)
               {
                   OnlineUserInfo oInfo = _onlineSession[_sKey];
                   if (oInfo == null)
                   {
                       _onlineSession.Remove(_sKey);
                   }
                   else
                   {
                       if (oInfo.UserID>0)
                       {
                           iRet++;
                       }
                   }
               }
           }
           return iRet;
       }
   }

   public OnlineUserInfo this[int UserID]
   {
       get
       {
           lock(this)
           {
               foreach (string _sKey in _onlineSession.Keys)
               {
                   OnlineUserInfo oInfo = _onlineSession[_sKey];
                   if (oInfo == null)
                   {
                       _onlineSession.Remove(_sKey);
                   }
                   else
                   {
                       if (oInfo.UserID == UserID)
                       {
                           return oInfo;
                       }
                   }
               }
               return null;
           }
       }
   }

   public OnlineUserInfo this[string sSessionID]
   {
       get
       {
           if (_onlineSession.ContainsKey(sSessionID))
               return _onlineSession[sSessionID];
           else