如何用Dictionary类记录在线用户

[gdmm919705]

我想创建一个Dictionary<string, Users> dictionary 添加到Application["UserList"] 记录在线用户

Users user = Session["CurrentUser"] as Users;
        if (Application["UserList"] != null)
        {

            Dictionary<string, Users> dictionary = Application["UserList"] as Dictionary<string, Users>;
            if (dictionary[user.Name] == null)
            {
                dictionary.Add(user.Name, user);
            }
            lblPeopleCount.Text = dictionary.Count.ToString();
        }
        else
        {
            Dictionary<string, Users> dictionary = new Dictionary<string, Users>();
            dictionary.Add(user.Name, user);
            Application["UserList"] = dictionary;
            lblPeopleCount.Text = dictionary.Count.ToString();
        }

第一个用户登录时正常，第二个用户登陆时在dictionary[user.Name] == null判断时抛异常：给定关键字不在字典中
应该如何判断：如果字典里不存在该用户则添加到字典里？

[houlinghouling]

if (!Dictionary.ContainsKey(user.Name))