判断用户是否存在

 /// <summary>
        /// 判断是否存在登录名为loginName的用户
        /// </summary>
        /// <param name="loginName">用户登录名</param>
        /// <returns>如果存在，返回true；否则，返回false</returns>
        //
        public static bool HasUser(string loginName)
        {
            Database db = new Database();

            string sql = "";
            sql = "Select * from [User] where [LoginName] = "
                + SqlStringFormat.GetQuotedString(loginName);

            DataRow row = db.GetDataRow(sql);
            if (row != null)
                return true;
            else
                return false;
        }