if (BLL.Users.CheckUser("张三", "123").Read())
            {
                Response.Write("登录成功！");
            }
            else
            {
                Response.Write("登录失败！");
            }

 /// <summary>
        /// 是否存在该记录
        /// </summary>
        public static SqlDataReader CheckUser(string uName, string uPwd)
        {
            return SQLServerDAL.Users.CheckUser(uName, uPwd);
        }

 public static SqlDataReader ExecuteReader(CommandType cmdType, string cmdText, params SqlParameter[] commandParameters)//设置断点看过N次了，commandParameters有值
        {
            SqlCommand cmd = new SqlCommand();
            SqlConnection conn = new SqlConnection(ConnectionStringLocalTransaction);

            // we use a try/catch here because if the method throws an exception we want to 
            // close the connection throw code, because no datareader will exist, hence the 
            // commandBehaviour.CloseConnection will not work
            try
            {
                PrepareCommand(cmd, conn, null, cmdType, cmdText, commandParameters);
                SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection);//断点到这里一看rdr=null;!!直接跳到Catch了！！！
                cmd.Parameters.Clear();
                return rdr;
            }
            catch
            {
                conn.Close();
                throw;
            }
        }