连接不上 exchange server

Question

最近要做个程序 连接exchange服务器 并新建一些日历事件
程序如下:
        static void Main(string[] args)
        {
            ExchangeServiceBinding binding = new ExchangeServiceBinding();
            binding.Url = "https://ent.enteduc.fr/ews/exchange.asmx";
            binding.Credentials = new NetworkCredential("username", "password");            //创建一个日历事件
            CalendarItemType newCalendar = new CalendarItemType();
            DateTime startTime = DateTime.Now;
            newCalendar.Start = startTime;
            newCalendar.End = startTime.AddHours(1);
            newCalendar.StartSpecified = newCalendar.EndSpecified = true;
            TimeSpan timeZone = (new TimeSpan(0)).Subtract(new TimeSpan(8, 0, 0));
            newCalendar.MeetingTimeZone = new TimeZoneType();
            newCalendar.MeetingTimeZone.BaseOffset = TimeSpanToXSDuration(timeZone);
            newCalendar.Subject = "a subjet";            CreateItemType createItemRequest = new CreateItemType();
            createItemRequest.SendMeetingInvitations = CalendarItemCreateOrDeleteOperationType.SendToNone;
            createItemRequest.SendMeetingInvitationsSpecified = true;
            createItemRequest.Items = new NonEmptyArrayOfAllItemsType();
            createItemRequest.Items.Items = new CalendarItemType[1];
            createItemRequest.Items.Items[0] = newCalendar;
            //将该日历存入服务器
            CreateItemResponseType response = binding.CreateItem(createItemRequest);
            
            ItemInfoResponseMessageType responseMessage = response.ResponseMessages.Items[0] as ItemInfoResponseMessageType;            if (responseMessage.ResponseCode != ResponseCodeType.NoError)
                Console.WriteLine("createItem failed with response code" + responseMessage.ResponseCode.ToString());
            else 
                Console.WriteLine("createItem succese");        }
但是运行程序一直不成功
显示:
System.Net.WebException: The request failed with a reponse null有谁来帮帮我吗 谢谢