NSURLSession GET 请求：DidCompleteWithError - 错误为空

目前我正在尝试从已弃用的 NSUrlConnection 切换到 NSUrlSession。我试图实现的是通过 GET 请求调用 IIS Web 服务，然后处理 XML 答案。我有两个问题：

1. 如果答案太长，DidReceiveData 中的 NSData 对象不会包含所有内容。答案在 1036 个字符后被裁剪。
2. 一切都完成后，虽然我已经处理了接收到的数据（无论它是否被裁剪），DidCompleteWithError 被调用。其中的“错误”参数为空，所以我不知道为什么要调用它以及如何处理它。因此，即使我收到了 DidReceiveData 中的所有数据并且一切正常，也会调用 DidCompleteWithError。

这是我的代码，感谢每一个帮助。提前致谢！

public class WebserviceCallerIOS : IWebServiceCaller
    {
        private NSMutableUrlRequest request;
        public NSUrlSessionDataTask dataTask;
        private Action<bool, string> completed;
        private string antwort;
        private NSUrlSession session;
        public NSOperationQueue myQueue;

        public void Invoke(string sUrl)
        {
            session = null;
            antwort = null;
            completed = null;
            myQueue = new NSOperationQueue();

            session = NSUrlSession.FromConfiguration(NSUrlSessionConfiguration.DefaultSessionConfiguration, (INSUrlSessionDelegate)new SessionDelegate((erfolg, body) => { completed(erfolg, body); }), myQueue);
            completed += WebserviceCallerIOS_Completed;

            try
            {
                request = CreateNativePostRequest(sUrl);
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                completed(false, "Error creating request: " + e);
            }

            dataTask = session.CreateDataTask(request);
            dataTask.Resume();
        }

        NSMutableUrlRequest CreateNativePostRequest(string url)
        {
            string converted = ((NSString)url).CreateStringByAddingPercentEscapes(NSStringEncoding.UTF8);
            var nsurl = NSUrl.FromString(converted);

            if (nsurl == null)
                throw new Exception("Invalid URL, could not create NSUrl from: '" + url + "'.");

            var req = new NSMutableUrlRequest(nsurl) {HttpMethod = "GET"};
            return req;
        }

private class SessionDelegate : NSUrlSessionDataDelegate, INSUrlSessionDelegate
        {
            private Action<bool, string> completed_callback;
            private NSData retData;
            private int status_code;

            public SessionDelegate(Action<bool, string> completed)
            {
                completed_callback = completed;
                retData = new NSData();
            }


            public override void DidReceiveResponse(NSUrlSession session, NSUrlSessionDataTask dataTask, NSUrlResponse response, Action<NSUrlSessionResponseDisposition> completionHandler)
            {
                var http_response = response as NSHttpUrlResponse;
                if (http_response == null)
                {
                    Console.WriteLine("Received non HTTP url response: '{0}'", response);
                    status_code = -1;
                    return;
                }


                status_code = (int)http_response.StatusCode;

                if (status_code == 200)
            {
                completionHandler(NSUrlSessionResponseDisposition.Allow);
            }
            else 
            {
                completionHandler(NSUrlSessionResponseDisposition.Cancel);
            }
        }


            public override void DidCompleteWithError(NSUrlSession session, NSUrlSessionTask task, NSError error)
            {
                completed_callback(false, error?.LocalizedDescription);
            }


            public override void DidReceiveData(NSUrlSession session, NSUrlSessionDataTask dataTask, NSData data)
            {
                retData = data;
                dataTask.Suspend();
                Console.WriteLine(retData);
                completed_callback(true, retData.ToString());
            }
    }
}