锁定Emulator(WindowsPhone8)屏幕时引发WebException

我有一个获取xml的web请求。这很好,但当我在应用程序请求服务器时按F12(锁定屏幕)。。。我遇到了WebException。
我使用taskCompletionSource对象。。。
这是我的代码

public async Task<String> Query(DataRequestParam dataRequestParam)
        {

            _dataRequestParam = dataRequestParam;

            try
            {
                Result = "";
                Result = await myDownloadString(dataRequestParam);

            }

            catch (WebException we)//ERROR IS CAUGHT HERE
            {
                throw new WebException(we.Message);

            }
            catch (Exception ex)
            {
                throw new MyException(ex.Message);

            }

            return Result;
        }

        public static Task<string> myDownloadString(DataRequestParam dataRequestParam)
        {
            var tcs = new TaskCompletionSource<string>();
            var web = new WebClient();

            if (!string.IsNullOrEmpty(dataRequestParam.AuthentificationLogin))
            {
                System.Net.NetworkCredential account = new NetworkCredential(dataRequestParam.AuthentificationLogin, dataRequestParam.AuthentificationPassword);
                web.Credentials = account;
            }

                web.DownloadStringCompleted += (s, e) =>
                {
                    if (e.Error != null) tcs.TrySetException(e.Error);
                    else if (e.Cancelled) tcs.TrySetCanceled();
                    else tcs.TrySetResult(e.Result);
                };

                web.DownloadStringAsync(dataRequestParam.TargetUri);
                return tcs.Task;

        }