我有一个桌面应用程序，它使用CustomWebClient对象并调用OpenReadAsync（）同时下载一个或多个小文件（jpg，大小小于400KB，一次不超过20个）。如果流程中没有问题，下载过程就可以了。我想将响应限制在一定时间（15秒），所以我引入了一个timeOut处理，它正在中止请求。甚至超时都在工作，之后我的“OpenReadCompletedEventHandler”方法正在接收System.Net.WebException：请求被中止：请求被取消（这是正确的行为）。 现在，我的问题是我想让用户尝试重新加载图片。因此，下一个webClient请求失败并出现相同的WebException。以下是我的代码。
这是我的自定义WebClient类（用于一次拥有2个以上的异步连接）：
internal class ExtendedWebClient : WebClient
{
private Timer _timer;
public int ConnectionLimit { get; set; }
public int ConnectionTimeout { get; set; }
public ExtendedWebClient()
{
this.ConnectionLimit = 2;
}
protected override WebRequest GetWebRequest(Uri address)
{
var request = base.GetWebRequest(address) as HttpWebRequest;
if (request != null){_timer = new Timer(TimeoutRequest, request, ConnectionTimeout, Timeout.Infinite);
request.ServicePoint.ConnectionLimit = this.ConnectionLimit;
request.ServicePoint.MaxIdleTime = 5000;
request.ServicePoint.ConnectionLeaseTimeout = 5000;
}
return request;
}
private void TimeoutRequest(object state)
{
_timer.Dispose();
_timer = null;
((WebRequest)state).Abort();
}
protected override void Dispose(bool disposing)
{
if (_timer != null)
{
_timer.Dispose();
_timer = null;
}
base.Dispose(disposing);
}
}
以下是使用我的自定义WebClient类下载文件的代码：
internal struct PageWaitHandleState
{
public int WaitHandleIndexInPage;
public bool ImageIsLoaded;
public string ErrMessage;
}
public Image[] downloadedImages;
private PageWaitHandleState[] waitHandlesInPage;
private OpenReadCompletedEventHandler[] downloadComplete;
private EventWaitHandle[] pagesEWH = null;
private EventWaitHandle[] downloadImageEvent;
private int availableImages = 1; // Set here to simplify, but as I stated in my description, it may be more than 1.
int downloadTimeOut = 15000;
int maxSimultaneousDownloads = 20;
private void DownloadImages(int pageIndex = 0, string[] imageUrl)
{
if (pagesEWH[pageIndex] != null)
{
ReloadImages(pageIndex, imageUrl); // Executed in the second request
return;
else
{
pagesEWH[pageIndex] = new EventWaitHandle[availableImages];
downloadedImages = new Image[availableImages];
downloadComplete = new OpenReadCompletedEventHandler[availableImages];
downloadImageEvent = new EventWaitHandle[availableImages];
waitHandlesInPage = new PageWaitHandleState[availableImages];
// Set the downloadComplete deletages
for (int i = 0; i < availableImages; i++)
{
downloadComplete[i] = ProcessImage;
}
}
for (int imgCounter = 0; i < availableImages; i++)
{
waitHandlesInPage[imgCounter] = new PageWaitHandleState() { ImageIsLoaded = false, WaitHandleIndexInPage = imgCounter, ErrMessage = null };
downloadImageEvent[imgCounter] = GrabImageAsync(imageUrl[imgCounter], downloadComplete[imgCounter], imgCounter, downloadTimeOut, maxSimultaneousDownloads);
pagesEWH[imgCounter] = downloadImageEvent[imgCounter];
}
offenderIndex++;
}
}
private static EventWaitHandle GrabImageAsync(string url, OpenReadCompletedEventHandler openReadCompletedEventHandler, int imgCounter, int downloadTimeOut, int maxSimultaneousDownloads)
{
var myClient = new ExtendedWebClient();
myClient.ConnectionLimit = maxSimultaneousDownloads;
myClient.ConnectionTimeout = downloadTimeOut;
myClient.OpenReadCompleted += openReadCompletedEventHandler;
var iewh = new ImageEventWaitHandle() { ewh = new EventWaitHandle(false, EventResetMode.ManualReset), ImageIndex = imgCounter };
myClient.OpenReadAsync(new Uri(url), iewh);
return iewh.ewh;
}
internal void ProcessImage(object sender, OpenReadCompletedEventArgs e)
{
ImageEventWaitHandle iewh = (ImageEventWaitHandle)e.UserState;
bool disposeObject = false;
try
{
if (e.Cancelled)
{
this.waitHandlesInPage[iewh.ImageIndex].ImageIsLoaded = false;
this.waitHandlesInPage[iewh.ImageIndex].ErrMessage = "WebClient request was cancelled";
}
else if (e.Error != null)
{
this.waitHandlesInPage[iewh.ImageIndex].ImageIsLoaded = false;
this.waitHandlesInPage[iewh.ImageIndex].ErrMessage = e.Error.Message;
iewh.ewh.Set();
this.downloadImageEvent[iewh.ImageIndex].Close();
}
else
{
using (Stream inputStream = e.Result)
using (MemoryStream ms = new MemoryStream())
{
byte[] buffer = new byte[4096];
int bytesRead;
int totalReadBytes = 0;
do
{
bytesRead = inputStream.Read(buffer, 0, buffer.Length); // Exception fired here with the second request
ms.Write(buffer, 0, bytesRead);
totalReadBytes += bytesRead;
} while (inputStream.CanRead && bytesRead > 0);
this.downloadedImages[iewh.ImageIndex] = Image.FromStream(ms);
this.waitHandlesInPage[iewh.ImageIndex].ImageIsLoaded = true;
this.waitHandlesInPage[iewh.ImageIndex].ErrMessage = null;
}
disposeObject = true;
}
}
catch (Exception exc)
{
this.downloadedImages[iewh.ImageIndex] = null;
}
finally
{
// Signal the wait handle
if (disposeObject)
{
iewh.ewh.Set();
((WebClient)sender).Dispose();
}
}
}
private void ReloadImages(int pageIndex, string[] imageUrl)
{
for (int imgCounter = 0; imgCounter < availableImages; imgCounter++)
{
this.downloadComplete[imgCounter] = this.ProcessImage;
this.waitHandlesInPage[imgCounter] = new PageWaitHandleState() { ImageIsLoaded = false, WaitHandleIndexInPage = imgCounter, ErrMessage = null };
this.downloadImageEvent[imgCounter] = GrabImageAsync(ImageUrl[imgCounter],this.downloadComplete[imgCounter], imgCounter, downloadTimeOut, maxSimultaneousDownloads);
this.pagesEWH[imgCounter] = this.downloadImageEvent[imgCounter];
}
}
最后，当我想访问图像时，我会使用以下方法检查它们是否准备就绪：
private bool ImagesInPageReady(int pageIndex, int recordsInCurrentPage)
{
if (_PagesEWH[pageIndex] != null)
{
int completedDownloadsCount = 0;
bool waitHandleSet;
// Wait for the default images first (imgCounter = 0). When moving page or asking for more pictures, then wait for the others.
for (int ewhIndexInPage = 0; ewhIndexInPage < recordsInCurrentPage; ewhIndexInPage++)
{
if (this.pagesEWH[ewhIndexInPage].WaitOne(this.downloadTimeOut))
{
if (this.WaitHandlesInPage[ewhIndexInPage].ImageIsLoaded)
{
completedDownloadsCount++;
}
}
else
{
this.pagesEWH[ewhIndexInPage].Set();
}
}
return (completedDownloadsCount > 0);
}
return false;
}
答案 0 :(得分：0)
@usr，谢谢你指出我正确的方向。 HttpClient就是解决方案。所以我基本上将我的HttpClient对象封装在一个新类中，与ProcessImage（）方法一起封装，并通过相同的方法触发公开和事件。