c# – 逃避异常和“功能评估超时”

在我的实用程序库(Shd.dll)中,我有一个名为AsyncOperation的类.简而言之,它是类型的基类,它封装了可能长时间运行的操作,在后台线程上执行它,并且它支持暂停/恢复,取消和进度报告. (它就像一个BackgroundWorker,只知道更多东西.)

在用户代码中,您可以像这样使用它：

class MyOperation : AsyncOperation
{
    public MyOperation() : base(null,AsyncOperationOptions.Cancelable | AsyncOperationOptions.Pausable) {}

    protected override void RunOperation(AsyncOperationState operationState,object userState)
    {
         ...
         operationState.ThrowIfCancelled();
    }
}

 var op = new MyOperation();
 op.Start();
 ...
 op.Cancel();

operationState.ThrowIfCancelled()正如它的名字所暗示的那样：如果另一个线程先前调用了Cancel(),它会抛出一个内部异常(AsyncOperationCancelException),然后由AsyncOperation类型处理,如下所示：

private void _DoExecute(object state)
{
    // note that this method is already executed on the background thread
    ...
    try
    {
        operationDelegate.DynamicInvoke(args); // this is where RunOperation() is called
    }
    catch(System.Reflection.TargetInvocationException tiex)
    {
        Exception inner = tiex.InnerException;
        var cancelException = inner as AsyncOperationCancelException;
        if(cancelException != null)
        {
             // the operation was cancelled
             ...
        }
        else
        {
            // the operation faulted
            ...
        }
        ...
    }
    ...
}

这非常有效.或者在过去的一年里我想到了,而我在很多场景中使用它.

实际问题

我正在构建一个使用System.Net.WebClient通过FTP上传潜在大量文件的类.此类是使用AsyncOperation基类构建的,如上所述.

对于准确的进度报告,我使用WebClient.UploadFileAsync(),这使代码复杂化,但相关部分如下所示：

private ManualResetEventSlim completedEvent = new ManualResetEventSlim(false);

private void WebClient_UploadProgressChanged(object sender,UploadProgressChangedEventArgs e)
{
    ...
    if (OperationState.IsCancellationRequested)
    {
        _GetCurrentWebClient().CancelAsync();
    }
}

private void WebClient_UploadFileCompleted(object sender,UploadFileCompletedEventArgs e)
{
    ...
    _UploadNextFile();
}

private void _UploadNextFile()
{
    if (OperationState.IsCancellationRequested || ...)
    {
        this.completedEvent.Set();
        return;
    }
    ...
}

protected override void RunOperation(AsyncOperationState operationState,object userState)
{
    ...
    _UploadNextFile();
    this.completedEvent.Wait();

    operationState.ThrowIfCancelled(); // crash
    ...
}

如您所见,我标记了发生崩溃的行.究竟发生了什么,当执行命中该行时(我在它上方放置了一个断点,所以我知道这是确切的行),Visual Studio 2010冻结了大约15秒,然后接下来我看到的是源代码of AsyncOperationState.ThrowIfCancelled()：

public void ThrowIfCancelled()
{
    if(IsCancellationRequested)
    {
        throw new AsyncOperationCancelException();
    }
} // this is the line the debugger highlights: "An exception of type AsyncOperationCancelException' occured in Shd.dll but was unhandled by user code."

我尝试将断点放到应该捕获异常的位置,但执行永远不会到达catch {}块.

另一个奇怪的是,最后它还写了以下内容：“功能评估已禁用,因为之前的功能评估已超时.”我用Google搜索了这个问题,并尝试了所有建议(禁用隐式属性评估,删除了所有断点),但到目前为止没有任何帮助.

以下是两个说明问题的屏幕截图：
http://dl.dropbox.com/u/17147594/vsd1.png
http://dl.dropbox.com/u/17147594/vsd2.png

我正在使用.NET 4.0.任何帮助将非常感谢.