c# - 为什么ActionBlock中的Try/catch有时抓不到这个错误？

Question

我正在编写的网络爬虫中使用 ActionBlock。

有时候打电话

actionBlock.Completion.Wait();

出现以下错误

One or more errors occurred.

内部异常是

System.Threading.Tasks.TaskCanceledException: A task was canceled.

这是下面的完整代码块。

actionBlock = new ActionBlock<URLsToCheckObject>(URLToCheck =>
{
    try
    {
        // get more urls etc here and post below

        actionBlock.Post(new URLsToCheckObject { URLAddress = CleanURL, Host = host });
        if (actionBlock.InputCount == 0) actionBlock.Complete();
    }
    catch (Exception ex)
    {
        try
        {
            Logger.AddToDebugLog("Block 3 catch...", WebsiteToCrawl);
        }
        catch { }
    }

}, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 3, CancellationToken = cancellationSource.Token });

actionBlock.Post(new URLsToCheckObject { URLAddress = WebsiteToCrawl.website, Host = host });

try
{
    Logger.AddToDebugLog("Waiting on action block..", WebsiteToCrawl);
    actionBlock.Completion.Wait();
}
catch (Exception ex)
{
    try
    {
        Logger.AddToDebugLog("Block 4 catch..." + ex.Message, WebsiteToCrawl);
    }
    catch { }

    try
    {
        Logger.AddToDebugLog("Block 4 catch..." + ex.InnerException, WebsiteToCrawl);
    }
    catch { }

    try
    {
        Logger.AddToDebugLog("Block 4 catch...", WebsiteToCrawl);
    }
    catch { }
}

为什么包裹 ActionBlock 全部内容的 try/catch 没有捕捉到这个异常？

Answer 1

异常通常来自:

cancellationToken.ThrowIfCancellationRequested();

因为您没有在 lambda 中使用 CancellationToken，但它用作以下参数:

new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 3, CancellationToken = cancellationSource.Token }

TaskCanceledException 不是来自您的 lambda(但可能来自 ExecutionDataflowBlockOptions 或 ActionBlock 内部)，因此它超出了您的范围try/catch 在 lambda 内部，因此它不会捕获。