await Task.WhenAll(tasks) 异常处理，记录来自任务的所有异常答案

【问题标题】：await Task.WhenAll(tasks) Exception Handling, log all exceptions from the tasksawait Task.WhenAll(tasks) 异常处理，记录来自任务的所有异常
【发布时间】：2015-05-03 02:25:54
【问题描述】：

我试图弄清楚如何从下面的代码中报告由任务列表引发的所有异常。

这段代码sn-p的基本思想是：用户向处理程序发送请求，处理程序创建消息tasks并将它们发送到一个类，该类将它们发送到外部系统。我包括了下面涉及的方法。

我必须做一些事情，因为我正在调试异常处理程序，并且任务 Exception 始终为 null，因为它们的状态似乎是 Waiting for Activiation，除非我在断点中停留的时间足够长。

// Handle the user request
public async void Handle(WriteScanToSys settings)
{
  _successfulScanIds = new List<int>();

  // create the messages as tasks
  var tasks = _factory.CreateMessage(settings).Select(msg => SendScans(msg));

  try
  {
    // wait for all of them to complete
    await Task.WhenAll(tasks); // I had ConfigureAwait(false) here, but took it off
  }
  catch (Exception)
  {
    foreach (var task in tasks.Where(t => t.Exception != null))
    {
      // ELMAH
      var errorLog = ErrorLog.GetDefault(null);
      errorLog.Log(new Error(task.Exception));
    }
  }

  // save to repository
}

// the task to perform
private async Task<IDictionary<string, object>> SendScans(IDictionary<string, object> data)
{
  object sysMsg = null;
  var response = await _msgCenter.SendMessage(data);
  response.TryGetValue("SystemMessage", out sysMsg);
  _successfulScanIds.Add(Convert.ToInt32(data["Id"]));
  return response;
}

// the communication with the external system (The message center class)
private async Task<IDictionary<string, object>> SendMessage(IDictionary<string, object> msg)
{
  var response = new Dictionary<string, object>();

  var response = await _client.sendAsync(
                          new BodyOfRequest(
                              // Compose Object
                          ));

  if (response.ScreenMessage != "SUCCESSFUL")
    throw new CustomException("The transaction for job " + job + " failed with msg: " + body.ScreenMessage);

  response.Add("SystemMessage", body.ScreenMessage);

  return response;
}

【问题讨论】：

标签： c# asynchronous exception-handling async-await

【解决方案1】：

你已经犯了懒惰的评估 - Select 的结果将在你每次迭代它时创建一组新的任务。你可以通过调用ToList()来解决这个问题：

var tasks = _factory.CreateMessage(settings)
                    .Select(msg => SendScans(msg))
                    .ToList();

这样，您等待的任务集将与您的 foreach 循环检查的任务集相同。

【讨论】：

你知道我想过这个问题，但当时因为一个我现在想不出的原因把它搞砸了。我应该知道这一点，尤其是当调试器在尝试钻取时无法评估任务时。有时我想它需要别人的眼睛。谢谢@JonSkeet

【解决方案2】：

您可以从Task.WhenAll-Task 获取异常（如果有），而不是遍历所有任务：

var taskResult = Task.WhenAll(tasks);
try
{
    await taskResult;
}
catch (Exception e)
{
    if (taskResult.IsCanceled)
    {
        // Cancellation is most likely due to a shared cancellation token. Handle as needed, possibly check if ((TaskCanceledException)e).CancellationToken == token etc.       
    }
    else if (taskResult.IsFaulted)
    {
        // use taskResult.Exception which is an AggregateException - which you can iterate over (it's a tree! .Flatten() might help)
        // caught exception is only the first observed exception
    }
    else
    {
        // Well, this should not really happen because it would mean: Exception thrown, not faulted nor cancelled but completed
    }
}

【讨论】：