asp.net mvc的4.5：让HostingEnvironment.QueueBackgroundWorkItem工作 [英] asp.net mvc 4.5: getting HostingEnvironment.QueueBackgroundWorkItem to work

问题描述

我试图完成一些轻量级的任务后的控制器已经完成了他的工作，我用 HostingEnvironment.QueueBackgroundWorkItem（）这一点。

I'm trying to finish some lightweight tasks after a controller has finished his work and I use HostingEnvironment.QueueBackgroundWorkItem() for this.

我看到一个奇怪的行为，所以我做的人工验证的概念，应用这一点。

I'm seeing a strange behavior, so I make an artificial proof-of-concept app for this.

在我的的global.asax.cs 我有这个漂亮的功能：

in my global.asax.cs I have this nice function:

public class MvcApplication : HttpApplication
{
    private static NLog.Logger logger = NLog.LogManager.GetCurrentClassLogger();

    public static void ContinueWorkOnBackground(Task workItem)
    {
        workItem.ContinueWith(t =>
        {
            if (t.IsFaulted)
            {
                var ex = t.Exception;
                logger.Error(ex);
            }
        });
        HostingEnvironment.QueueBackgroundWorkItem(_ => workItem);
    }

在我的控制器，我创建了一个工作项目，并抛出有：

And in my controller I create a workitem and throw there:

public ActionResult About()
{
    logger.Info("About");
    ViewBag.Message = "Your application description page.";
    MvcApplication.ContinueWorkOnBackground(TestWorkItem());
    return View();
}

private async Task TestWorkItem()
{
    logger.Trace("TestWorkItem");
    await Task.Delay(500);
    logger.Trace("let's fail");
    throw new NotImplementedException();
}

我看到日志消息TestWorkItem，但从来没有让我们失败，并在错误没有消息。

I see the message "TestWorkItem" in logs, but never "let's fail" and no message on error.

我也做了 HostingEnvironment.QueueBackgroundWorkItem（（Func键＆LT;的CancellationToken，任务＆GT;）（_ =＆GT;的工作项目））; ，以确保通话一点也不含糊。

I also did HostingEnvironment.QueueBackgroundWorkItem((Func<CancellationToken,Task>)(_ => workItem)); to make sure the call is not ambiguous.

我该如何解决这个问题？

How do I fix this?

推荐答案

您的问题是， TestWorkItem 正在捕获当前请求的背景下，并试图恢复该请求上下文其之后的await 。 （我解释此捕获/恢复的工作原理在我的博客） 。由于请求已完成，请求上下文不再受它试图恢复时间存在。

Your problem is that TestWorkItem is capturing the current request context, and attempting to resume on that request context after its await. (I explain how this capture/resume works on my blog). Since the request has completed, the request context no longer exists by the time it attempts to resume.

这样做的最好的解决方法是让 QueueBackgroundWorkItem 执行code的请求上下文之外。因此，首先改变你的辅助方法，采取了 Func键＆LT;任务＆gt;将代替工作：

The best fix for this is to allow QueueBackgroundWorkItem to execute the code outside of a request context. So, first change your helper method to take a Func<Task> instead of a Task:

public static void ContinueWorkOnBackground(Func<Task> workItem)
{
  HostingEnvironment.QueueBackgroundWorkItem(async _ =>
  {
    try
    {
      await workItem();
    }
    catch (Exception ex)
    {
      logger.Error(ex);
    }
  });
}

然后就可以调用它是这样：

Then you can call it as such:

public ActionResult About()
{
  logger.Info("About");
  ViewBag.Message = "Your application description page.";
  MvcApplication.ContinueWorkOnBackground(() => TestWorkItem());
  return View();
}

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