异步代码，无需等待完成 [英] Async code without waiting for completion

问题描述

我目前遇到IIS崩溃的问题，并在事件日志中留下了以下消息.他们并不能帮助我找到错误的实际来源，但是一些研究表明，这只是生成任务的一种情况，而不是等待结果，当它们在父流程完成后最终完成时就无法执行与导致空引用异常的父线程相关联.正确吗?

I'm currently having issues with IIS crashing out leaving the following messages in the event log. They're not too helpful in directing me to the actual source of the error but a bit of research suggests this is just a case of spawning tasks but not waiting for the result, when they do eventually complete if the parent process has completed it cannot be associated with the parent thread which causes a null reference exception. Is that correct?

在大多数情况下，我已经删除或添加了等待操作的地方，但是在某些地方它很方便.在会话开始时，一个这样的例子就是内存缓存会话特有的一些细节，但是它们并不是立即重要的，因此我启动了一项任务来完成这项工作，但不要等待它.

For the most part I have removed or added awaits where this was going on but there are some areas where it is handy. One such example is on session start is memory caching a few details unique to the session, however they're not vital immediately so I spin up a task to do this work but don't await it.

我添加了ConfigureAwait(false)，这是否足以防止将来出现此错误?听起来这将删除我认为可以防止该错误的线程切换.

I have added ConfigureAwait(false), is this sufficient to prevent the error in future? It sounds like this will remove the thread switching which I assume will prevent the error.

Task.Run(() =>
{
    // Do caching here

}).ConfigureAwait(false);

ASP.NET 4.0.30319.0

ASP.NET 4.0.30319.0

Exception: System.NullReferenceException

Message: Object reference not set to an instance of an object.

StackTrace:    at System.Web.ThreadContext.AssociateWithCurrentThread(Boolean setImpersonationContext)
   at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext)
   at System.Web.LegacyAspNetSynchronizationContext.CallCallbackPossiblyUnderLock(SendOrPostCallback callback, Object state)
   at System.Web.LegacyAspNetSynchronizationContext.CallCallback(SendOrPostCallback callback, Object state)
   at System.Threading.Tasks.AwaitTaskContinuation.RunCallback(ContextCallback callback, Object state, Task& currentTask)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.Tasks.AwaitTaskContinuation.<ThrowAsyncIfNecessary>b__1(Object s)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

.NET运行时

Application: w3wp.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.NullReferenceException
Stack:
   at System.Threading.Tasks.AwaitTaskContinuation.<ThrowAsyncIfNecessary>b__1(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

应用程序错误

Faulting application name: w3wp.exe, version: 7.5.7601.17514, time stamp: 0x4ce7afa2
Faulting module name: KERNELBASE.dll, version: 6.1.7601.18015, time stamp: 0x50b8479b
Exception code: 0xe0434352
Fault offset: 0x0000000000009e5d
Faulting process id: 0x1d98
Faulting application start time: 0x01ceaf8ea10ece66
Faulting application path: c:\windows\system32\inetsrv\w3wp.exe
Faulting module path: C:\Windows\system32\KERNELBASE.dll
Report Id: 341b9a76-1b82-11e3-8e17-005056be0005

推荐答案

在追踪了这个问题的早晨之后，我最终发现有一条路径可以允许父母绕开等待，以解决此问题.快速重构以防止这种情况解决了该问题.

After a morning of chasing this problem around I eventually discovered there was a path which could allow for the parent to bypass the await creating this problem. A quick refactoring to prevent this solved the problem.

以最简单的形式导致问题的代码如下:

In its most simple form the code causing the problem was along the lines of this:

var getDataTask = getData();

if(some_condition == true) {
    return some_object;
}

getDataTask.Wait();

return getDataTask.result;

如果some_condition == true，该方法将返回而无需等待getDataTask完成.重构以阻止该问题的修复.

If some_condition == true the method would return without waiting for getDataTask to complete. Refactoring to stop that fixed it.

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