我想以异步方式记录一个MVC 4控制器动作针对.NET 4.0使用AsyncTargetingPack内部的一些信息到SQL Server。我会直接跳到.NET 4.5，但我的应用程序住在Azure和we're还在等待更新 ...

I am trying to asynchronously log some information to SQL Server inside of an MVC 4 controller action targeting .NET 4.0 using the AsyncTargetingPack. I would jump straight to .NET 4.5 but my app lives in Azure and we're still waiting for the update...

这code按预期工作（一排被写入我的数据库抛出没有例外）：

This code works as expected (a row is written to my database with no exceptions thrown):

public class SystemActionLogger : ISystemActionLogger
{
    private readonly ActionBlock<Tuple<SystemAction, object>> actionBlock;

    public SystemActionLogger(ISystemActionLogEntryRepository repository)
    {
        actionBlock = new ActionBlock<Tuple<SystemAction, object>>(
            entry => TaskEx.Run(async () =>
                {
                    string data = await JsonConvert.SerializeObjectAsync(entry.Item2);
                    await repository.PersistAsync(new SystemActionLogEntry(entry.Item1, data));
                }));
    }

    public void Log(SystemAction systemAction, object data)
    {
        actionBlock.Post(new Tuple<SystemAction, object>(systemAction, data));
    }
}

这code抛出一个NullReferenceException：

And this code throws a NullReferenceException:

public class SystemActionLogger : ISystemActionLogger
{
    private readonly ActionBlock<Tuple<SystemAction, object>> actionBlock;

    public SystemActionLogger(ISystemActionLogEntryRepository repository)
    {
        actionBlock = new ActionBlock<Tuple<SystemAction, object>>(async entry =>
            {
                string data = await JsonConvert.SerializeObjectAsync(entry.Item2);
                await repository.PersistAsync(new SystemActionLogEntry(entry.Item1, data));
            });
    }

    public void Log(SystemAction systemAction, object data)
    {
        actionBlock.Post(new Tuple<SystemAction, object>(systemAction, data));
    }
}

的NullReferenceException：对象未设置为一个对象的实例

NullReferenceException: "Object reference not set to an instance of an object."

Server stack trace: 
   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.Runtime.CompilerServices.TaskAwaiter.<>c__DisplayClassa.<OnCompletedInternal>b__0(Task param0)

Exception rethrown at [0]: 
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__1(Object state)
   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()

我不可见性的例外，因为它是所有外部code。我不明白为什么code中的第二块失败。这是code我原来写的。

I have no visibility into the exception as it is all external code. I don't understand why the second block of code fails. This is the code I originally wrote.

我究竟做错了什么？

推荐答案

我只是有一个非常类似的问题（从AssociateWithCurrentThread使用日志记录任务时NullReference）。

I was just having a very similar issue (NullReference from AssociateWithCurrentThread when using a logging task).

我遇到的问题是，原来的动作并没有等待完成测井任务，所以当日志完成，并试图重新加入原来的线程nullReference被抛出，因为原来的线程终止。

The issue I was having was that the original action did not await the completion of the logging task, so when the log finishes and tries to rejoin the original thread a nullReference is thrown because the original thread has terminated.

这是解决了，我要确保请求控制器等待记录功能。

This was solved for me by making sure that the request controller awaited the logging function.