异步日志记录抛出NullReferenceException

Question

我试图使用AsyncTargetingPack异步将一些信息记录到针对.NET 4.0的MVC 4控制器操作中的SQL Server.我会直接跳到.NET 4.5,但我的应用程序存在于Azure中,我们仍在等待更新 ...

此代码按预期工作(一行写入我的数据库,没有抛出异常):

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));
    }
}

并且此代码抛出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));
    }
}

NullReferenceException:"对象引用未设置为对象的实例."

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()

我没有对异常的可见性,因为它是所有外部代码.我不明白为什么第二个代码块失败.这是我最初写的代码.

我究竟做错了什么？

Answer 1

我只是遇到了一个非常类似的问题(使用日志记录任务时来自AssociateWithCurrentThread的NullReference).

我遇到的问题是原始操作没有等待日志记录任务的完成,因此当日志完成并尝试重新加入原始线程时,抛出nullReference,因为原始线程已终止.

这是通过确保请求控制器等待日志记录功能来解决的.

Answer 2

数据流仅适用于 .NET 4.5。事实上，您在 .NET 4.0 上运行它不受支持，这可能就是您看到虚假异常的原因。