使用任务获取“未将对象引用设置为对象的实例”

Question

首先，我已经在许多 NullReferenceException 问题中搜索了 SO。这里和这里

当我尝试调用时收到错误“对象引用未设置为对象的实例” Task.WaitAll(tasks);

我确信在尝试调用方法之前我正在初始化所有对象。下面是代码片段：

public IList<ResourceFreeBusyDto> GetResourceFreeBusy(int requesterId, int[] resourceIds, DateTime start, DateTime end)
    {
        IList<ResourceFreeBusyDto> result = new List<ResourceFreeBusyDto>();

        ValidateFreeBusyInputs(resourceIds, start, end);

        List<Task<IList<ResourceFreeBusyDto>>> tasks = new List<Task<IList<ResourceFreeBusyDto>>>();
        TimeSpan timeout = new TimeSpan(0,0,30); // 30 seconds          

        // Split resources to persons and meetingRooms
        List<int> personIds;
        List<int> meetingRoomIds;

        SplitResourceIds(resourceIds, out personIds, out meetingRoomIds);

        // Go online for persons
        if (personIds.Count > 0)
        {
            //result.AddRange(GetResourceFreeBusyOnline(requesterId, personIds.ToArray(), start, end)); // paralelizovat
            Task<IList<ResourceFreeBusyDto>> task = Task.Factory.StartNew(() => GetResourceFreeBusyOnline(requesterId, personIds.ToArray(), start, end));
            tasks.Add(task);
        }

        // Go online for meetingrooms if they are not cached in DB
        if (meetingRoomIds.Count > 0)
        {
            DateTime? lastModifiedMeetingRoomFreeBusy = new DateTime();

            lastModifiedMeetingRoomFreeBusy = freeBusyRepository.GetMinTimeStamp();

            if (lastModifiedMeetingRoomFreeBusy.Value.AddMinutes(1) < DateTime.UtcNow || lastModifiedMeetingRoomFreeBusy == null)
            {
                //result.AddRange(GetResourceFreeBusyOnline(requesterId, meetingRoomIds.ToArray(), start, end)); // mrIDs
                Task<IList<ResourceFreeBusyDto>> task = Task.Factory.StartNew(() => GetResourceFreeBusyOnline(requesterId, resourceIds, start, end));
                tasks.Add(task);
            }
            else
            {
                //result.AddRange(GetMeetingRoomsFreeBusyCached(requesterId, meetingRoomIds.ToArray(), start, end)); // mrIDs
                Task<IList<ResourceFreeBusyDto>> task = Task.Factory.StartNew(() => GetMeetingRoomsFreeBusyCached(requesterId, resourceIds, start, end));
                tasks.Add(task);
            }
        }

        bool status = false;

        try
        {
            var a = tasks.ToArray();
            Task.WaitAll(a);
            status = Task.WaitAll(tasks.ToArray(), timeout);
        }
        catch (Exception ex)
        {

            Log.Fatal(ex);
        }


        if (status == false)
        {
            throw new ApplicationException(
            string.Format("Timeout expired." +
            " The timeout period elapsed prior to completion of the asynchronous importing task executing" +
            " or the server is not responding. Try it later!"));
        }
        else
        {
            foreach (Task<IList<ResourceFreeBusyDto>> task in tasks)
            {
                result.AddRange(task.Result);
            }
        }

        return result;

    }

笔记

var a = tasks.ToArray();
Task.WaitAll(a);

是抛出异常的测试。var a = tasks.ToArray();正在通过而没有错误。

这里抛出异常：

Task.WaitAll(a); 

和这里

status = Task.WaitAll(tasks.ToArray(), timeout);

你能解释一下发生了什么吗？我可以在tasks初始化的调试过程中看到。

PS注释行例如：result.AddRange(GetResourceFreeBusyOnline(requesterId, meetingRoomIds.ToArray(), start, end));是在单线程中调用方法的行，该线程无错误地传递并返回预期结果。

堆

2016-12-22 13:24:18,844 [9] FATAL Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks - System.AggregateException: One or more errors occurred. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSimpleSessionStorage()
   at SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSessionForKey(String factoryKey)
   at SharpArch.NHibernate.NHibernateSession.CurrentFor(String factoryKey)
   at SharpArch.NHibernate.NHibernateRepositoryWithTypedId`2.get_Session()
   at SharpArch.NHibernate.LinqRepositoryWithTypedId`2.FindAll()
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.GetResourceFreeBusyOnline(Int32 requesterId, Int32[] resourceIds, DateTime start, DateTime end) in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 238
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.<>c__DisplayClass7_0.<GetResourceFreeBusy>b__0() in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 83
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout)
   at System.Threading.Tasks.Task.WaitAll(Task[] tasks)
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.GetResourceFreeBusy(Int32 requesterId, Int32[] resourceIds, DateTime start, DateTime end) in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 113
---> (Inner Exception #0) System.NullReferenceException: Object reference not set to an instance of an object.
   at SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSimpleSessionStorage()
   at SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSessionForKey(String factoryKey)
   at SharpArch.NHibernate.NHibernateSession.CurrentFor(String factoryKey)
   at SharpArch.NHibernate.NHibernateRepositoryWithTypedId`2.get_Session()
   at SharpArch.NHibernate.LinqRepositoryWithTypedId`2.FindAll()
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.GetResourceFreeBusyOnline(Int32 requesterId, Int32[] resourceIds, DateTime start, DateTime end) in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 238
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.<>c__DisplayClass7_0.<GetResourceFreeBusy>b__0() in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 83
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()<---

---> (Inner Exception #1) System.NullReferenceException: Object reference not set to an instance of an object.
   at SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSimpleSessionStorage()
   at SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSessionForKey(String factoryKey)
   at SharpArch.NHibernate.NHibernateSession.CurrentFor(String factoryKey)
   at SharpArch.NHibernate.NHibernateRepositoryWithTypedId`2.get_Session()
   at SharpArch.NHibernate.LinqRepositoryWithTypedId`2.FindAll(ILinqSpecification`1 specification)
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.GetMeetingRoomsFreeBusyCached(Int32 requesterId, Int32[] meetingRoomIds, DateTime start, DateTime end) in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 196
   at Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.<>c__DisplayClass7_0.<GetResourceFreeBusy>b__2() in C:\_Hg\Tieto.MyMeetings\source\server\Tieto.MyMeetings.Tasks\FreeBusy\FreeBusyTasks.cs:line 103
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()<---

Answer 1

Task.WaitAll 不会抛出此异常。它将重新抛出其任务之一引发的异常。如果没有完整的异常和调用堆栈（由 返回Exception.ToString()），则无法确定，但标准指南也适用于此处 - 在某个地方，以某种方式使用了未初始化的变量或参数。

例如：

List<int> personIds;
List<int> meetingRoomIds;

SplitResourceIds(resourceIds, out personIds, out meetingRoomIds);

很可能将personIds和 都设置meetingRoomIds为空。这意味着

var task = Task.Factory.StartNew(() => GetResourceFreeBusyOnline(requesterId,
                                                                 personIds.ToArray(), 
                                                                 start, end));

Task.WaitAll将在任务内抛出异常，该异常将在调用时重新引发。

要解决此问题，只需遵循重复问题中的建议即可。检查参数是否为空，调试代码，检查异常的完整调用堆栈。Task.WaitAll将抛出一个AggregateException，其中包含其InnerExceptions属性中的所有基础异常。

至少您应该捕获AggregateException以分别处理和记录任务中引发的异常。

最后，使用async/await, Task.Run和await Task.WhenAll代替 StartNew 和 Task.WaitAll。await解开AggregateException并引发底层异常，这使得调试变得更加容易。

更新

从调用堆栈来看，Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.GetMeetingRoomsFreeBusyCached调用SharpArch.NHibernate.LinqRepositoryWithTypedId'2.FindAll并传递 null 参数或包含 null 的项目列表。这些值可能用于创建会话密钥，因为SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSessionForKey在引发异常之前调用。

找到确切的问题需要调试代码并单步执行GetMeetingRoomsFreeBusyCached