首先，我已经在许多 NullReferenceException 问题中搜索过 SO。 here和 here
当我尝试调用 Task.WaitAll(tasks);
我确定在尝试调用方法之前我正在初始化所有对象。以下是代码片段:
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;
}
注意
var a = tasks.ToArray();
Task.WaitAll(a);
是抛出异常的测试。 var a = tasks.ToArray(); 顺利通过。
这里抛出异常:
Task.WaitAll(a);
在这里
status = Task.WaitAll(tasks.ToArray(), timeout);
你能解释一下发生了什么吗？我可以在调试期间看到 tasks 已初始化。
附言例如注释行:result.AddRange(GetResourceFreeBusyOnline(requesterId, meetingRoomIds.ToArray(), start, end)); 是在单线程中调用方法的行，它没有错误地传递并返回预期的结果。
堆栈
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()<---
最佳答案
Task.WaitAll 不会抛出这个异常。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 Exception.ToString() 返回)，则无法确定，但标准指南也适用于此 - 在某处，不知何故，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);
可以将 personIds 和 meetingRoomIds 都设置为 null。这意味着
var task = Task.Factory.StartNew(() => GetResourceFreeBusyOnline(requesterId,
personIds.ToArray(),
start, end));
将在任务中抛出异常，当调用 Task.WaitAll 时将重新引发异常。
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 AggregateException其中包含其 InnerExceptions 中的所有基础异常属性(property)。
至少您应该捕获 AggregateException 以单独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 解包 AggregateException 并引发底层异常，这使调试变得更加容易。
更新
从调用堆栈看来，Tieto.MyMeetings.Tasks.FreeBusy.FreeBusyTasks.GetMeetingRoomsFreeBusyCached 调用 SharpArch.NHibernate.LinqRepositoryWithTypedId'2.FindAll 并传递一个 null参数或包含 null 的项目列表。这些值可能用于创建 session key ，因为在抛出异常之前调用了 SharpArch.NHibernate.Web.Mvc.WebSessionStorage.GetSessionForKey。
找到确切的问题需要调试代码并进入 GetMeetingRoomsFreeBusyCached
关于c# - 使用获取 "Object reference not set to an instance of an object"的任务，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/41282921/