创建线程时的NullReferenceException [英] NullReferenceException when creating a thread
问题描述
我在看着这线程创建一个简单的线程池。
$ B:在那里,我遇到 @ MilanGardian对.NET 3.5 回应这是优雅,并担任我的目的就$ b
使用系统;
使用System.Collections.Generic;
使用的System.Threading;
命名空间SimpleThreadPool
{
公共密封类游泳池:IDisposable的
{
邪乎(INT大小)
{
本._workers =新的LinkedList<螺纹>();
为(VAR I = 0; I<大小; ++ I)
{
变种工人=新主题(this.Worker){名称= string.Concat(工人,一世) };
worker.Start();
this._workers.AddLast(工人);
}
}
公共无效的Dispose()
{
VAR waitForThreads = FALSE;
锁(this._tasks)
{
如果(this._disposed!)
{
GC.SuppressFinalize(本);
this._disallowAdd = TRUE; //等待所有任务完成处理,而不是让更多的新任务
,而(this._tasks.Count大于0)
{
Monitor.Wait(this._tasks);
}
this._disposed = TRUE;
Monitor.PulseAll(this._tasks); //唤醒所有工人(他们都将在这一点上活跃;设置标志将导致然后来完成,这样我们就可以加入他们的行列)
waitForThreads = TRUE;
}
}
如果(waitForThreads)
{
的foreach(VAR在this._workers工人)
{
worker.Join() ;
}
}
}
公共无效QueueTask(行动任务)
{
锁(this._tasks)
{
如果(this._disallowAdd){抛出新的InvalidOperationException异常(这池实例是在布置,不能再增加的过程); }
如果(this._disposed){抛出(这池实例已被释放)新的ObjectDisposedException; }
this._tasks.AddLast(任务);
Monitor.PulseAll(this._tasks); //因为脉冲计数任务改为
}
}
私人无效工人()
{
行动任务= NULL;
,而(真)//循环,直到线程池设
{
锁(this._tasks)//找到一个任务需要是原子
{
,而(真)//等待轮到我们在_workers队列和可用的任务
{
如果(this._disposed)
{
的回报;
}
如果(零= this._workers.First&放大器;!&放大器; object.ReferenceEquals(Thread.CurrentThread,this._workers.First.Value)及&放大器; this._tasks.Count> 0)//我们只能声明一个任务如果轮到我们了(这工作者线程处于_worker队列中的第一项),并有一个任务可以
{
=任务this._tasks.First.Value ;
this._tasks.RemoveFirst();
this._workers.RemoveFirst();
Monitor.PulseAll(this._tasks); //脉冲电流,因为(一)劳动者改变(让下一个可用的工人睡会拿起它的任务)
中断; //我们发现了一个任务来处理,从上面的突破,而(真)循环
}
Monitor.Wait(this._tasks); //去睡觉,要么不轮到我们甚至没有任务来处理
}
}
任务(); //过程中发现任务
this._workers.AddLast(Thread.CurrentThread);
任务= NULL;
}
}
私人只读的LinkedList<螺纹> _工人; //工作者线程队列准备好处理动作
私人只读LinkedList的<作用> _tasks =新的LinkedList<作用>(); //行动由工作线程
私人布尔_disallowAdd进行处理; //处理队列时设置为true但仍存在悬而未决
私人布尔_disposed任务; //处理队列时设置为true,并没有更多的任务正在等待
}
公共静态类节目
{
静态无效的主要()
{
使用(VAR池=新池(5))
{
变种随机=新的随机();
动作< INT>随机数发生器=(指数=>
{
Console.WriteLine({0}:索引工作{1},Thread.CurrentThread.Name,指数);
Thread.sleep代码( random.Next(20,400));
Console.WriteLine({0}结束{1},Thread.CurrentThread.Name,指数);
});
为(VAR I = 0; I< 40; ++ I)
{
变种I1 = I;
pool.QueueTask(()=>随机化器(I1));
}
}
}
}
}
我使用这个如下:
静态无效的主要(字串[] args)
$ p $
{
...
:
在(keepRunning)
{
...
pool.QueueTask(()=> DoTask(eventObject)传递;
}
:
}
私有静态无效DoTask(的EventObject E)
{
//做一些计算
pool.QueueTask(()=> DoAnotherTask(eventObject)传递); //这是一个相对较小的计算
}
<$ C $:p>
我运行的代码,两天后得到以下异常C>未处理的异常:System.NullReferenceException:对象不设置到对象的实例
在System.Collections.Generic.LinkedList`1.InternalInsertNodeBefore(LinkedListNode`1节点,LinkedListNode`1 newNode)$ b $。 b。在System.Collections.Generic.LinkedList`1.AddLast(T值)
在MyProg.Pool.Worker()
在System.Threading.ThreadHelper.ThreadStart_Context(对象状态)
。在System.Threading.ExecutionContext.Run(ExecutionContext中的ExecutionContext,ContextCallback回调,对象状态)
在System.Threading.ThreadHelper.ThreadStart()
我无法弄清楚是什么原因造成这是我无法再得到这个错误。如何解决这个问题有什么建议?
解决方案好像进入
_workers
链表未正确同步。考虑这种情况:
让我们假设,在某些时候
this._workets
列表中包含了一个项目
首先线程调用
this._workers.AddLast(Thread.CurrentThread);
,但在一个非常特殊的地方被中断 - 在addlast仅()
方法:公共无效addlast仅(一个LinkedListNode< T>节点)
{
this.ValidateNewNode(节点);
如果(this.head == NULL)
{
this.InternalInsertNodeToEmptyList(节点);
}
,否则
{
//这里我们被中断 - 这样的名单不为空,
//但它会很快被和this.head变空
// InternalInsertNodeBefore()没有想到,
this.InternalInsertNodeBefore(this.head,节点);
}
node.list =(LinkedList的< T>)这一点;
}
其他线程调用
this._workers.RemoveFirst() ;
。没有锁()
周围的语句,以便它完成,现在列表是空的。addlast仅()
现在应该呼叫InternalInsertNodeToEmptyList(节点);
,但它不能作为条件已经评价
把一个简单的
锁(this._tasks)
单左右this._workers.AddLast( )
行应该防止这样的场景。
其他不良情况包括添加项目到同一个列表,由两个线程同时进行。
I was looking at this thread on creating a simple thread pool. There, I came across @MilanGardian's response for .NET 3.5 which was elegant and served my purpose:
using System; using System.Collections.Generic; using System.Threading; namespace SimpleThreadPool { public sealed class Pool : IDisposable { public Pool(int size) { this._workers = new LinkedList<Thread>(); for (var i = 0; i < size; ++i) { var worker = new Thread(this.Worker) { Name = string.Concat("Worker ", i) }; worker.Start(); this._workers.AddLast(worker); } } public void Dispose() { var waitForThreads = false; lock (this._tasks) { if (!this._disposed) { GC.SuppressFinalize(this); this._disallowAdd = true; // wait for all tasks to finish processing while not allowing any more new tasks while (this._tasks.Count > 0) { Monitor.Wait(this._tasks); } this._disposed = true; Monitor.PulseAll(this._tasks); // wake all workers (none of them will be active at this point; disposed flag will cause then to finish so that we can join them) waitForThreads = true; } } if (waitForThreads) { foreach (var worker in this._workers) { worker.Join(); } } } public void QueueTask(Action task) { lock (this._tasks) { if (this._disallowAdd) { throw new InvalidOperationException("This Pool instance is in the process of being disposed, can't add anymore"); } if (this._disposed) { throw new ObjectDisposedException("This Pool instance has already been disposed"); } this._tasks.AddLast(task); Monitor.PulseAll(this._tasks); // pulse because tasks count changed } } private void Worker() { Action task = null; while (true) // loop until threadpool is disposed { lock (this._tasks) // finding a task needs to be atomic { while (true) // wait for our turn in _workers queue and an available task { if (this._disposed) { return; } if (null != this._workers.First && object.ReferenceEquals(Thread.CurrentThread, this._workers.First.Value) && this._tasks.Count > 0) // we can only claim a task if its our turn (this worker thread is the first entry in _worker queue) and there is a task available { task = this._tasks.First.Value; this._tasks.RemoveFirst(); this._workers.RemoveFirst(); Monitor.PulseAll(this._tasks); // pulse because current (First) worker changed (so that next available sleeping worker will pick up its task) break; // we found a task to process, break out from the above 'while (true)' loop } Monitor.Wait(this._tasks); // go to sleep, either not our turn or no task to process } } task(); // process the found task this._workers.AddLast(Thread.CurrentThread); task = null; } } private readonly LinkedList<Thread> _workers; // queue of worker threads ready to process actions private readonly LinkedList<Action> _tasks = new LinkedList<Action>(); // actions to be processed by worker threads private bool _disallowAdd; // set to true when disposing queue but there are still tasks pending private bool _disposed; // set to true when disposing queue and no more tasks are pending } public static class Program { static void Main() { using (var pool = new Pool(5)) { var random = new Random(); Action<int> randomizer = (index => { Console.WriteLine("{0}: Working on index {1}", Thread.CurrentThread.Name, index); Thread.Sleep(random.Next(20, 400)); Console.WriteLine("{0}: Ending {1}", Thread.CurrentThread.Name, index); }); for (var i = 0; i < 40; ++i) { var i1 = i; pool.QueueTask(() => randomizer(i1)); } } } } }
I am using this as follows:
static void Main(string[] args) { ... ... while(keepRunning) { ... pool.QueueTask(() => DoTask(eventObject); } ... } private static void DoTask(EventObject e) { // Do some computations pool.QueueTask(() => DoAnotherTask(eventObject)); // this is a relatively smaller computation }
I am getting the following exception after running the code for about two days:
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. at System.Collections.Generic.LinkedList`1.InternalInsertNodeBefore(LinkedListNode`1 node, LinkedListNode`1 newNode) at System.Collections.Generic.LinkedList`1.AddLast(T value) at MyProg.Pool.Worker() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart()
I am unable to figure out what is causing this as I am unable to get this error again. Any suggestions on how to fix this?
解决方案Seems like access to
_workers
linked list is not properly synchronized. Consider this scenario:Lets assume that at some point
this._workets
list contains one item.First thread calls
this._workers.AddLast(Thread.CurrentThread);
but gets interrupted at a very special place - insideAddLast()
method:public void AddLast(LinkedListNode<T> node) { this.ValidateNewNode(node); if (this.head == null) { this.InternalInsertNodeToEmptyList(node); } else { // here we got interrupted - the list was not empty, // but it would be pretty soon, and this.head becomes null // InternalInsertNodeBefore() does not expect that this.InternalInsertNodeBefore(this.head, node); } node.list = (LinkedList<T>) this; }
Other thread calls
this._workers.RemoveFirst();
. There is nolock()
around that statement so it completes and now list is empty.AddLast()
now should callInternalInsertNodeToEmptyList(node);
but it can't as the condition was already evaluated.Putting a simple
lock(this._tasks)
around singlethis._workers.AddLast()
line should prevent such scenario.Other bad scenarios include adding item to the same list at the same time by two threads.
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