C# 创建线程时出现NullReferenceException

我在创建一个简单的线程池时看到了这一点。在那里，我遇到了一个优雅且符合我目的的：

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));
                }
            }
        }
    }
}

在运行代码大约两天后，我遇到以下异常：

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()

---

我无法找出原因，因为我无法再次获得此错误。有没有关于如何解决这个问题的建议？

我想我找到了问题所在。代码示例缺少一个

lock（）

锁应该围绕

此扩展或包装。\u workers.AddLast（Thread.CurrentThread）

如果查看修改
LinkedList
（Pool.QueueTask）的其他代码，它将被包装在
锁中
 对
\u workers
链接列表的访问似乎没有正确同步。考虑这种情况：

让我们假设在某个点上，此.uWorkets
列表包含一项

第一个线程调用this.\u workers.AddLast（thread.CurrentThread）但在一个非常特殊的位置被中断-在
AddLast（）内
方法：

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;
}

public void AddLast（LinkedListNode节点）
{
this.ValidateNewNode（节点）；
if（this.head==null）
{
this.InternalInsertNodeToEmptyList（节点）；
}
其他的
{
//这里我们被打断了-列表不是空的，
//但它很快就会变成空的
//InternalInsertNodeBefore（）不希望
this.InternalInsertNodeBefore（this.head，node）；
}
node.list=（LinkedList）这个；
}

其他线程调用
this.\u workers.RemoveFirst（）。该语句周围没有
lock（）
，所以它完成了，现在列表为空
AddLast（）
现在应该调用
InternalInsertNodeToEmptyList（节点），但无法执行，因为条件已被评估

在单个
this.\u workers.AddLast（）
行周围放置一个简单的
锁（this.\u tasks）
，可以防止出现这种情况

其他不好的情况包括由两个线程同时将项添加到同一列表。
堆栈跟踪指向
this.\u workers.AddLast（Thread.CurrentThread）作为罪魁祸首。我在C#中与LinkedList的合作不多，但也许它在顺序上出错了，并且不是线程安全的。几乎所有
NullReferenceException
的情况都是一样的。请参阅“”以获取一些提示。我认为您遇到了NULLReferenceException错误，因为您尚未首先初始化或正确创建对象。尝试使用
新建创建对象
-这可能有助于+1谢谢。这似乎解决了问题。如果我再次观察到这一点，我将进行更新。+1感谢您抽出时间。将语句封装在锁中解决了问题。
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
            {
            ....
            }
        }

        task(); // process the found task
        this._workers.AddLast(Thread.CurrentThread);
        task = null;
    }
}

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;
}