With all due respect to the great answers provided already, there are times that I simply want a thread-safe IList. Nothing advanced or fancy. Performance is important in many cases but at times that just isn't a concern. Yes, there are always going to be challenges without methods like "TryGetValue" etc, but most cases I just want something that I can enumerate without needing to worry about putting locks around everything. And yes, somebody can probably find some "bug" in my implementation that might lead to a deadlock or something (I suppose) but lets be honest: When it comes to multi-threading, if you don't write your code correctly, it is going deadlock anyway. With that in mind I decided to make a simple ConcurrentList implementation that provides these basic needs.

为了它的价值:我做了一个基本的测试，添加10,000,000项到常规列表和ConcurrentList，结果是:

列表完成时间:7793毫秒。 并发完成时间:8064毫秒。

public class ConcurrentList<T> : IList<T>, IDisposable
{
    #region Fields
    private readonly List<T> _list;
    private readonly ReaderWriterLockSlim _lock;
    #endregion

    #region Constructors
    public ConcurrentList()
    {
        this._lock = new ReaderWriterLockSlim(LockRecursionPolicy.NoRecursion);
        this._list = new List<T>();
    }

    public ConcurrentList(int capacity)
    {
        this._lock = new ReaderWriterLockSlim(LockRecursionPolicy.NoRecursion);
        this._list = new List<T>(capacity);
    }

    public ConcurrentList(IEnumerable<T> items)
    {
        this._lock = new ReaderWriterLockSlim(LockRecursionPolicy.NoRecursion);
        this._list = new List<T>(items);
    }
    #endregion

    #region Methods
    public void Add(T item)
    {
        try
        {
            this._lock.EnterWriteLock();
            this._list.Add(item);
        }
        finally
        {
            this._lock.ExitWriteLock();
        }
    }

    public void Insert(int index, T item)
    {
        try
        {
            this._lock.EnterWriteLock();
            this._list.Insert(index, item);
        }
        finally
        {
            this._lock.ExitWriteLock();
        }
    }

    public bool Remove(T item)
    {
        try
        {
            this._lock.EnterWriteLock();
            return this._list.Remove(item);
        }
        finally
        {
            this._lock.ExitWriteLock();
        }
    }

    public void RemoveAt(int index)
    {
        try
        {
            this._lock.EnterWriteLock();
            this._list.RemoveAt(index);
        }
        finally
        {
            this._lock.ExitWriteLock();
        }
    }

    public int IndexOf(T item)
    {
        try
        {
            this._lock.EnterReadLock();
            return this._list.IndexOf(item);
        }
        finally
        {
            this._lock.ExitReadLock();
        }
    }

    public void Clear()
    {
        try
        {
            this._lock.EnterWriteLock();
            this._list.Clear();
        }
        finally
        {
            this._lock.ExitWriteLock();
        }
    }

    public bool Contains(T item)
    {
        try
        {
            this._lock.EnterReadLock();
            return this._list.Contains(item);
        }
        finally
        {
            this._lock.ExitReadLock();
        }
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        try
        {
            this._lock.EnterReadLock();
            this._list.CopyTo(array, arrayIndex);
        }
        finally
        {
            this._lock.ExitReadLock();
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        return new ConcurrentEnumerator<T>(this._list, this._lock);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return new ConcurrentEnumerator<T>(this._list, this._lock);
    }

    ~ConcurrentList()
    {
        this.Dispose(false);
    }

    public void Dispose()
    {
        this.Dispose(true);
    }

    private void Dispose(bool disposing)
    {
        if (disposing)
            GC.SuppressFinalize(this);

        this._lock.Dispose();
    }
    #endregion

    #region Properties
    public T this[int index]
    {
        get
        {
            try
            {
                this._lock.EnterReadLock();
                return this._list[index];
            }
            finally
            {
                this._lock.ExitReadLock();
            }
        }
        set
        {
            try
            {
                this._lock.EnterWriteLock();
                this._list[index] = value;
            }
            finally
            {
                this._lock.ExitWriteLock();
            }
        }
    }

    public int Count
    {
        get
        {
            try
            {
                this._lock.EnterReadLock();
                return this._list.Count;
            }
            finally
            {
                this._lock.ExitReadLock();
            }
        }
    }

    public bool IsReadOnly
    {
        get { return false; }
    }
    #endregion
}

    public class ConcurrentEnumerator<T> : IEnumerator<T>
{
    #region Fields
    private readonly IEnumerator<T> _inner;
    private readonly ReaderWriterLockSlim _lock;
    #endregion

    #region Constructor
    public ConcurrentEnumerator(IEnumerable<T> inner, ReaderWriterLockSlim @lock)
    {
        this._lock = @lock;
        this._lock.EnterReadLock();
        this._inner = inner.GetEnumerator();
    }
    #endregion

    #region Methods
    public bool MoveNext()
    {
        return _inner.MoveNext();
    }

    public void Reset()
    {
        _inner.Reset();
    }

    public void Dispose()
    {
        this._lock.ExitReadLock();
    }
    #endregion

    #region Properties
    public T Current
    {
        get { return _inner.Current; }
    }

    object IEnumerator.Current
    {
        get { return _inner.Current; }
    }
    #endregion
}

ConcurrentList(作为一个可调整大小的数组，而不是一个链表)不容易用非阻塞操作编写。它的API不能很好地转换为“并发”版本。

你用ConcurrentList做什么?

在线程世界中，随机访问容器的概念并不像它看起来那样有用。该声明

  if (i < MyConcurrentList.Count)  
      x = MyConcurrentList[i]; 

总的来说仍然不是线程安全的。

与其创建ConcurrentList，不如尝试使用现有的内容构建解决方案。最常见的类是ConcurrentBag，尤其是BlockingCollection。

有些人强调了一些好处(以及我的一些想法):

It could looklikes insane to unable random accesser (indexer) but to me it appears fine. You only have to think that there is many methods on multi-threaded collections that could fail like Indexer and Delete. You could also define failure (fallback) action for write accessor like "fail" or simply "add at the end". It is not because it is a multithreaded collection that it will always be used in a multithreaded context. Or it could also be used by only one writer and one reader. Another way to be able to use indexer in a safe manner could be to wrap actions into a lock of the collection using its root (if made public). For many people, making a rootLock visible goes agaist "Good practice". I'm not 100% sure about this point because if it is hidden you remove a lot of flexibility to the user. We always have to remember that programming multithread is not for anybody. We can't prevent every kind of wrong usage. Microsoft will have to do some work and define some new standard to introduce proper usage of Multithreaded collection. First the IEnumerator should not have a moveNext but should have a GetNext that return true or false and get an out paramter of type T (this way the iteration would not be blocking anymore). Also, Microsoft already use "using" internally in the foreach but sometimes use the IEnumerator directly without wrapping it with "using" (a bug in collection view and probably at more places) - Wrapping usage of IEnumerator is a recommended pratice by Microsoft. This bug remove good potential for safe iterator... Iterator that lock collection in constructor and unlock on its Dispose method - for a blocking foreach method.

这不是答案。这只是一些不适合特定场合的注释。

... My conclusion, Microsoft has to make some deep changes to the "foreach" to make MultiThreaded collection easier to use. Also it has to follow there own rules of IEnumerator usage. Until that, we can write a MultiThreadList easily that would use a blocking iterator but that will not follow "IList". Instead, you will have to define own "IListPersonnal" interface that could fail on "insert", "remove" and random accessor (indexer) without exception. But who will want to use it if it is not standard ?

没有ConcurrentList的原因是因为它根本不能被写入。原因是在IList中有几个重要的操作依赖于索引，而这根本行不通。例如:

int catIndex = list.IndexOf("cat");
list.Insert(catIndex, "dog");

作者所追求的效果是在“cat”之前插入“dog”，但在多线程环境中，这两行代码之间的列表可能发生任何事情。例如，另一个线程可能执行list. removeat(0)，将整个列表向左移动，但关键的是，catIndex不会改变。这里的影响是Insert操作实际上将“狗”放在猫之后，而不是在它之前。

The several implementations that you see offered as "answers" to this question are well-meaning, but as the above shows, they don't offer reliable results. If you really want list-like semantics in a multithreaded environment, you can't get there by putting locks inside the list implementation methods. You have to ensure that any index you use lives entirely inside the context of the lock. The upshot is that you can use a List in a multithreaded environment with the right locking, but the list itself cannot be made to exist in that world.

如果你认为你需要一个并发列表，实际上只有两种可能:

你真正需要的是一个ConcurrentBag 您需要创建自己的集合，可能使用List和自己的并发控制来实现。

If you have a ConcurrentBag and are in a position where you need to pass it as an IList, then you have a problem, because the method you're calling has specified that they might try to do something like I did above with the cat & dog. In most worlds, what that means is that the method you're calling is simply not built to work in a multi-threaded environment. That means you either refactor it so that it is or, if you can't, you're going to have to handle it very carefully. You you'll almost certainly be required to create your own collection with its own locks, and call the offending method within a lock.

我试过一段时间(也:在GitHub)。我的实现出现了一些问题，我在这里就不赘述了。让我告诉你，更重要的是，我学到了什么。

首先，你不可能得到一个完整的IList<T>的无锁和线程安全的实现。特别是，随机插入和删除是行不通的，除非你也忘记了O(1)随机访问(也就是说，除非你“欺骗”，只是使用某种链表，让索引糟糕透顶)。

我认为可能值得的是一个线程安全的IList<T>的有限子集:特别是一个允许添加并通过索引提供随机只读访问的子集(但没有Insert、RemoveAt等，也没有随机写访问)。

This was the goal of my ConcurrentList<T> implementation. But when I tested its performance in multithreaded scenarios, I found that simply synchronizing adds to a List<T> was faster. Basically, adding to a List<T> is lightning fast already; the complexity of the computational steps involved is miniscule (increment an index and assign to an element in an array; that's really it). You would need a ton of concurrent writes to see any sort of lock contention on this; and even then, the average performance of each write would still beat out the more expensive albeit lockless implementation in ConcurrentList<T>.

在相对罕见的情况下，列表的内部数组需要调整自身的大小，您确实需要付出一点代价。因此，最终我得出结论，这是一个适合的场景，其中仅添加ConcurrentList<T>集合类型是有意义的:当您希望保证在每次调用中添加元素的开销较低时(因此，与平摊性能目标相反)。

它并不是一个像您想象的那样有用的类。