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。

在读的数量远远超过写的情况下，或者(无论多么频繁)写是非并发的情况下，在写时复制方法可能是合适的。

实现如下所示

lockless blazingly fast for concurrent reads, even while concurrent modifications are ongoing - no matter how long they take because "snapshots" are immutable, lockless atomicity is possible, i.e. var snap = _list; snap[snap.Count - 1]; will never (well, except for an empty list of course) throw, and you also get thread-safe enumeration with snapshot semantics for free.. how I LOVE immutability! implemented generically, applicable to any data structure and any type of modification dead simple, i.e. easy to test, debug, verify by reading the code usable in .Net 3.5

为了让写时复制工作，你必须让你的数据结构有效地保持不可变，也就是说，在你让它们对其他线程可用之后，任何人都不允许改变它们。当你想修改的时候，你

克隆结构 对克隆进行修改 原子地交换对修改后的克隆的引用

Code

static class CopyOnWriteSwapper
{
    public static void Swap<T>(ref T obj, Func<T, T> cloner, Action<T> op)
        where T : class
    {
        while (true)
        {
            var objBefore = Volatile.Read(ref obj);
            var newObj = cloner(objBefore);
            op(newObj);
            if (Interlocked.CompareExchange(ref obj, newObj, objBefore) == objBefore)
                return;
        }
    }
}

使用

CopyOnWriteSwapper.Swap(ref _myList,
    orig => new List<string>(orig),
    clone => clone.Add("asdf"));

如果你需要更多的性能，它将有助于非泛化方法，例如，为你想要的每一种类型的修改(添加，删除，…)创建一个方法，并硬编码函数指针克隆和op。

注意事项#1:确保没有人修改(假定)不可变的数据结构是你的责任。在泛型实现中我们无法阻止这种情况，但当专门化到List<T>时，可以使用List. asreadonly()来防止修改。

注意事项2 .注意列表中的值。上面的copy on write方法只保护它们的列表成员关系，但如果你放的不是字符串，而是一些其他可变对象，你必须注意线程安全(例如锁定)。但这与这个解决方案是正交的，例如，可变值的锁定可以很容易地使用而没有问题。你只需要意识到这一点。

注意事项#3如果您的数据结构很大，并且您经常修改它，那么从内存消耗和复制所涉及的CPU成本两方面考虑，全部在写时复制的方法可能是不可取的。在这种情况下，你可能想要使用MS的不可变集合。

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 ?

我实现了一个类似于Brian的方法。我的情况不同:

我直接管理数组。 我没有在try块中输入锁。 我使用yield return生成枚举器。 我支持锁递归。这允许在迭代期间从列表中读取。 我尽可能使用可升级的读锁。 DoSync和GetSync方法允许需要独占访问列表的顺序交互。

代码:

public class ConcurrentList<T> : IList<T>, IDisposable
{
    private ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);
    private int _count = 0;

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

    public int InternalArrayLength
    { 
        get
        { 
            _lock.EnterReadLock();
            try
            {           
                return _arr.Length;
            }
            finally
            {
                _lock.ExitReadLock();
            }
        }
    }

    private T[] _arr;

    public ConcurrentList(int initialCapacity)
    {
        _arr = new T[initialCapacity];
    }

    public ConcurrentList():this(4)
    { }

    public ConcurrentList(IEnumerable<T> items)
    {
        _arr = items.ToArray();
        _count = _arr.Length;
    }

    public void Add(T item)
    {
        _lock.EnterWriteLock();
        try
        {       
            var newCount = _count + 1;          
            EnsureCapacity(newCount);           
            _arr[_count] = item;
            _count = newCount;                  
        }
        finally
        {
            _lock.ExitWriteLock();
        }       
    }

    public void AddRange(IEnumerable<T> items)
    {
        if (items == null)
            throw new ArgumentNullException("items");

        _lock.EnterWriteLock();

        try
        {           
            var arr = items as T[] ?? items.ToArray();          
            var newCount = _count + arr.Length;
            EnsureCapacity(newCount);           
            Array.Copy(arr, 0, _arr, _count, arr.Length);       
            _count = newCount;
        }
        finally
        {
            _lock.ExitWriteLock();          
        }
    }

    private void EnsureCapacity(int capacity)
    {   
        if (_arr.Length >= capacity)
            return;

        int doubled;
        checked
        {
            try
            {           
                doubled = _arr.Length * 2;
            }
            catch (OverflowException)
            {
                doubled = int.MaxValue;
            }
        }

        var newLength = Math.Max(doubled, capacity);            
        Array.Resize(ref _arr, newLength);
    }

    public bool Remove(T item)
    {
        _lock.EnterUpgradeableReadLock();

        try
        {           
            var i = IndexOfInternal(item);

            if (i == -1)
                return false;

            _lock.EnterWriteLock();
            try
            {   
                RemoveAtInternal(i);
                return true;
            }
            finally
            {               
                _lock.ExitWriteLock();
            }
        }
        finally
        {           
            _lock.ExitUpgradeableReadLock();
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        _lock.EnterReadLock();

        try
        {    
            for (int i = 0; i < _count; i++)
                // deadlocking potential mitigated by lock recursion enforcement
                yield return _arr[i]; 
        }
        finally
        {           
            _lock.ExitReadLock();
        }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }

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

    private int IndexOfInternal(T item)
    {
        return Array.FindIndex(_arr, 0, _count, x => x.Equals(item));
    }

    public void Insert(int index, T item)
    {
        _lock.EnterUpgradeableReadLock();

        try
        {                       
            if (index > _count)
                throw new ArgumentOutOfRangeException("index"); 

            _lock.EnterWriteLock();
            try
            {       
                var newCount = _count + 1;
                EnsureCapacity(newCount);

                // shift everything right by one, starting at index
                Array.Copy(_arr, index, _arr, index + 1, _count - index);

                // insert
                _arr[index] = item;     
                _count = newCount;
            }
            finally
            {           
                _lock.ExitWriteLock();
            }
        }
        finally
        {
            _lock.ExitUpgradeableReadLock();            
        }


    }

    public void RemoveAt(int index)
    {   
        _lock.EnterUpgradeableReadLock();
        try
        {   
            if (index >= _count)
                throw new ArgumentOutOfRangeException("index");

            _lock.EnterWriteLock();
            try
            {           
                RemoveAtInternal(index);
            }
            finally
            {
                _lock.ExitWriteLock();
            }
        }
        finally
        {
            _lock.ExitUpgradeableReadLock();            
        }
    }

    private void RemoveAtInternal(int index)
    {           
        Array.Copy(_arr, index + 1, _arr, index, _count - index-1);
        _count--;

        // release last element
        Array.Clear(_arr, _count, 1);
    }

    public void Clear()
    {
        _lock.EnterWriteLock();
        try
        {        
            Array.Clear(_arr, 0, _count);
            _count = 0;
        }
        finally
        {           
            _lock.ExitWriteLock();
        }   
    }

    public bool Contains(T item)
    {
        _lock.EnterReadLock();
        try
        {   
            return IndexOfInternal(item) != -1;
        }
        finally
        {           
            _lock.ExitReadLock();
        }
    }

    public void CopyTo(T[] array, int arrayIndex)
    {       
        _lock.EnterReadLock();
        try
        {           
            if(_count > array.Length - arrayIndex)
                throw new ArgumentException("Destination array was not long enough.");

            Array.Copy(_arr, 0, array, arrayIndex, _count);
        }
        finally
        {
            _lock.ExitReadLock();           
        }
    }

    public bool IsReadOnly
    {   
        get { return false; }
    }

    public T this[int index]
    {
        get
        {
            _lock.EnterReadLock();
            try
            {           
                if (index >= _count)
                    throw new ArgumentOutOfRangeException("index");

                return _arr[index]; 
            }
            finally
            {
                _lock.ExitReadLock();               
            }           
        }
        set
        {
            _lock.EnterUpgradeableReadLock();
            try
            {

                if (index >= _count)
                    throw new ArgumentOutOfRangeException("index");

                _lock.EnterWriteLock();
                try
                {                       
                    _arr[index] = value;
                }
                finally
                {
                    _lock.ExitWriteLock();              
                }
            }
            finally
            {
                _lock.ExitUpgradeableReadLock();
            }

        }
    }

    public void DoSync(Action<ConcurrentList<T>> action)
    {
        GetSync(l =>
        {
            action(l);
            return 0;
        });
    }

    public TResult GetSync<TResult>(Func<ConcurrentList<T>,TResult> func)
    {
        _lock.EnterWriteLock();
        try
        {           
            return func(this);
        }
        finally
        {
            _lock.ExitWriteLock();
        }
    }

    public void Dispose()
    {   
        _lock.Dispose();
    }
}

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
}

System.Collections.Generic。List<t>对于多个读取器来说已经是线程安全的。试图使它对多个写入器是线程安全的是没有意义的。(原因Henk和Stephen已经提到了)