No ConcurrentList<T> in .Net 4.0?

I was thrilled to see the new System.Collections.Concurrent namespace in .Net 4.0, quite nice! I've seen ConcurrentDictionary, ConcurrentQueue, ConcurrentStack, ConcurrentBag and BlockingCollection.

One thing that seems to be mysteriously missing is a ConcurrentList<T>. Do I have to write that myself (or get it off the web :) )?

Am I missing something obvious here?

I gave it a try a while back (also: on GitHub). My implementation had some problems, which I won't get into here. Let me tell you, more importantly, what I learned.

Firstly, there's no way you're going to get a full implementation of IList<T> that is lockless and thread-safe. In particular, random insertions and removals are not going to work, unless you also forget about O(1) random access (i.e., unless you "cheat" and just use some sort of linked list and let the indexing suck).

What I thought might be worthwhile was a thread-safe, limited subset of IList<T>: in particular, one that would allow an Add and provide random read-only access by index (but no Insert, RemoveAt, etc., and also no random write access).

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>.

In the relatively rare event that the list's internal array needs to resize itself, you do pay a small cost. So ultimately I concluded that this was the one niche scenario where an add-only ConcurrentList<T> collection type would make sense: when you want guaranteed low overhead of adding an element on every single call (so, as opposed to an amortized performance goal).

It's simply not nearly as useful a class as you would think.

What would you use a ConcurrentList for?

The concept of a Random Access container in a threaded world isn't as useful as it may appear. The statement

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

as a whole would still not be thread-safe.

Instead of creating a ConcurrentList, try to build solutions with what's there. The most common classes are the ConcurrentBag and especially the BlockingCollection.

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.

And for what its worth: I did a basic test of adding 10,000,000 items to regular List and ConcurrentList and the results were:

List finished in: 7793 milliseconds. Concurrent finished in: 8064 milliseconds.

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
}

The reason why there is no ConcurrentList is because it fundamentally cannot be written. The reason why is that several important operations in IList rely on indices, and that just plain won't work. For example:

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

The effect that the author is going after is to insert "dog" before "cat", but in a multithreaded environment, anything can happen to the list between those two lines of code. For example, another thread might do list.RemoveAt(0), shifting the entire list to the left, but crucially, catIndex will not change. The impact here is that the Insert operation will actually put the "dog" after the cat, not before it.

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.

If you think you need a concurrent list, there are really just two possibilities:

1. What you really need is a ConcurrentBag
2. You need to create your own collection, perhaps implemented with a List and your own concurrency control.

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.

ConcurrentList (as a resizeable array, not a linked list) is not easy to write with nonblocking operations. Its API doesn't translate well to a "concurrent" version.

In cases where reads greatly outnumber writes, or (however frequent) writes are non-concurrent, a copy-on-write approach may be appropriate.

The implementation shown below is

• 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

For copy-on-write to work, you have to keep your data structures effectively immutable, i.e. no one is allowed to change them after you made them available to other threads. When you want to modify, you

1. clone the structure
2. make modifications on the clone
3. atomically swap in the reference to the modified clone

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;
        }
    }
}

Usage

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

If you need more performance, it will help to ungenerify the method, e.g. create one method for every type of modification (Add, Remove, ...) you want, and hard code the function pointers cloner and op.

N.B. #1 It is your responsibility to make sure the no one modifies the (supposedly) immutable data structure. There's nothing we can do in a generic implementation to prevent that, but when specializing to List<T>, you could guard against modification using List.AsReadOnly()

N.B. #2 Be careful about the values in the list. The copy on write approach above guards their list membership only, but if you'd put not strings, but some other mutable objects in there, you have to take care of thread safety (e.g. locking). But that is orthogonal to this solution and e.g. locking of the mutable values can be easily used without issues. You just need to be aware of it.

N.B. #3 If your data structure is huge and you modify it frequently, the copy-all-on-write approach might be prohibitive both in terms of memory consumption and the CPU cost of copying involved. In that case, you might want to use MS's Immutable Collections instead.

System.Collections.Generic.List<t> is already thread safe for multiple readers. Trying to make it thread safe for multiple writers wouldn't make sense. (For reasons Henk and Stephen already mentioned)

Some people hilighted some goods points (and some of my thoughts):

• 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.

That is not an answer. This is only comments that do not really fit to a specific place.

... 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 ?

I implemented one similar to Brian's. Mine is different:

• I manage the array directly.
• I don't enter the locks within the try block.
• I use yield return for producing an enumerator.
• I support lock recursion. This allows reads from list during iteration.
• I use upgradable read locks where possible.
• DoSync and GetSync methods allowing sequential interactions that require exclusive access to the list.

The code:

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();
	}
}

In sequentially executing code the data structures used are different from (well written) concurrently executing code. The reason is that sequential code implies implicit order. Concurrent code however does not imply any order; better yet it implies the lack of any defined order!

Due to this, data structures with implied order (like List) are not very useful for solving concurrent problems. A list implies order, but it does not clearly define what that order is. Because of this the execution order of the code manipulating the list will determine (to some degree) the implicit order of the list, which is in direct conflict with an efficient concurrent solution.

Remember concurrency is a data problem, not a code problem! You cannot Implement the code first (or rewriting existing sequential code) and get a well designed concurrent solution. You need to design the data structures first while keeping in mind that implicit ordering doesn’t exist in a concurrent system.

lockless Copy and Write approach works great if you're not dealing with too many items. Here's a class I wrote:

public class CopyAndWriteList<T>
{
    public static List<T> Clear(List<T> list)
    {
        var a = new List<T>(list);
        a.Clear();
        return a;
    }

    public static List<T> Add(List<T> list, T item)
    {
        var a = new List<T>(list);
        a.Add(item);
        return a;
    }

    public static List<T> RemoveAt(List<T> list, int index)
    {
        var a = new List<T>(list);
        a.RemoveAt(index);
        return a;
    }

    public static List<T> Remove(List<T> list, T item)
    {
        var a = new List<T>(list);
        a.Remove(item);
        return a;
    }

}

example usage: orders_BUY = CopyAndWriteList.Clear(orders_BUY);

I'm surprised no-one has mentioned using LinkedList as a base for writing a specialised class.

Often we don't need the full API's of the various collection classes, and if you write mostly functional side effect free code, using immutable classes as far as possible, then you'll actually NOT want to mutate the collection favouring various snapshot implementations.

LinkedList solves some difficult problems of creating snapshot copies/clones of large collections. I also use it to create "threadsafe" enumerators to enumerate over the collection. I can cheat, because I know that I'm not changing the collection in any way other than appending, I can keep track of the list size, and only lock on changes to list size. Then my enumerator code simply enumerates from 0 to n for any thread that wants a "snapshot" of the append only collection, that will be guaranteed to represent a "snapshot" of the collection at any moment in time, regardless of what other threads are appending to the head of the collection.

I'm pretty certain that most requirements are often extremely simple, and you need 2 or 3 methods only. Writing a truly generic library is awfully difficult, but solving your own codes needs can sometimes be easy with a trick or two.

Long live LinkedList and good functional programming.

Cheers, ... love ya all! Al

p.s. sample hack AppendOnly class here : https://github.com/goblinfactory/AppendOnly