Correct way to implement a resource pool

Question

I\'m trying to implement something that manages a pool of resources such that the calling code can request an object and will be given one from the pool if it\'s available,

Answer 1

As was already mentioned in the comments, a counting semaphore is your friend. Combine this with a concurrent stack and you have got a nice simple, thread safe implementation, where you can still lazily allocate your pool items.

The bare-bones implementation below provides an example of this approach. Note that another advantage here is that you do not need to "contaminate" your pool items with an InUse member as a flag to track stuff.

Note that as a micro-optimization, a stack is preferred over a queue in this case, because it will provide the most recently returned instance from the pool, that may still be in e.g. L1 cache.

public class GenericConcurrentPool : IDisposable where T : class
{
    private readonly SemaphoreSlim _sem;
    private readonly ConcurrentStack _itemsStack;
    private readonly Action _onDisposeItem;
    private readonly Func _factory;

    public GenericConcurrentPool(int capacity, Func factory, Action onDisposeItem = null)
    {
        _itemsStack = new ConcurrentStack(new T[capacity]);
        _factory = factory;
        _onDisposeItem = onDisposeItem;
        _sem = new SemaphoreSlim(capacity);
    }

    public async Task CheckOutAsync()
    {
        await _sem.WaitAsync();
        return Pop();
    }

    public T CheckOut()
    {
        _sem.Wait();
        return Pop();
    }

    public void CheckIn(T item)
    {
        Push(item);
        _sem.Release();
    }

    public void Dispose()
    {
        _sem.Dispose();
        if (_onDisposeItem != null)
        {
            T item;
            while (_itemsStack.TryPop(out item))
            {
                if (item != null)
                    _onDisposeItem(item);
            }
        }
    }

    private T Pop()
    {
        T item;
        var result = _itemsStack.TryPop(out item);
        Debug.Assert(result);
        return item ?? _factory();
    }

    private void Push(T item)
    {
        Debug.Assert(item != null);
        _itemsStack.Push(item);
    }
}