LimitedPool<T>

public class LimitedPool<T> : IDisposable where T : class
{
    readonly Func<T> _valueFactory;
    readonly Action<T> _valueDisposeAction;
    readonly TimeSpan _valueLifetime;
    readonly ConcurrentStack<LimitedPoolItem<T>> _pool;
    bool _disposed;

    public LimitedPool(Func<T> valueFactory, Action<T> valueDisposeAction, TimeSpan? valueLifetime = null)
    {
        _valueFactory = valueFactory;
        _valueDisposeAction = valueDisposeAction;
        _valueLifetime = valueLifetime ?? TimeSpan.FromHours(1);
        _pool = new ConcurrentStack<LimitedPoolItem<T>>();
    }

    public int IdleCount => _pool.Count;

    public LimitedPoolItem<T> Get()
    {
        LimitedPoolItem<T> item;
        // try to get live cached item
        while (!_disposed && _pool.TryPop(out item))
        {
            if (!item.Expired)
                return item;
            // dispose expired item
            item.Dispose();
        }
        // since no cached items available we create a new one
        return new LimitedPoolItem<T>(_valueFactory(), disposedItem =>
        {
            if (disposedItem.Expired)
            {
                // item has been expired, dispose it forever
                _valueDisposeAction(disposedItem.Value);
            }
            else
            {
                // item is still fresh enough, return it to the pool
                if (!_disposed)
                    _pool.Push(disposedItem);
            }
        }, _valueLifetime);
    }

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

    void Dispose(bool disposing)
    {
        if (disposing)
        {
            _disposed = true;
            var items = _pool.ToArray();
            foreach (var item in items)
                _valueDisposeAction(item.Value);
        }
    }
}

public class LimitedPoolItem<T> : IDisposable
{
    readonly Action<LimitedPoolItem<T>> _disposeAction;

    readonly TimeSpan _lifetime;
    bool _expired;

    public T Value { get; }

    internal bool Expired
    {
        get
        {
            if (_expired)
                return true;
            _expired = _stopwatch.Elapsed > _lifetime;
            return _expired;
        }
    }
    readonly Stopwatch _stopwatch;

    internal LimitedPoolItem(T value, Action<LimitedPoolItem<T>> disposeAction, TimeSpan lifetime)
    {
        _disposeAction = disposeAction;
        _lifetime = lifetime;
        Value = value;
        _stopwatch = new Stopwatch();
        _stopwatch.Start();
    }

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

    void Dispose(bool disposing)
    {
        if (disposing)
        {
            _disposeAction(this);
        }
    }
}