FastList.cs

    public class FastList<T>
    {
        private readonly List<T> _internalList = new List<T>();
        public int Count => _count;
        private int _count;
        private int _freeIndex;

        public T this[int index] => _internalList[index];

        public int Add(T input)
        {
            while (true)
            {
                if (_freeIndex == _count)
                {
                    _internalList.Add(input);
                    return _count++;
                }
                else
                {
                    if (_internalList[_freeIndex] == null || _internalList.Equals(default(T)))
                    {
                        _internalList[_freeIndex] = input;
                        return _freeIndex++;
                    }
                    else
                    {
                        _freeIndex++;
                    }
                }
            }
        }

        public void Remove(int index)
        {
            _internalList[index] = default(T);
            Interlocked.Exchange(ref _freeIndex, index);
        }

        public bool Exists(int index)
        {
            if (_count < index || _internalList[index] == null || _internalList[index].Equals(default(T)))
                return false;
            return true;
        }

        public bool TryGetValue(int key, out T value)
        {
            if (!Exists(key))
            {
                value = default(T);
                return false;
            }
            else
            {
                value = _internalList[key];
                return true;
            }
        }

        public bool TryPickValue(int key, out T value)
        {
            if (!Exists(key))
            {
                value = default(T);
                return false;
            }
            else
            {
                value = _internalList[key];
                Remove(key);
                return true;
            }
        }
    }