public void Insert(int index, T item) {
            // Note that insertions at the end are legal.
            if ((uint) index > (uint)_size) {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_ListInsert);
            }
            Contract.EndContractBlock();
            if (_size == _items.Length) EnsureCapacity(_size + 1);
            if (index < _size) {
                Array.Copy(_items, index, _items, index + 1, _size - index);
            }
            _items[index] = item;
            _size++;            
            _version++;
        }

        private T[] _items;