Queue implemented in stacks

QueueByStacks.cs

using System.Collections.Generic;

class QueueByStacks<T>
{
    private Stack<T> _inStack, _outStack;

    public QueueByStacks()
    {
        _inStack = new Stack<T>();
        _outStack = new Stack<T>();
    }

    public void Enqueue(T item)
    {
        _inStack.Push(item);
    }

    public T Dequeue()
    {
        if (_outStack.Count > 0)
        {
            return _outStack.Pop();
        }

        while (_inStack.Count > 0)
        {
            _outStack.Push(_inStack.Pop());
        }

        return _outStack.Count > 0? _outStack.Pop() : throw new InvalidOperationException("Nothing to dequeue");
    }
}