/// <summary>
/// 接收一个访问者，使所有的Element都执行这个动作
/// </summary>
/// <param name="visitor"></param>
public void Accept(Visitor visitor)
{
foreach (var item in _list)
{
item.Accept(visitor);
}
}
public override void Accept(Visitor visitor)
{
visitor.CallByElementB(this);
Console.WriteLine();
}
public abstract void Accept(Visitor visitor);
