说明：  C#中使用命令模式实现Undo/Redo 撤销/重做 原理很简单，通过Command模式把每一步操作封装成一个可undo的命令（包含do和redo两个操作）。并将每一步操作执行后用栈保存起来，undo的时候就以此将Command依次出栈，并执行undo操作。
(The principle is very simple, by the Command pattern to encapsulate each step into an undo command (containing both do and redo operations). After performing with a stack and save it, undo one step every time you turn the stack in order to Command and perform undo operations.)