using System;

using System.Collections;

using System.Collections.Generic;

​

public interface ICommand

{

    void Execute();

}

​

public interface IUndoableCommand: ICommand

{

    void Unexecute();

}

​

public class Document

{

    public string Content {get; set;}

    public void MakeBold()

    {

        Content = "<b>" + Content + "</b>";

    }

}

​

public class BoldCommand: IUndoableCommand

{

    private Document document;

    private History history;

    private string prevContent;

    public BoldCommand(Document document, History history)

    {

        this.document= document;

        this.history = history;

    }

    public void Execute()

    {

        this.prevContent = document.Content;

        document.MakeBold();

        history.Push(this);

    }

    public void Unexecute()

    {

        document.Content = this.prevContent;

    }

}

​

public class UndoCommand: ICommand

{

    private History history;

    public UndoCommand(History history)

    {

        this.history = history;

    }

​

    public void Execute()

    {

        var command = history.Pop();

        command.Unexecute();

    }

}

​

public class History

{

    private Stack<IUndoableCommand> stack = new Stack<IUndoableCommand>();

    public void Push(IUndoableCommand command)

    {

        stack.Push(command);

    }

    public IUndoableCommand Pop()

    {

        return stack.Pop();

    }

}

​

public class Program

{

    public static void Main()

    {

        var history = new History();

        var document = new Document();

        document.Content = "Hello World";

        Console.WriteLine(document.Content);

        var command = new BoldCommand(document, history);

        command.Execute();

        Console.WriteLine(document.Content);

        var undoCommand = new UndoCommand(history);

        undoCommand.Execute();

        Console.WriteLine(document.Content);

    }

}