C# Online Compiler | .NET Fiddle

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);
	}
}
Cached Result
result: 0.428571428571429