C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;

public class EditorState 
{
	private string content;

	public EditorState(string content) {
		this.content = content;
	}

	public string Content {
		get 
		{
			return content;
		}
	}
}

public class Editor 
{
	public string Content {get; set; }

	public EditorState createState() {
		return new EditorState(Content);
	}

	public void restore(EditorState state) {
		Content = state.Content;
	}
}

public class History 
{
	private List<EditorState> states = new List<EditorState>();

	public void push(EditorState state) {
		states.Add(state);
	}

	public EditorState pop() {
		var lastIndex = states.Count - 1;
		var lastState = states[lastIndex];
		states.Remove(lastState);

		return lastState;
	}
}
					
public class Program
{
	public static void Main()
	{
		var editor = new Editor();
		var history = new History();

		editor.Content = "hello";
		history.push(editor.createState());

		editor.Content = "hello world";
		history.push(editor.createState());

		editor.Content = "hello world design patterns";
		editor.restore(history.pop());
		Console.WriteLine(editor.Content);
	}
}
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