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The memento designpattern is a pattern thathelps to save the objectinternal state in an externalplace enabling us to restorethe state later when

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Licence CPOL

First Posted 1 Oct 2008

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Memento PatternBy ASP.NET Community, 12 Oct 2013

Editorial NoteThis articles was originally at wiki.asp.net but has now been given a new home on CodeProject. Editing rights forthis article has been set at Bronze or above, so please go in and edit and update this article to keep it fresh andrelevant.

The memento design pattern is a pattern that helps to save the object internal state in an external place enabling us to restore the state later when needed. The memento pattern doesn’t violate encapsulation of the internal state. The pattern is rarely used but it’s very helpful in scientific computing or in computer games (saving of check points in the game for example). Use Cases for the Memento Pattern You should use the pattern in the following cases:

You need to save object’s state and use the saved state later in order to restore the saved state.You don’t want to expose the internal state of your object.

UML Diagram

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Memento Pattern - CodeProject

http://www.codeproject.com/Articles/667701/Memento-Pattern[12/10/2013 22:24:03]

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Memento Pattern UML

Example in C# The following code is an example of how to implement the pattern:

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#region Originator

public class Originator<T>{

#region Properties

public T State { get; set; }

#endregion

#region Methods

/// <summary> /// Creates a new memento to hold the current /// state /// </summary> /// <returns>The created memento</returns> public Memento<T> SaveMemento() { return (new Memento<T>(State)); }

/// <summary> /// Restores the state which is saved in the given memento /// </summary> /// <param name="memento" />The given memento

public void RestoreMemento(Memento<T> memento) { State = memento.State; }

Memento Pattern - CodeProject

http://www.codeproject.com/Articles/667701/Memento-Pattern[12/10/2013 22:24:03]

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#endregion

}

#endregion

#region Memento

public class Memento<T>{

#region Properties

public T State { get; private set; }

#endregion

#region Ctor

/// <summary> /// Construct a new memento object with the /// given state /// </summary> /// <param name="state" />The given state public Memento(T state) { State = state; }

#endregion

}

#endregion

#region Caretaker

public class Caretaker<T>{ #region Properties public Memento<T> Memento { get; set; } #endregion}

#endregion

Memento Pattern - CodeProject

http://www.codeproject.com/Articles/667701/Memento-Pattern[12/10/2013 22:24:03]

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The given example shows the three parts of the pattern: the Originator, the Memento and the Caretaker. The Caretaker is the repository for the Memento. You can also see that once the Memento object is created you can’t change the saved state and in order to save a new Memento you need to create it again. Lets look at an example of how to use the pattern in code:

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As you can see the saved state is inserted to the Caretaker object and than you can change the state of the Originator to whatever you need. In order to restore the Originator state you use the restore method and pass it the Caretaker’s saved Memento.

Summary To sum up the post, the memento pattern is is used for saving encapsulated object state in an external object. The state can then be restored by demand. As I wrote the places that you’ll want to use the pattern are in scientific computing or in computer games but it can be helpful also in other places.

LicenseThis article, along with any associated source code and files, is licensed under The Code Project Open License(CPOL)

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Originator<string> org = new Originator<string>();org.State = "Old State";// Store internal state in the caretaker objectCaretaker<string> caretaker = new Caretaker<string>();caretaker.Memento = org.SaveMemento();Console.WriteLine("This is the old state: {0}", org.State);org.State = "New state";Console.WriteLine("This is the new state: {0}", org.State);// Restore saved state from the caretakerorg.RestoreMemento(caretaker.Memento);Console.WriteLine("Old state was restored: {0}", org.State);// Wait for userConsole.Read();

Memento Pattern - CodeProject

http://www.codeproject.com/Articles/667701/Memento-Pattern[12/10/2013 22:24:03]

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