.NET Design Patterns

Memento


 Definition
 UML diagram
 Participants
 Structural code in C#
 Real-world code in C#
 .NET Optimized code in C#



Definition

Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

Frequency of use:
Low




UML class diagram






Participants


    The classes and objects participating in this pattern are:

  • Memento  (Memento)
    • stores internal state of the Originator object. The memento may store as much or as little of the originator's internal state as necessary at its originator's discretion.
    • protect against access by objects of other than the originator. Mementos have effectively two interfaces. Caretaker sees a narrow interface to the Memento -- it can only pass the memento to the other objects. Originator, in contrast, sees a wide interface, one that lets it access all the data necessary to restore itself to its previous state. Ideally, only the originator that produces the memento would be permitted to access the memento's internal state.
  • Originator  (SalesProspect)
    • creates a memento containing a snapshot of its current internal state.
    • uses the memento to restore its internal state
  • Caretaker  (Caretaker)
    • is responsible for the memento's safekeeping
    • never operates on or examines the contents of a memento.



Structural code in C#


This structural code demonstrates the Memento pattern which temporary saves and restores another object's internal state.

                 

using System;

 

namespace DoFactory.GangOfFour.Memento.Structural

{

  /// <summary>

  /// MainApp startup class for Structural

  /// Memento Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    static void Main()

    {

      Originator o = new Originator();

      o.State = "On";

 

      // Store internal state

      Caretaker c = new Caretaker();

      c.Memento = o.CreateMemento();

 

      // Continue changing originator

      o.State = "Off";

 

      // Restore saved state

      o.SetMemento(c.Memento);

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Originator' class

  /// </summary>

  class Originator

  {

    private string _state;

 

    // Property

    public string State

    {

      get { return _state; }

      set

      {

        _state = value;

        Console.WriteLine("State = " + _state);

      }

    }

 

    // Creates memento

    public Memento CreateMemento()

    {

      return (new Memento(_state));

    }

 

    // Restores original state

    public void SetMemento(Memento memento)

    {

      Console.WriteLine("Restoring state...");

      State = memento.State;

    }

  }

 

  /// <summary>

  /// The 'Memento' class

  /// </summary>

  class Memento

  {

    private string _state;

 

    // Constructor

    public Memento(string state)

    {

      this._state = state;

    }

 

    // Gets or sets state

    public string State

    {

      get { return _state; }

    }

  }

 

  /// <summary>

  /// The 'Caretaker' class

  /// </summary>

  class Caretaker

  {

    private Memento _memento;

 

    // Gets or sets memento

    public Memento Memento

    {

      set { _memento = value; }

      get { return _memento; }

    }

  }

}


Output
State = On
State = Off
Restoring state:
State = On




Real-world code in C#


This real-world code demonstrates the Memento pattern which temporarily saves and then restores the SalesProspect's internal state.

                 

using System;

 

namespace DoFactory.GangOfFour.Memento.RealWorld

{

  /// <summary>

  /// MainApp startup class for Real-World

  /// Memento Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    static void Main()

    {

      SalesProspect s = new SalesProspect();

      s.Name = "Noel van Halen";

      s.Phone = "(412) 256-0990";

      s.Budget = 25000.0;

 

      // Store internal state

      ProspectMemory m = new ProspectMemory();

      m.Memento = s.SaveMemento();

 

      // Continue changing originator

      s.Name = "Leo Welch";

      s.Phone = "(310) 209-7111";

      s.Budget = 1000000.0;

 

      // Restore saved state

      s.RestoreMemento(m.Memento);

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Originator' class

  /// </summary>

  class SalesProspect

  {

    private string _name;

    private string _phone;

    private double _budget;

 

    // Gets or sets name

    public string Name

    {

      get { return _name; }

      set

      {

        _name = value;

        Console.WriteLine("Name:  " + _name);

      }

    }

 

    // Gets or sets phone

    public string Phone

    {

      get { return _phone; }

      set

      {

        _phone = value;

        Console.WriteLine("Phone: " + _phone);

      }

    }

 

    // Gets or sets budget

    public double Budget

    {

      get { return _budget; }

      set

      {

        _budget = value;

        Console.WriteLine("Budget: " + _budget);

      }

    }

 

    // Stores memento

    public Memento SaveMemento()

    {

      Console.WriteLine("\nSaving state --\n");

      return new Memento(_name, _phone, _budget);

    }

 

    // Restores memento

    public void RestoreMemento(Memento memento)

    {

      Console.WriteLine("\nRestoring state --\n");

      this.Name = memento.Name;

      this.Phone = memento.Phone;

      this.Budget = memento.Budget;

    }

  }

 

  /// <summary>

  /// The 'Memento' class

  /// </summary>

  class Memento

  {

    private string _name;

    private string _phone;

    private double _budget;

 

    // Constructor

    public Memento(string name, string phone, double budget)

    {

      this._name = name;

      this._phone = phone;

      this._budget = budget;

    }

 

    // Gets or sets name

    public string Name

    {

      get { return _name; }

      set { _name = value; }

    }

 

    // Gets or set phone

    public string Phone

    {

      get { return _phone; }

      set { _phone = value; }

    }

 

    // Gets or sets budget

    public double Budget

    {

      get { return _budget; }

      set { _budget = value; }

    }

  }

 

  /// <summary>

  /// The 'Caretaker' class

  /// </summary>

  class ProspectMemory

  {

    private Memento _memento;

 

    // Property

    public Memento Memento

    {

      set { _memento = value; }

      get { return _memento; }

    }

  }

}


Output
Name:   Noel van Halen
Phone:  (412) 256-0990
Budget: 25000

Saving state --

Name:   Leo Welch
Phone:  (310) 209-7111
Budget: 1000000

Restoring state --

Name:   Noel van Halen
Phone:  (412) 256-0990
Budget: 25000




.NET Optimized code in C#


The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc. You can find an example on our Singleton pattern page.

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