• Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
• The intent of the Memento pattern is to provide storage as well as restoration of an object.
  • The mechanism in which you store the object’s state depends on the required duration of persistence which could be a few seconds, a few days, or possibly years.
  • Storage options include memory (for example Session), a scratch file, or a database.
• In an abstract sense you can view a database as an implementation of the Memento design pattern. However, the most common reason for using this pattern is to capture a snapshot of an object’s state so that any subsequent changes can be undone easily if necessary.
  • In .NET the most common implementation is when serializing and de-serializing objects to save and restore an object’s state.
• Essentially, a Memento is a small repository that stores an object’s state.
  • Scenarios in which you may want to restore an object into a state that existed previously include: saving and restoring the state of a player in a computer game or the implementation of an undo operation in a database.
• The Memento pattern creates a snapshot of an object’s state and then offers the ability to restore it to its original state.
• This is what the serialization architecture of .NET offers and therefore qualifies as an example of Memento in the .NET Framework.

Participants

• Memento (Memento)
  • Stores internal state of the Originator object.
    • The memento may store as much or as little of the originators internal state as necessary.
  • 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 its internal state.
• Originator (SalesProspect)
  • Creates a memento containing a snapshot of its current internal state.
  • Uses the memento to restore its internal state.
• Caretaker (MementoHolder)
  • Is responsible for the memento's safekeeping.
  • Never operates on or examines the contents of a memento.

var salesProspect = new SalesProspect
{
	Name = "Noel van Halen",
	Phone = "(412) 256-0990",
	Budget = 25000.0
};

var salesProspectMemento = salesProspect.Copy();
var prospectMemory = new MementoHolder(salesProspectMemento);

salesProspect.Name = "Leo Welch";
salesProspect.Phone = "(310) 209-7111";
salesProspect.Budget = 1000000.0;

salesProspect.Restore(prospectMemory.Memento);

public class SalesProspect
{
	public string Name { get; set; }
	public string Phone { get; set; }
	public double Budget { get; set; }

	public Memento Copy() => new Memento
	{
		Name = Name,
		Phone = Phone,
		Budget = Budget
	};

	public void Restore(Memento memento)
	{
		Name = memento.Name;
		Phone = memento.Phone;
		Budget = memento.Budget;
	}
}

public class Memento
{
	public string Name { get; set; }
	public string Phone { get; set; }
	public double Budget { get; set; }      
}

public class MementoHolder
{
	public Memento Memento { set; get; }
	public MementoHolder(Memento memento) => Memento = memento;
}

Rules of Thumb

• Command and Memento act as magic tokens to be passed around and invoked at a later time.
  • In Command, the token represents a request; in Memento, it represents the internal state of an object at a particular time.
  • Polymorphism is important to Command, but not to Memento because its interface is so narrow that a memento can only be passed as a value.
• Command can use Memento to maintain the state required for an undo operation.
• Memento is often used in conjunction with Iterator.
  • An Iterator can use a Memento to capture the state of an iteration.
  • The Iterator stores the Memento internally.