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Null object pattern - design pattern

Acest design pattern ar trebui sa ne ajute pentru a elimina sau cel putin sa reducem numarul de verificare de genul:
If( obj != null )
//Ceva cod.
Pentru a putea folosi acest design pattern este nevoie sa declaram o instanta a unui obiect( sau o clasa) care sa joace rolul obiectului care reprezinta valorea null. Exista doua varinte de implementare, de la caz la caz se poate folosi una din ele.
In prima varianta se foloseste o instanta a clasei care joaca rolul de valoarea NULL. Aceasta este oferita dezvoltatorului prin intermediul unei propietati statice.
public class Car
public static readonly Car NullObj;

static Car
NullObj = new Car();
//Setare valori, propietati cu valori default.
Deoarece am declarat field-ul ca fiind read-only, setarea acestuia se va putea face doar in constructor. Codul pe care o sa il folosim acuma o sa fie asemanator cu:
if( car == Car.NullObj )
//Executa ceva
In cazul in care vrem sa apelam metode sau propietati, putem le apelam fara sa facem verificarea daca obiectul este NULL sau nu. O sa putem scrie fara probleme cod de genul:
O alta varianta care se foloseste cand lucram cu clase abstracte sau cu interfete este sa ne declaram o clasa care sa reprezinte valoarea NULL. Iar aceasta clasa o sa poata fi folosita cand nu avem o instanta a clasei dorite si care poate sa aiba un comportament mai special, de exemplu sa faca o logare.
public abstract class Car
public static readonly Car NullObj;

static Car()
NullObj = new NullCar();
//Initializare custom daca este nevoie;

public class NullCar : Car
//Implementare custom pentru clasele abstracte sau virtuale.
Toate valoriile care o sa reprezinte instance NULL din clasa Car o sa fie de tip NullCar.
Acest pattern nu se poate sa fie folosit in orice caz, dar sunt cazuri cand se preteaza foarte bine. Un caz pe care l-am identificat este cand avem un API( un serviciu) care se ocupa de notificari. Mecanismul prin care se face notificati poate sa fie configurabil( SMS,MSM, email, etc) si chiar schimbat fara sa oprim aplicatia. Pot sa apara cazuri cand acest mecanism sa nu fie configurat, caz in care ar fi nevoie sa verificam inante de a apela metoda care face notificarea daca valoarea este NULL sau nu. Pentru acest caz am putea sa avem ceva de genul:
public interface INotification
void Notify();
public class NullNotification : INotification
public void Notify
//Se poate face un login.
In acest caz, daca nu avem nici un mecanism de notificare configurat, putem ca in NullNotification sa logam aceasta problema.


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