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Covarianta si contravarianta in .NET 4.0

Covarianta si contravarinata au existat de la .NET 1.0. Schimbarea pe care a adus-o .NET 4.0 a fost ca a introdus aceste funcționalități si in tipurile generice.
In teorie, un tip T este mai mare decat S, daca si numai daca S este un subtip( derivat) din T.
Din acest enunt putem sa ajungem la concluzia ca exista o oarecare unitate de masura prin care se pot descrie tipurile( in forma unui arbore). Putem sa consideram ca T este clasa de baza, iar S este clasa derivata.
Covarianta: daca Generic este mai mare sau egal decat Generic. Covarianta se folosește in special cand vrem ca de la ceva generic sa ajungem la ceva mai particular.
Covarianta ne permite sa tratam un obiect ca si cum ar fi mai sus in ierarhie. Din aceasta cauza se ajunge ca proprietățile sa fie read-only din tipul generic.
De exemplu daca avem un tip de baza Animal si doua clase care moștenesc aceasta clasa( Caine, Sarpe), in mod normal nu am putea sa avem:
public class Animal{}
public class Caine:Animal{}
public class Sarpe:Animal{}

public interface IBox<TAnimal>
{
TAnimal Obiect { get; }
}
...
IBox<Caine> caineBox = null;
IBox<Animal> animalBox = caineBox;
Pentru a putea face acest lucru avem nevoie de:
public interface IBox
{
TAnimal Obiect { get; }
}
Trebue sa tinem cont ca covarianta nu ne permite sa avem:

  • setter pe proprietatea Obiect;
  • o metoda care sa aibe ca parametru de intrare un parametru de tip TAnimal;
  • o metoda care sa aibe ca parametru de intrare un parametru de tip referinta(ref) de tip TAnimal;
Putem sa avem in schimb o metoda ce returneaza tipul TAnimal.
interface IBox
{
TAnimal GetAnimal(); // permis
void Proceseaza(TAnimal value); // nu este permis
void Schimba(ref TAnimal reference); // nu este permis

TAnimal Obiect
{
get; // permis
set; // nu este permis
}
}
Contravarianta: daca Generic este mai mic sau egal decat Generic. Contravarianta se folosește cand vrem ca de la un tip particular sa ajungem la un tip generic. Prin acest mecanism putem sa setam valori in clasa generica.
Contravarianta apare in momentul in care vrem sa avem ceva de genul:
public interface IBox
{
void Adauga(TAnimal animal);
}
...
IBox animalBox = Init();
animalBox.Adauga(new Sarpe());
Operatiile care nu sunt permise pentru contravarianta sunt:
  • o metoda nu poate returna un obiect de tip TAnimal;
  • o metoda nu poate sa accepte ca parametru de intrare un parametru de tip referinta(ref);
interface IBox
{
TAnimal GetAnimal(); // nu este permis
void Proceseaza(TAnimal value); // permis
void Schimba(ref TAnimal reference); // nu este permis

TAnimal Obiect
{
get; // nu este permis
set; // este permis
}
}
Este bine de stiut faptul ca contravarianta poate fi folosita cand este nevoie de nevoie de trecerea de la ceva mai particular la ceva mai generic. Covarianta este asemanatoare cu READ ONLY iar contravarianta cu WRITE ONLY.
Pentru mai multe informatii: http://community.devexpress.com/blogs/paulk/archive/2010/06/18/covariance-and-contravariance-in-net-4-0.aspx
O prezentare foarte interesanta: http://blog.mbharrington.org/2011/02/21/covariance-and-contravariance-in-net-4/

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