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Dispose and finalize

Am avut o discutie zilele astea cu un coleg despre Dispose() si Finalize(), despre modul in care se apeleaza cele doua. O sa prezint pe scurt cele doua metode.
Pentru a implementa metoda Dispose() este nevoie sa implementa interfata IDispose(). Aceasta metoda nu primeste nici un parametru. Se apeleaza de catre dezvoltator cand se doreste sa se elibere resurse folosite de aceasta entitate.
Resursele unmanaged sunt eliberate prin intermediul acestei metode si reprezinta reponsabilitatea utilizatorului.
Pana aici totul este destul de clar.
public class ProcessData: IDispose
{
private Stream _fileStream;
//....
public void Dispose()
{
_fileStream.Dispose();
}
}
Inainte sa explic ce face metoda Finalize si de cine este apelata vreau sa explic cum functioneaza pe scurt GC:
  • Cauta toate resursele (obiectele) managed care sunt referite in codul managed;
  • Incearca sa faca Finalize pe obiectele care nu mai sunt referite in cod;
  • Elibereaza obiectele care nu mai sunt referentiate;
  • Elibereaza spatiul din memorie ocupat de acestea;
Metoda Finalize() este apelata de catre GAG si reprezinta procesul in care GC elibereaza resulsele inainte sa distruga instanta unui obiect. Acesta ar trebuii sa elibereze doar resulsele externe care sunt referentiate. Aceasta metoda este apelata in mod automat, nu se poate controla (in general) momentul in care se apeleaza.
Trebuie avut grija cand se implementeaza aceasta metoda deoarece ordinea in care care se apeleaza Finalize() nu poate sa fie controlata.
Cea ce este bine de stiut, este ca in momentul in care se implementeaza Finalize() si Dispose() se recomanda ca Finalize() sa execute si codul care apare in Dispose(). GC nu o sa apeleze niciodata Dispose().
Totodata metoda Finalize() trebuie sa apeleze obligatoriu base.Finalize(), recomanda intr-un bloc finnaly:
protected override void Finalize()
{
try
{
//Code Cleanup
}
finally
{
base.Finalize();
}
}
Urmatoarele reguli trebuie respectate cand se implementeaza Finalize():
  • Trebuie sa fie mereu protected;
  • Trebuie sa apeleze base.Finalize();
  • Trebuie sa elibere resursele unmanaged;
  • .NET nu garanteaza ca Finalize() se va executa pentru fiecare instanta separat intr-un anumit moment;
  • Nu faceti new aceasta metoda;
  • Nu aruncati exceptii din Finalize();
  • Nu il definiti in date de tip value type;
  • Nu implementati Finalize() cu body gol, deoarece se consuma resurse fara rost;
Acest mecanism trebuie sa fie considerat un mecanism de fallback. Pentru eliberea resurselor se recomanda sa se folosesca Dispose(), iar Finalize() sa apeleze la randul lui codul care se executa in Dispose().
Dar Dispose() nu face release la obiect. In cazul in care dispose-ul elibereaza resursele, atunci nu mai este nevoie sa executam si Finalize(). Pentru a spune la GC sa nu mai ruleze Finalize() trebuie sa scriem urmatoarea linie de cod in metoda Dispose():
GC.SuppressFinalize(this).
Mai jos gasiti un exemplu cum sa se apeleze Dispose():
class StoreManager: IDisposable
{
private bool isDisposed = false;

~StoreManager: ()
{
Dispose(false);
}

protected void Dispose(bool disposing)
{
if (disposing)
{
// Dispose la resurse
}

isDisposed = true;
}

public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}

Parca Finalize() si Dispose() sunt mai clare acuma.

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