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Dispose pattern - using statement

In .NET nu trebuie sa facem dispose manual la obiectele pe care le iniţializam. Acesta face automat dispose la obiectele pe care nu le mai folosim. GC( garbage collector) se ocupa de acest lucru, stergand din memorie obiectele pe care nu le mai folosim.
Totuşi, exista momente când dorim sa controlam aceasta acţiune. De exemplu sunt cazuri când accesam baza de date, fişiere şi/sau resurse din COM. Ori avem alte obiecte în interiorul obiectului nostru care folosesc acest mecanism şi dorim sa facem dispose la ele. In aceste cazuri dorim sa controlam acţiunea de dispose. Cand spun sa o putem controla ma refer la doua lucruri separate:
  • momentul când se face "dispose";
  • acţiunea care sa se execute când se face dispose;
Pentru a putea controla acest mecanism trebuie sa implementam interfaţa IDispose. Nimic complicat pana aici, avem doar o metoda void Dispose() pe care trebuie sa o implementam. In interiorul ei putem sa facem acţiunea de dispose.
public class ProcessData: IDispose
{
private Stream _fileStream;
//....
public void Dispose()
{
_fileStream.Dispose();
}
}
Pentru a putea folosii acest pattern, putem sa folosim doua mecanisme.
Primul din ele este sa iniţializam obiectul într-un block try/catch, iar în zona de finally sa apelam Dispose().
var processData = null;
try
{
processData = new ProcessData();
//...
}
//Catch este optional.
catch(Exception ex)
{
...
}
finally
{
if(processData != null)
{
processData.Dispose();
}
}
Al doilea mecanism, care este mult mai elegant este prin folosirea lui using. Este doar un syntax sugar, care apelează in mod automat Dispose() în momentul în care se iese din block-ul using. Poate sa fie folosit doar pentru obiectele ce implementează interfata IDispose.
using(processData=new ProcessData()
{
//Executa ceva.
}
In mod automat după ce se iese din block-ul using, se va apela metoda Dispose(). Aceasta se va apela şi in momentul in care se arunca o excepţie din interiorul block-ului using.
Atenţie, nu înseamna ca după ce am ieşit din block-ul care aparţine la using, obiectul processData a fost eliberat din memorie. GC nu eliberează automat resursele ocupate de către acest obiect. Obiectul este marcat pentru ştergere, iar GC o sa ajungă într-un anumit moment sa facă dispose la acest obiect. using ne garantează doar ca metoda Dispose() o sa fie apelata în momentul în care se iese din acest block. In interiorul metodei Dispose() trebuie sa facem eliberarea resurselor pe care le ţinem ocupate sau pe care le putem controla

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