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Saturday, 10 March 2012

Resources from base class

Posted on 12:25 by Unknown
Pornim de la urmatoarea problema:
O clasa de baza expune printr-o propietate o resursa de orice fel. Prin modul in care este implementata clasa de baza, clasele care mostenesc clasa noastra de baza nu sunt obligate sa initializeze propietatea noastra.
class BasePC
{
public string SomeData { get; private set; }

public BasePC(){ }

public BasePC(string someData)
{
SomeData = someData;
}
}

class DesktopPC : BasePC
{
public DesktopPC()
: base()
{

}
}
Ce parere aveti despre aceasta propietate neinitializata. Este corect ca sa existe propietati in clasa de baza care sa nu fie initializate?
Nu ar fi prima data cand am avea aceasta problema. Exista diferite modalitati sa verificam daca aceasta propietate poate sa fie folosita sau nu, dar ...
In exemplul dat mai sus, cel care o sa foloseasca clasa DesktopPC se va astepta ca resursele care ii sunt puse la dispozitie sa fie deja initializate si sa le poata folosi fara nici o problema.
In exemplul dat mai sus, vorbim doar de un string, dar la fel de bine putea sa fie orice alt obiect sau orice alta resursa.
In urma acestei greseli pot sa apara nenumarate erori, iar fix-urile pentru ele sa arate destul de ciudat.
Problema este la nivelul clasei de baza, care pune la dispozitie o resursa care nu este initializata. Pentru a evita aceasta problema clasa de baza ar trebui sa initializeze toate resursele.
class BasePC
{
public string SomeData { get; private set; }
public BasePC(string someData = null)
{
SomeData = someData ?? "DefaultValue";
}
}

class DesktopPC : BasePC
{
public DesktopPC()
: base()
{

}
}
Dar o sa spuneti ca exista cazuri in clasele derivate cand aceasta resursa nu este folosita si nu este necesar sa fie initializate. In acest caz problema poate sa fie de la design. Aceasta resursa nu ar trebuii sa apara in clasa de baza ci intr-o clasa intermediara, iar in functie de ce date sunt necesare o clasa derivata sa mosteneasca din clasa de baza sau din clasa intermediara( atat clasa de baza cat si cea derivata pot sa fie clase abstracte - din aceasta cauza am folosit termenul de clasa intermediara). O alta solutie ar fi ca clasa noastra de baza sa poate sa aibe mai multe stari, pe baza carora cel care foloseste clasa noatra sa stie daca resursa este disponibila sau nu.

Orice resursa( proprietate) pe care o expunem dintr-o clasa de baza ar trebui sa fie initializata si gata sa fie folosita. Acest lucru nu poate sa fie facut de fiecare data( depinde de la caz la caz), dar cand ajungem intr-un caz in care aceasta nu poate sa fie initializata, ar trebui sa ne gandim daca putem sa evitam acest caz, deoarece in acelasi punct din doua clase derivate ce implementeaza aceiasi clasa de baza ma astept sa pot accesa aceleasi resurse din clasa de baza.
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