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Field like events and polymorphic invocation

Ce vi se pare gresit in codul de mai jos:
public class Base
{
public virtual event EventHandler EventOne;
}
public class Derived : Base
{
public override event EventHandler EventOne;
public void DoAction()
{
...
EventOne(this, args);
}
}
In clasa de baza avem un eveniment la care se poate face override in orice clasa derivata. Desi la prima vedere totul pare sa fie in ordine daca ne uitam mai atent, putem sa observam o problema care poate sa apara din cauza la override.
EventOne o sa existe in doua locatii diferite, atat in clasa de baza cat si in clasa derivata. Din aceasta cauza o sa fie momente cand EventOne din clasa de baza nu o sa fie niciodata instantiat.
Din aceasta cauza, in cazul in care evenimentul este apelat din clasa de baza ne putem trezi cu un comportament ciudat, precum NullReferenceException sau sa se execute cod la aruncarea unui eveniment pe care noi nu ne-am astepta sa se execute.
Ce putem sa facem ca sa rezolvam aceasta problema?
Avem la indemana doua solutii. Prima varianta este sa eliminam virtual si override. O sa fie nevoie sa ne declaram o metode de tip "event-firing" in clasa de baza. Aceasta metoda o sa fie adaugata in clasa de baza si tot ce va face este sa arunce evenimentul nostru. In clasa derivata nu se va folosii direct evenimentul ci se va apela metoda pe care noi am declarato in clasa de baza. Iar aceata la randul ei o sa arunce evenimentul. Mai jos puteti sa gasiti implementarea propusa:
public class Base
{
public event EventHandler EventOne;
public void FireEventOne(object sender, EventArgs args)
{
if (EventOne != null)
{
EventOne(sender, args);
}
}
}
public class Derived
{
public void DoAction()
{
...
FireEventOne(this, args);
}
}

In varianta data mai sus, clasa derivata nu mai poate sa faca override la eveniment, aceasta putand doar sa se inscrie la acest eveniment. Field-ul EventOne nu o sa mai fie duplicat in clasa de baza. Prim aceasta metoda am eliminat orice comportament ciudat care ar fi putut sa apara.
Daca avem nevoie sa facem override, atunci va recomand sa mergeti pe varianta in care in clasa de baza evenimentul sa fie declarat ca si abstract( nu consider aceasta variata ca fiind cea mai sanatoasa).
public class Base
{
public abstract event EventHandler EventOne;
}
Va recomand sa va ganditi de doua ori daca chiar aveti nevoie de un eveniment abstract. S-ar putea ca design-ul pe care l-ati ales sa nu fie cel mai bun.

Comments

  1. Pai cred ca prima intrebare ar fi: de ce ar avea nevoie cineva sa declare un event .. virtual si sa faca override la el?

    La urma urmei, event EventHandler nu e decat o colectie de delegates..

    Eventual daca vrea sa customizeze modul in care sunt stocati acei delegates folosing add/remove accessors sau sa fie thread-safe, da' ..

    ReplyDelete
  2. Si mie mi se pare un pic trasa de par. Iar FireEventOn - numita , de obicei, OnEventOne e ceea ce fac de obicei.

    ReplyDelete
  3. In doua proiecte diferite am vazut problema asta. Partea buna ii ca ReSharper-ul urla destul de tare cand vede probleme de genul acesta.

    ReplyDelete
  4. La http://blogs.msdn.com/b/samng/archive/2007/11/26/virtual-events-in-c.aspx se pare ca explica un nene de la Microsoft mai in detaliu care e problema, si de ce nu o pot fixa cum ar vrea..

    ReplyDelete

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